State Geologicai, 4ij aarriaon Digitized by tiie Internet Arciiive in 2012 witii funding from University of Illinois Urbana-Champaign http://archive.org/details/geologymineralre37cady d.l STATE OF ILLINOIS DEPARTMENT OF REGISTRATION AND EDUCATION Division of the STATE GEOLOGICAL SURVEY FRANK W. DE WOLF, Chief BULLETIN No. 37 'pf) GEOLOGY AND MINERAL RESOURCES OF THE HENNEPIN AND LA SALLE QUADRANGLES BY GILBERT H. CADY Work in Cooperation with, the U. S. Geological Survey PRINTED BY AUTHORITY OF THE STATE OF ILLINOIS URBANA, ILLINOIS 1919 Springfield, III. Illinois State Journal Co.. State Printers. 19 19 2 01N4— 3M State of Illinois DEPARTMENT OF REGISTRATION AND EDUCATION Division of the STATE GEOLOGICAL SURVEY Fkank W. D-eWolf, Chief Committee of the Board of Xatukal Eesources AND Conservation Francis W. SiiEPAimsoN^ Chairman Director of Registration and Education Kendric C. Babcock Be presenting the President of the University of Illinois EoLLiN D. Salisbury Geologist LETTER OF TRANSMITTAL State Geological Survey Division, June 20, 1919. Ftancis W. Shepardson, Chairman, and Members of the Board of Natural Re- sources and Conservation. Gentlemen: — I submit heiewitli a report on the Geology and Mineral Resources of the Hennepin and La Salle Quadrangles, by Gilbert H. Cady, and recommend that it be published as Bulletin No. 37. The field work was done under the joint auspices of the U. S. Geological Survey and the Illinois Geological Survey Division. Much of the Hennepin Quadrangle was surveyed by U. S. Grant of Northwestern University, and the renjainder of that quadrangle and all of the La Salle quadrangle by Mr. Cady. I believe the report will appeal to many classes of readers both in Hlinois and without, as the area presents unusual problems for the geologist and attractive opportunities for industrial development, and is widely known not only by reason of the concentration there of mineral resources and industries, but perhaps as much for a historic and scenic interest rivalling that of any other part of Hlinois. Very respectfully, Frank W. DeWolf, Chief. ^'I'^i CONTENTS PAGE Chapter I. — Introduction 13 Importance of area 13 Location and extent of area 13 Acl<;nowledgments 15 Chapter II. — General topographic and geologic relationships of north central Illinois 17 Preliminary statement 17 Topography 17 Drainage 17 General stratigraphic relationships 18 General structural relationships 21 Chapter III. — Topography of the Hennepin and La Salle Quadrangles 23 Relief 23 General features 23 Upland prairies 23 Morainic hills 24 Valley features 25 Minor topographic features 27 Drainage 27 Culture 28 Chapter IV. — Stratigraphy 29 General character of the rocks 29 Cambrian system 32 Ordovician system 33 Lower Ordovician series 33 Prairie du Chien group 33 Oneota formation 33 New Richmond formation 34 Shakopee dolomite 35 Distribution and physiographic expression 35 Fossils and correlation 35 Lithologic character 36 Stratigraphic relations 36 Middle Ordovician series 37 St. Peter sandstone : c7 Character and distribution 37 Stratigraphic relations 39 Fossils and correlation , . 40 Platteville and Galena formations 40 Names and relation 40 Thickness 40 5 CONTENTS PAGE Chapter IV. — Stratigrarhy — Continued Thickness 40 Description and subdivision -J i Fossils and correlation 43 Stratigraphic relations 44 Upper Ordovician series 45 Maquoketa formation 45 Name and distribution 45 Thickness and character io Correlation and stratigraphic relations 46 Silurian and Devonian (?) systems 46 Thickness and lithologic character 46 Carboniferous system 47 Pennsylvanian series 47 General statement 47 Subdivisions 48 Pottsville formation 48 Thickness and lithology of the Pottsville between Bureau and Oglesby 48 Pottsville formation along and east of the La Salle anti- cline 50 Stratigraphic sequence 50 Lithologic character 52 Correlation 53 Carbondale formation 54 General features 54 La Salle (No. 2 ) coal 54 Strata between La Salle coal and Vermilionville sandstone 56 Strata from the Vermilionville sandstone to Springfield (H'arrisburg or No. 5) coal 57 Springfield (No. 5) coal 58 Carbondale strata above Springfield (No. 5) coal 59 McLeansboro formation 59 Name and definition 59 Strata between the base of the McLeansboro and the Streator (No. 7 ) coal 59 Streator (No. 7) coal 61 Strata between the Streator coal and the Lonsdale lime- stone G2 Lonsdale limestone 63 Strata between the Lonsdale limestone and the La Salle limestone 64 La Salle limestone 65 McLeansboro strata above the La Salle limestone 69 Tertiary (?) system 69 CONTENTS i PAGE Chapter IV. — Stratigraphy— Concluded Quaternary system 70 Pleistocene series 70 Pre-Illinoian drift and Yarmouth ( ?) soil 71 Pre-Illinoian loess and silt 72 Illinoian drift 72 Sangamon interglacial deposits 75 Loess and silt between the Illinoian and Wisconsin till sheets... 76 Early Wisconsin drift 77 Thickness and distribution 77 Till ^ 78 Sand and gravel 79 Moraines 81 Post-Early Wisconsin deposits 81 Gravel 81 Loess 82 Recent series 83 Dune sand 83 Alluvium 83 Chapter V.— Structure 85 Definition 85 Structure contour map 85 Uses of structure contour map 85 Accuracy of structure contours 86 Structure of the quadrangles 87 chapter VI. — Geologic history 91 Imperfection of the record 91 Paleozoic era 91 Cambrian period 91 Ordovician period 91 Lower and Middle Ordovician epochs 91 Upper Ordovician epoch 93 Silurian period 93 Devonian period 93 Carboniferous period 93 Mississippian epoch 93 Pennsylvanian epoch 94 Pottsville erosion an:l deposition 94 Carbondale deposition 94 McLeansboro deposition 95 Post Carboniferous deformation 95 Mesozoic era 95 Cenozoic era 96 Tertiary period 96 Development of relief 96 Quaternary period 97 Pleistocene epoch 97 Pre-Illinoian Kansan (?) and Yarmouth (?) time 97 Illinoian time 98 8 CONTENTS PAGE Chapter VI. — Geologic history — Concluded Sangamon-Peorian time 98 Wisconsin time 99 Recent epoch 101 Chapter VII. — Economic geology 103 Coal 103 Pottsville coal 103 Carbondale coals 103 La Salle (No. 2) coal 103 Springfield (No. 5) coal 104 McLeansboro coals 106 Streator (No. 7 ) coal 106 Coal beds above Streator (No. 7) coal. 107 Mining conditions 108 Chemical analyses 108 Clay and shale 110 Pennsylvanian clay and shale 110 Pottsville clay 110 Carbondale and McLeansboro clay and shales Ill Quaternary clays 114 Cement material 114 Natural cement 114 Portland cement 115 Limestone 116 Sandstone 116 Sand and gravel 116 Oil and gas 117 Indurated strata 117 Gas in the drift 117 Water resources 118 Surface water , 118 Underground water 118 Springs 118 Springs in the drift 118 Springs in the rock 118 Water table 120 Wells in tne drift and the alluvium 120 Shallow wells in the rock 122 Deep wells in the rock 122 Quality of the water 125 Soils 125 ILLUSTRATIONS PLATE PAGE I, Map showing surficial geology and rock outcrops of Hennepin and La Salle quadrangles In pocket II. Map showing economic, structural, and areal geology of Hennepin and La Salle quadrangles In pocket III. Map showing the structure of the St. Peter sandstone in the Hennepin and La Salle quadrangles 86 IV. Contours on La Salle (No. 2) coal in Ladd mine (furnished by F. 0. Chadwick, Engineer) 90 V. Map showing the pre-glacial surface and the altitude of buried soils in the Hennepin and La Salle quadrangles 9(^ VL Map showing the approximate outcrops of Springfield (No. 5) coal and Streator (No. 7) coal on the Hennepin and La Salle quadrangles 106 FIGURE 1. Map showing the location of the Hennepin and La Salle quadrangles... 14 2. Diagram illustrating an erosional unconformity, such as that existing between the St. Peter sandstone and Shakopee dolomite 18 3. Diagram illustrating a structural unconformity, such as that existing between the coal-bearing strata and underlying rocks as at Split Rock and Deer Park 19 4. Map showing the distribution of moraines in northeastern Illinois 20 5. Profile across a typical valley 25 6. Ridge (hog-back) on the upland east of the Little Vermilion River caused by the outcropping of the La Salle limestone along the La Salle anti- cline 26 7. The rock dump at Mine No. 3, Spring Valley Coal Company 27 8. Generalized columnar section of the rocks of the Hennepin and La Salle quadrangles 31 9. Shakopee dolomite outcropping along the north bluff of Illinois River between Split Rock and Utica 34 K). Shakopee dolomite, showing the brecciated structure 35 11. The contact of St. Peter sandstone and Lower Magnesian limestone near Split Rock (vertical scale about 16 feet to 1 inch) 36 12. Unconformity between St. Peter sandstone and Platteville-Galena dolomite at Deer Park 37 13. Deer Park Canyon looking downstream. The cliffs are St. Peter sand- stone 38 14. Galena-Platteville thin-bedded limestone exposed in Clayton's Ravine in the NB. % sec. 31, Deer Park Township 42 15. The irregular surface of Galena-Platteville lim.estone upon which was de- posited the Pottsville clay, as exposed in the bottom of an abandoned clay pit in section 15, Vermilion Township 44 9 10 ILLUSTRATIONS FIGURE PAGE IG. Boulder of pisolitic limestone from the Pottsville formation of south- western Illinois. Similar to boulders found in the Starved Rock region 51 17. Pottsville clays overlying the Platteville-Galena dolomite in a clay pit west of Starved Rock Park. Shows the irregularity of the surface of the dolomite 53 IS. Black fissile shale or "slate" 15 to 18 feet above the La Salle (No. 2) coal 55 19. Ironstone concretions or "niggerheads" in the black fissile shale above the La Salle (No. 2) coal SW. 14 sec. 9, Vermilion Township 56 20. Sketch map of part of the workings of the mine of Matthiessen and Hegeler Zinc Company in SE. i/4 sec. 9, Peru Township, in the nortli- west part of the mine, showing distribution of various kinds of roof rock CO 21. Streator (No. 7) coal cut by a "horseback" in the M. and H. mine, La Salle 61 22. Lonsdale limestone at Coal Hollow. The ledge is about 4 feet in thick- ness 63 23. La Salle limestone in the west quarry of the Chicago Portland Cement Company, near La Salle 65 24. Strata at the horizon of the La Salle limestone in a gulley in Peru, show- ing the more siliceous phase 66 25. Buried peat of the Sangamon interglacial epoch, lying above Illinoian till and below loess along East Bureau Creek near the center of N. ^Z. sec. 8, Selby Township 73 26. "Forest bed" below loess near the same location as the buried peat of figure 25 75 27. Loess above gravel of probable Illinoian age, east of Princeton 76 28. Gravel pits east of Spring Valley 79 29. Farm Ridge moraine north of Illinois Valley 81 30. Normal faults in Black Hollow mine 88 31 Thrust fault in the Oglesby mine 89 32. Step fault in Dalzell shaft. Spring Valley Coal Company 89 33. Map of Illinois showing the position of the Hennepin and La Salle quad- rangles with reference to the extent of the "Coal Measures" 102 34. Graphic sections of La Salle (No. 2) coal bed in Hennepin and La Salle quadrangles 105 35. Graphic sections of Springfield (No. 5) and Streator (No. 7) coal beds in Hennepin and La Salle quadrangles 106 3(; Fissure in the St. Peter sandstone parallel to and near the crest of the anticline. Several springs issue from this crack 119 TABLES PAGE 1. Names and locations of the shipping mines operating in the La Salle and Hennepin quadrangles and the depths, thicknesses, and elevations of the coal beds 107 2. Proximate analyses of coals in La Salle and Hennepin quadrangles 109 3. Analyses of Utica natural cement rock 115 4. Analyses of the La Salle limestone 115 5. Analyses of artesian waters in La Salle and Hennepin quadrangles 126 UNIVERSITY OF ILLINOIS LIBRARY OCT X * K GEOLOGY AND MINERAL RESOURCES OF THE HENNEPIN AND LA SALLE QUADRANGLES By Gilbert H. Cady CHAPTER I— IXTEODrCTIOX Importance of Area The area included in the Hennepin and La Salle quadrangles represents one of the wealthiest agricultural, manufacturing, and mining communities within the middle west. Valuable farm lands, world famous manufacturing industries, and extensive coal mining operations combine to make the area one which can hardly be equalled in commercial importance in the State out- side of the large cities of Chicago and East St. Louis. In no small degree have geological factors contributed to the importance of the region, for the manufacturing, as well as the agricultural and mining wealth is drawn largely from the natural resources of the region. That such a region possesses geo- logic interest is to be expected, and this interest is greatly enhanced by the presence in the area of unusual structural conditions, and by exposures of rocks and buried glacial deposits that are more numerous and better uncovered than is commonly the case in the Prairie Province. Further interest attaches to the region because of its nearness to Starved Eock State Park, the western end of which extends into the La Salle quad- rangle. Deer Park a companion area of nearly equal scenic interest and beauty lies along Vermilion River wholly within the La Salle quadrangle. The valleys of the Illinois and the Vermilion and to a less degree the other valleys tributary to the Illinois possess scenic attractiveness of exceptional quality in contrast to the monotony of the landscape of the surrounding plains. Location and Extent of Area Parts of La Salle, Bureau, and Putnam counties and the cities of La Salle, Peru, Oglesby, Spring Valley, and Hennepin are included within these tAvo quadrangles (fig. 1). A quadrangle is the unit of area chosen by the United States Geological Survey in the preparation of a topographic and geologic atlas of the entire country, and consists of a rectangular area of the eartli^s surface bounded by meridians and parallels, and hence does not con- 13 14 HENNEPIN AND LA SALLE QUADRANGLES form to state, county, or township boundaries. Each quadrangle measures 15 minutes of latitude by 15 minutes of longitude, a distance of approxi- mately 17 miles north and south by 13 miles east and west. The Hennepin and La Salle quadrangles are bounded on the north and south respectively by parallels 41° 30' and 41° 15'. The former parallel passes through the village of Triumph and the latter through the town of Hennepin. The east Fig. 1. — Map showing the location of the Hennepin and La Salle quadrangles. boundary of the La Salle quadrangle is the meridian 89° which passes one- half mile east of Utica, and the west boundary of the Hennepin quadrangle, which passes through the village of Tiskilwa, is the meridian 89° 30'. The area of two quadrangles at this latitude and longitude is about 448% square miles. The district lies across the valley of the Illinois, the Hennepin quad- rangle including that ])art of the valley known as the "Big Bend,^' and the La Salle quadrangle extending along the "Upper Illinois" to the head of navigation at TTtica. The city of La Salle is 99 miles west of Chicago on the INTRODUCTION 15 Kock Island Eailroacl, and Princeton is 104 miles from Chicago, on the Chicago, Burlington and Quinc}' Eailroad. What was formerly the main line of the Illinois Central Eailroad, but what is now the Freeport-Centralia ciivision, crosses the Illinois Eiver at La Salle. The terminus of the old Illinois Michigan Canal is at Peru and the Illinois and Mississippi or '"Ilennepin" Canal joins Illinois Eiver near Bureau and extends up the valley of Bureau Creek, paralleling the main line of the Eock Island Eail- road. The Chicago and Northwestern, the Chicago, Milwaukee, and St. Paul, and the Chicago, Indiana, and Southern railroads also enter the area. The Illinois Valley Electric Eailroad of the McKinley System does an import- ant interurban business in the district. Wagon roads are excellent, and there are several miles of the so-called ^SState-aid" roads radiating from the differ- ent towns. There is hardly a part of the district that is not readily acces- sible to automo))ile at any time. ACKXOWLEDGMENTS The field work on the La Salle and Hennepin quadrangles was carried on in cooperation with the U. S. Geological Survey, and the results will be published as a folio^ of the Geologic Atlas of the United States. The Henne- pin quadrangle was surveyed largely by U. S. Grant. The eastern part of this quadrangle and all of La Salle quadrangle were surveyed by G. H. Cady. Assistance was given by T. E. Savage who collected some and determined all the fossils, and by Frank Leverett and W. C. Alden who gave aid and criticism in the study of the glacial deposits. Acknowledgment is herewith made of suggestions received from reading a manuscript, since published, by Carl Sauer^ on the geography of the upper Illinois Valley especially concern- ing the history of the Illinois Valley. The maps and sketches illustrating the discussion were, in part, prepared by Miss Helen Skewes, of the Survey staff. To her and to other members of the State Geological Survey I am gratefully indebted for helpful criticism of the manuscript. For field super- vision and for numy suggestions in the organization of the report I owe thanks to Prof. V. S. Grant, with whom, originally, collaboration was intended. 1 This folio is one of a series prepaied in coopei^ation with the U. S. Geological Sur- vey. The reports and maps were submitted by the State. Five folios have previously been published, namely Murphysboro-Herrin, No. 185 ; Tallula-Springfield, No. 188 ; Belle- ville-Breese, No. 195; Galena-Elizabeth, No. 200; and Colchester-Macomb, No. 208. 2 Sauer, C. O., Geography of the upper Illinois Valley and history of development : State Geological Survey Bull. 27, 1916. CHAPTER II— GENERAL TOPOGRAPHIC AND GEOLOGIC RELATIONSHIPS Preliminary Statement These two quadrangles form a part of the Glaciated Plains province which includes the Prairie Plains and Lake Plains of Powell. All parts of the province have had a general history in common^ recorded in the rocks and in the topography/ Topography The part of the Glacial Plains province inckided in and adjacent to this area forms the north central part of Illinois. The relief of this part of the State is not great, ranging from ahout 400 feet above sea level in the Illinois valley to about 1,000 feet on the higher hills. The greater part of the area is monotonously level or only gently rolling. The principal feature of the region is the valley of the Illinois. It is a flat-bottomed, steep-sided, and fairly regular valley, 1 to 3 miles in width and 250 feet and less in depth. Surface features of the region, here as elsewhere in the north central part of Illinois, aside from those due to stream erosion, are caused mainly by the irregular disposition of the glacial drift. Numerous morainic 'belts sub- parallel to the shore of Lake Michigan pass through Illinois rising locally as much as 200 to 300 feet above the drift plains separating them. They vary greatly in width from a fraction of a mile to possibly 25 or 30 miles, the average width being probably 2 or 3 miles. The moraines are con- spicuous topographic features of the northern and central part of the State. in a region otherwise generally level. Drainage North central Illinois lies entirely within the drainage basin of the Illinois, its northern boundary being arbitrarily regarded as the water shed of Rock River. The region is fairly well drained, but swamps are not, uncommon in the river plains and the heads of some of the streams. The drift surface is sufficiently old so that the streams have drained many of the original irregularities in the drift, but here and there inter stream areas are broad and characterized by shallow ponds and swamps. The streams in general occupy youthful rather than mature valleys. ^ An outline of the geography and geology of the Glacial Plains province can be found in folios 185, 188, 195, 200, and 208. 17 —2 GB 18 HENNEPIN AND LA SALLE QUADRANGLES The pre-glacial drainage differed greatly from that of the present. It is believed that the Eock and possibly the Mississippi flowed south through Bureau County and continued southward in the approximate position of the present Illinois. Other streams tributary to this master stream entered it at various places^ the position of which can be determined to some extent by outcrop but chiefly by drilling. Whether the Illinois existed east of Bureau County as a single pre-glacial stream is a debated question. General Stratigraphic Eelationships Each Paleozoic system is represented in the strata underlying north central Illinois as far south as the south line of McLean County or as far west as Peoria County. The Cambrian system does not outcrop but is known from drilling and probably underlies the whole region. Strata of Ordovician age include two limestone members, one shale member, and a sandstone mem- FiG. 2. — Diagram illustrating an erosional unconformity, such as that existing between the St. Peter sandstone and Shakopee dolomite. Reproduced by permis- sion of the University of Chicago Press. ber. The older limestone is the "Lower Magnesian^^ limestone of the upper Mississippi Valley. This is overlain by the St. Peter sandstone. The second limestone, which succeeds the sandstone, is of middle Ordovician age and is made up of the Platteville and Galena formations. These formations of the Ordovician probably underlie all the northern and central parts of the State except where locally removed by erosion. The Cincinnatian or upper Ordo- vician series, represented by the Maquoketa shales and shaly limestone, over- lies the middle Ordovician limestones. These strata have been eroded off or were never deposited over the crest of the anticline in the northern part of the State, at least. Eocks of the Silurian system underlie the southern part of the area but outcrop along the flanks of the anticline to the north. The Niagaran dolo- GENERAL TOPOGRAPHIC AND GEOLOGIC RELATIONSHIPS 19 mite, which is the third of the important limestone formations of the Pale- ozoic group, is the only formation of the Silurian system known with reasonable probability to underlie the region. The knowledge of the distribu- tion of this formation is based upon drilling. The middle Devonian system is represented by the fourth important Pale- ozoic limestone formation. This formation does not extend east of the axis of the anticline and is possibly absent over much of the area to the west. Within north central Illinois it is known only from drilling. The upper part of the Devonian system is locally represented by shale. Strata of Mississippian or Lower Carboniferous age probably underlie the southern and western part of the region^ if drill cuttings from deep wells are correctly interpreted. This series contains the fifth important lime- stone member of the Paleozoic group in the interior province of the United States. This limestone formation and some of the underlying shales may extend into the region. The strata of the Pennsylvanian or Upper Carboniferous series overlie Fig. 3. — Diagram illustrating a structural unconformity, such as that existing between the coal-bearing strata and underlying rocks as at Split Rock and Deer Park. Reproduced by permission of the University of Chicago Press. the Mississippian and the older rocks and occur as surface formations below the drift over much of north central Illinois, the outcrop crossing the north- ern part of La Salle and Bureau counties. The Pennsylvanian series is made up largely of shale, but includes much sandstone, and lesser amounts of limestone and coal. Many beds are lenticular but certain strata especially beds of coal and limestone, are continuous over thousands of square miles. Because of the frequent oscillation of the sea over the interior region and the constantly changing position of the strand line that resulted, in no one locality in the interior region is there a complete stratigraphic record of the Paleozoic era. The many breaks in the record are marked by repeated erosional unconformities (fig. 2), by many instances of stratigraphic break 20 HENNEPIN AND LA SALLE QUADRANGLES apparent only from the interruption in the life record^ and in some places by angular or structural unconformities (fig. 3). There are at least five widesj^reacl and marked unconformities in the Paleozoic era. The interruption at the top of the Cambro-Ordovician lime- stone is general over large areas of the interior of the United States. The second break is below the Ordovician shale carrying the Richmond fauna. It is less conspicuous than tlie other four. The third is above the Ordovician (Richmond) shale and l)e]ow tlie Silurian. There is a fourth oreueral inter- CHICAGO Fig. 4. — Map showing the distribution of moraines in northeastern Illinois. After Leverett, Frank, the Illinois Glacial Lobe: U. S. Geological Survey Mono- graph XXXVIII. Reproduced by permission of the University of Chicago Press. ruption in the lower Devonian^ for in this region the upper or middle Devonian rocks commonly rest upon the Xiagaran lime-stone. The fifth and most conspicuous unconformity in the Paleozoic group of the interior GENERAL TOPOGRAPHIC AND GEOLOGIC RELATIONSHIPS 21 region occurs throughout the province at the base of the Pennsylvanian series. This unconformity results from the overlap of the Pennsylvanian rocks upon the older, and more or less deformed, strata of various ages from Missis- sippi an to early Ordovician. Strata of Mesozoic or Tertiary age are not known certainly to exist in the region. It is possible that scattered bodies of gravel found below the drift may represent rewashed if not original late Tertiary deposition. Deposits of glacial origin representing several stages of Pleistocene glaciation, the latest being the Wisconsin, and several interglacial epochs, have been found in the region. The areal distribution of the drift sheets and moraines in northern Illinois, is shown in Figure 4. General STRUCTurvAL Eelationships The structure of northern Illinois including the north central part of the State is dominated by the general southward dip of the strata from the Wisconsin state line toward the center of the eastern interior coal basin. This general southward dip is modified by a broad arching along an axis bearing about N". 30° W., which crosses the Illinois valley about 1 mile east of I^a Salie and which is known as the La Salle anticline. Because of these structural relationships older strata tend to be exposed to the north and also along the axis of the transverse La Salle anticline. The prevailing dips are low, gen- erally less than 1 degree. However, along a belt of steeply dipping strata on the west side of the axis of the La Salle anticline unusual inclination, as high as 52 degrees, has been observed, and strata commonly dip as much as 12 to 30 degrees. So far as known the strata have not been faulted, except possibly very locally and with off-sets of but a few feet. CHAPTER III— TOPOGRAPHY OF THE HENNEPIN AND LA SALLE QUADRANGLES Relief genekal features The Hennepin and La Salle region has comparatively low relief, and as a rule, moderate slopes. Its altitude is low, although the area is far from the sea. The greatest altitude, about 810 feet above sea level, is near the north- east corner of the Hennepin quadrangle; the lowest point, about 435 feet above sea level, is in the channel of the Illinois at Hennepin on the south line of the area. The surface except where cut by the valleys of the Illinois and its tributaries is that of a drift-formed plain of upland prairie from 650 to 750 feet above sea level. Above the general plain three morainic ridges rise 20 to 100 feet. Two of them cross the Hennepin quadrangle from the north- east to the southwest, and the other extends along the east side of the La Salle area. Into the upland prairie the Illinois has carved a valley which is in some places more than 2 miles wide and more than 200 feet deep. It is the dominant topographic feature of the region; tributary streams, how- ever, have also attacked the upland prairie and the moraines which rise above it, and have contributed to the topographic relief. The area contains several topographic types — upland prairies, morainic hills with associated kames, valley features which include, besides the valleys, erosional and gradational terraces, flood plains, wash slopes, and alluvial fans, and finally, minor topographic features represented by dunes, hog-backs, and mine dumps. UPLAND PRAIRIES The uplands comprise more than half of the area of the quadrangles, are flat except where crossed by morainic hills, and are largely prairie. The remarkably level character of the intermorainic upland plains is due to the presence of a mantle of nearly flat-surfaced glacial drift. The La Salle quadrangle has the more extensive flat upland, with a general slope from 700 feet above sea level near Arlington to 650 or 660 feet along the valley of Tomahawk Creek. Along the south boundary the upland elevation varies from about 700 feet near Granville to 620 near Lowell; and above the Illinois bluffs it averages 600 to 620 feet. The continuity of the flat upland on the Hennepin quadrangle is interrupted by the Arlington moraine, but in the northwest quarter of the area and between the Dover and Coal Hollow ridges the surface is very level. In Dover Township the plain has 23 24 HENNEPIN AND LA SALLE QUADRANGLES an elevation varying from 730 feet on the north to 700 feet on the south. The Maiden plain and the plain east of Arlington vary from 700 to 660 feet above sea level. The headwaters of many of the streams on the quadrangle rise in low, poorly drained^ swamp-like areas of considerable extent, upon the uplands, so that definite divides between streams are not discernible. MORAINIC HILLS Above the general upland surface rise several ridges having a north- easterly or southeasterly trend. These ridges are formed of glacial drift, are without rock cores, and possess subdued undulating topography. They are regarded as weak morainic ridges. Two belts of moraines are discernible. That to the west has its best development near Arlington, where it is about 3 miles in width and has a relief of about 100 to 125 feet above the upland prairie, rising to an altitude of about 810 feet. West of Arlington the moraine separates into two ridges which continue southwest in slightly diverging directions, and with con- stantly decreasing relief. Upon the west ridge, the Dover moraine, is situated the village of Dover and the city of Princeton. South of Princeton this moraine is scarcely discernible as a ridge because of dissection, but knob- and-kettle topography persists to the valley of Bureau Creek. South, across the valley of Bureau Creek, a weak ridge with morainic expression and pos- sibly the continuation of the Dover moraine can be followed east to Illinois Valley. The east or Coal Hollow moraine lies between East Bureau Creek and Brush Creek passing near but east of Coal Hollow. Toward Illinois Valley it becomes less conspicuous as a ridge but the topography is morainic to the bluff line. Another ridge, the "backbone" extends southeast from sees. 4 and 9, Waltham Township, north of IJtica, to the Vermilion Eiver east of La Salle. It is interrupted by Illinois Valley but south of the Illinois extends to the southeast as the Farm Ridge moraine, only the west fringe of which is in the La Salle quadrangle. This moraine has an altitude of about 700 feet above sea level in this area and a relief of 50 to 60 feet above the adjacent upland prairies. The surface of each ridge is undulating, commonly in open curves, but locally there are present the small hills and basins of typical knob-and-kettle topography. In the vicinity of Arlington in sec. 7, T. 17 N., R. 11 E., one such kettle of considerable size has until recently been occupied by a lake or swamp. Near Zearing in sees. 21 and 22, Berlin Township, and also near Hallo- way ville in sees. 14 and 15, Selby Township, small hills of sand and gravel having a relief of 25 to 40 feet above the surroundings lie on or adjacent TOPOGRAPHY 25 to the Coal Hollow moraine. These features are indicated on the areal geology map as kames, and are thought to have l3een formed by streams flowing in the ice near the front of the glacier. VALLEY FEATURES The topographic features of the valleys include not only the erosion slopes and cliffs, but also terraces due to erosion, others due to deposition, and finally the extensive flood plain. The surface of the quadrangles is fairly well dissected but with the excep- tion of the Illinois none of the valleys is large. The valley of the Illinois varies in width from about 1 mile near Spring Valley to II/2 miles at Utica and 21/4 miles at Hennepin. It is about 200 feet deep. In general, tributary valleys cut into the rock, such as the lower part of Little and Big Vermilion rivers, Pecumsaugan Creek, Cedar Creek, and Clark Eun, are narrow and steep sided, some being gorges or canyons. Such valleys are confined to the La Salle quadrangle. Other valleys, especially those in Hennepin quadrangle, Fig. 5. — Profile across a typical valley. but also the upper parts of the valleys in La Salle quadrangle, which are cut into the glacial drift, are broader and have less precipitous walls. Steep cuts in the drift, however, are not uncommon along these streams. In some places broad swampy areas connect the heads of valleys. For example, in Troy Crove Township (T. 35 X., E. 1, E.), the tributaries that enter Little Ver- milion Eiver from the west and the headwaters of Spring Creek together drain an extensive flat area. Similar divides separate the headwaters of Cedar and Allforks creeks, and of Xegro and Brush creeks. Bordering the larger valleys is found a stream-worn bench, which along the Illinois lies outside the inner valley at an altitude of 590 to 610 feet. The head portions of other valleys are commonly open and have gently-sloping sides. Downstream a narrow, steep-sided valley is commonly cut below the floor of the wider valley (fig. 5). The bench thus located outside the inner gorge shows evidences of stream scour upon the bed rock in places, as near the Pecumsaugan. The chief features of this -bench seem to be. due to erosion rather than to gradation. Similar, but gradational terraces, that are probably contemporaneous 26 HENNEPIN AND LA SALLE QUADRANGLES with one another, are distributed at altitudes varying from 560 to 580 feet along the Illinois and at higher elevations along the tributary streams. One such feature known as the Hennepin terrace, borders the Illinois northeast from Hennepin for 4 or 5 miles. It represents a former alluvial filling of Illinois Valley, probably a valley train originating at the Marseilles moraine, which forms the west slope of the Fox Eiver drainage basin. The terrace is apparently similar in character and in history to others at a similar height which border the Illinois between Hennepin and Peoria, It is represented also along most of the large tributaries. Lower and younger terraces are also Fig. 6. — Ridge (hog-back) on the upland east of the Little Vermilion River caused by the outcropping of the La Salle limestone along the La Salle anticline. very common. In the vicinity of Depue, for instance, such a terrace lies 10 to 20 feet above the flood plain of the Illinois, and 460 to 480 feet above sea level. All of the valleys are characterized by terraces, but correlation is diffi- cult. A flood plain occupies the floor of most of the valleys within the area. The most notable example is Illinois Valley. The flood plain of the Illinois begins in sec. 18, Utica Township, and continues downstream beyond the limits of the quadrangles. Above sec. 18 the river is entrenched in the valley floor which here is rock and represents the floor of the channel of an ancient river of large size. The flood plain of the Illinois is conspicuously regular in width and is remarkable for the straightness of its boundaries, but many irregularities due to cusps and valley embayments, in places deep and amphi- TOPOGBAPHY 27 theater-like, affect the flood phiins of the smaller streams. The valley floors are not uncommonly built np at the foot of the valley bluffs by wash and fans ; particularly is this true in Illinois Yalley, and in places abandoned channels are occupied by beds of peat. MINOR TOPOGRAPHIC FEATURES Miscellaneous topographic features include dunes, which are distributed here and there over the Hennepin terrace as shown on the areal geology map ; the out-cropping edges of the upturned La Salle limestone, which forms small hog-backs in sees. 11, 13, and 14, La Salle Township (fig. 6) ; and, finally, mine dumps. These last are conical piles of rock refuse supplied from the mines, not uncommonly reaching a height of 100 feet or more (fig. 7). These Fig. 7. — The rock dump at Mine No. 3, Spring Valley Coal Company. are visible for considerable distances if located upon the upland prairie, as at Cherry and at Ladd. Drainage The greater part of the quadrangles is well drained, but somewhat exten- sive flood plain areas along the Illinois are subject to frequent overflow, and part of the upland is so flat that the water from heavy rains disappears slowly. The run-off from the upland is more rapid near Illinois Valley than it is at a distance, where the headwaters of several streams terminate in swampy areas that form indistinct divides between adjacent drainage basins. The master stream of the area toward which all others flow is Illinois River. Of the smaller streams the most important is Vermilion Eiver, which although it drains only about 18 square miles of La Salle quadrangle is the trunk stream for a large part of southern La Salle County and for large areas in Livingston County. In the Hennepin quadrangle Bureau Creek is the largest tributary of the Illinois, but like the Vermilion it drains a large outside area. Other streams draining large parts of the Hennepin and La Salle quad- 28 HEX.NEPIN AND LA SALLE QUADRANGLES rangles north of the Illinois are Little Vermilion River^ Spring Creek, N'egro Creek, East Bureau Creek, and its tributary Brush Creek. These latter two streams drain a large part of the Hennepin quadrangle. On the south side of the Illinois N'allcy prom incut streams are Cedar Creek, and Allforks C*reek. Culture All the large towns of the area except Princeton are located along the edge of Illinois Valley. I^a Salle^ Peru, Depue, and Hennepin are located at points where the river swings toward the bluff. Peru and Hennepin had an important stage in their histories when considerable traffic went up and down the Illinois. The practically abandoned water front of Peru is one of its places of historical interest. Princeton, the one large town off Illinois River, is the center of a large farming community in Bureau County. The popula- tions of La Salle, Pern, Spring Valley, and Princeton, the four largest cities in the area in 1910, were 11,537, 7,984, 7,035, and 4,131 respectively. The population of the rural communities is relatively dense, the township without cities or villages generally having between GOO and 1,000 people. The occupations of the people can be roughly divided as follows: farming and its attendant industries, mercantile pursuits, coal mining, zinc smelting, the manufacture of cement, clock making, and the minor industries, such as the manufactuie'of brick and tile and of plows and farm implements, and clay mining. The area is well ])rovided with transportation facilities. The highways are numerous and well maintained. The great amount of available gravel affords abundant cJH'ai) road metal. Direct roads between cities and- towns are usually well ballasted. Dirt roads are frequently dragged and scraped. Several railroad lines enter the area, viz: Chicago, Rock Island and Pacific Railway; Chicago, Burlington and Quincy Railway; Illinois Central Railroad; Chicago and Northwesterii Railway; Chicago, Milwaukee and St. Paul Rail- way; Chicago, Indiana and Southern Railroad; La Salle and Bureau Counties Railroad. The main lines of the first three railroads in the list pass through the area. The last line named is a freight road only. An electric interurban line is operated between Princeton, Bureau, Spring Valley, Ladd, Peru, La Salle, Utica, and other towns lying to the east as far as Joliet. There are two canals : one in the La Salle quadrangle (Illinois and Michigan Canal) passes along the foot of the Illinois 1)1 n IT as far as Peru; the other, in the Hennepin quad- rangle (Illinois and Mississij)pi Canal), begins at Illinois River near Bureau and runs westward up the valley of Bureau Creek past Tiskilwa. Xeither of these canals is much used at present except by pleasure boats. Illinois River is navigable tlie whole length of the area by shallow di'aught boats, and by larger l)oats bcdow Peru. CHAPTEE lY— STEATIGEAPHY General Character of tele Eocks The rocks of the Hennepin and La Salle quadrangles^ including both those that outcrop and those that are knoAvn from well records only, are sedi- mentary and consist in part of indurated strata, Carboniferous and older, and in part of unconsolidated sediments of Quaternary and later age. (See fig. 8). The rocks known from outcrop or only from drilling include in ascending order the Oneota dolomite, the Xew Eichmond sandstone and the Shakopee dolomite of the Prairie du Chien group of Lower Ordovician age; the St. Peter sandstone, and the Platteville-Galena limestone and dolomites of Middle Ordovician age ; the Maquoketa shales and limestone of Upper Ordovician age ; the Silurian dolomite, probably of Niagaran age; possibly Devonian lime- stone; and the Pottsville, Carbondale, and McLeansboro formations of the Pennsylvanian series. Of these the Oneota dolomite, New Eichmond sand- stone, Maquoketa formation, and the Silurian and Devonian rocks are known only from drilling explorations. The total explored thickness of indurated strata is about 2,300 feet. The unconsolidated boulder tills, gravels, silts, and buried soils represent the deposition of probably three periods of glaciation in the Quaternary period. There is, in addition some indication of a thin deposit of gravel overlying the Pennsylvanian rocks and underlying the Quaternary deposits. The maximum known thickness of the sediments of glacial origin is 343 feet. Within Illinois Valley and its tributary valleys there have accumulated partly on glacial drift and partly on the older rocks fluviatile sediments com- posed of sand and gravel, silt, and alluvium to a maximum known thickness of about 100 feet. Thus the total known thickness of consolidated and rmconsolidated sedi- ments amounts to about 2,800 feet. If to this thickness is added the additional thickness of other strata probably underlying the area and known from drilling in the adjacent Ottawa quadrangle the total thickness of sedimentary strata known in the region amounts to 3,500 to 3,600 feet. The following section is a compilation of the records of several wells in the area showing the succession of the pre-Pennsylvanian strata encountered. The succession from the Penns^dvanian contact to the base of the Niagaran is taken from the record of the Mineral Point Zinc Company's well at Depue.^ The strata will be described from below upward in the order of deposition. ^ Udden, J. A., Some deep borings in lUinois, lU. State Geological Survey Bull. 24, p. 49, 1914. 29 30 HENNEPIN AND LA SALLE QUADRANGLES The Maquoketa, Galena, and Platteville succession is from the record of the Chicago Portland Cement Company's well at Oglesby. The Shakopee and New Richmond sections are from the record of the well of the Illinois Zinc Company at Peru, and the Oneota section is from the record of the well at St. Bede College, Spring Valley. Section of the pre-Pennsylvanian strata in the Hennepin and LaSalle quadrangles compiled from the records of several artesian wells . System, formation and bed Thickness Actual depth Total thickness Pennsylvanian series: (Pennsylvanmn-pre-Pennsylvaman Feet contact) Shale, gray, with pyrite and coal and white, dolomitic limestone Devonian or Silurian system (462 feet) : Limestone, dolomitic, white and gray 34 Limestone, grayish white, cherty 13 Dolomite, porous, with drusy quartz and chert. . . 85 Dolomite, porous, and cavern clay and sand 20 Dolomite, porous, with quartz and chert 15 Limestone, dolomitic, cream-white, cherty 47 Niagaran series: Limestone, dolomitic, straw-colored, porous 6 Limestone, dolomitic, like above, with pyrite 7 Limestone, dolomitic, like above; fossils. 17 Limestone, dolomitiC; light gray, porous 18 Limestone, dolomitic, white, fine 23 Limestone, dolomitic, straw colored 27 Limestone, dolomitic, white, compact 20 Limestone, dolomitic, straw-gray, coarse 17 Limestone, dolomitic; shale, green and gray 28 Limestone, dolomitic, gray 22 Limestone, dolomitic, white, compact; some chert. 55 Limestone, dolomitic, gray, with pyrite 8 Ordovician system (1351 feet) : Upper Ordovician series: Maquoketa formation (190 feet) : Upper shale member: Limestone, green, porous, shale 10 Shale, fine, greenish 20 Shale, hard, fine, greenish gray 5 Shale, fine, light blue, calcareous 5 Upper limestone member: Shale, calcareous, with small amounts of white crystalline limestone 20 Limestone, coarse gray; shale, gray 20 Middle shale member: Shale, fine, bluish 20 Lower limestone member: Limestone, gray to white, crystalline 25 Limestone, gray to white, crystalline, and shale, bluish gray, calcareous 10 No sample 10 Shale, gray, calcareous; limestone, white, crystal- line . 20 Lower shale member: Shale, bluish gray, non-calcareous 25 Feet 422 435 520 540 555 602 608 615 632 650 673 700 720 737 765 787 842 850 970 990 995 1.000 1,020 1,040 1,060 1,085 1,095 1,105 1,125 1,150 Feet 34 47 132 152 167 214 220 227 244 262 285 312 332 349 377 799 454 462 472 492 497 502 522 542 562 587 597 607 627 652 STRATIGRAPHY 31 200 400 600 800 1000 Till Sandy shale -1400 1600h-h-r Limestone 1800 Shaly limestone 2000 SuHace loess 4 fe«t Wisconsin till 0- 140 feet Sangamon soil and tilt 0-25 feet Illinoian till 0-lSO feet »'W !' ,',T/7 T"S^ ■£^X1 Yarmouth ? soil and silt 0-.'5 leet Pre-IIlinoian till 50 leet Unconformity La Salle limestone i iber Lonsdale limestone member Streator Nu 7 Coal 39 inches Springlicld Nu 5 Coal Veriuilionville sand- stone lentil La Salle No 2 Coal 42 inches Unconformity- 1- ° p ^3S Niagara Dolomite 125-250 feet Maquoketa Shale 180 feet Unconforinity . Galena Dolomite Plattevllle Limestone Galena Plattevllle 335 370 feet Unconformity St. Peter Sandstone 125- IbO feet Shakopee Dolomite 176-232 feet New Richmond Sandstone 80-188 feet Oneota Dolomite 215 feet Jordan Sandstone 120 feet ■g.5 Fig. 8. — Generalized columnar section of the rocks of the Hennepin and La Sall€ quadrangles. 32 HENNEPIN AND LA SALLE QUADRANGLES Section of the pre-Pennsylvanian Strata in the Hennepin and La Salle quadrangles — Concluded System, formation and bed Thickness Middle Ordovician series: Galena and Platteville formations (380 feet) : Galena: Dolomite, calcareous, brown: magnesia decreases downward Limestone, dolomitic, brownish Limestone, dolomitic, sandy, brown Platteville?: Limestone, thin bedded, soft Limestone, dolomitic, sandy, buff Dolomite and shale Dolomite, dark brown St. Peter sandstone (120 feet): Sandstone, white, colorless Lower Ordovician series: Prairie du Chien group (661 feet) : Shakopee formation (190 feet) : Dolomite, light brownish gray; shale, gray Sandstone, colorless, coarse Dolomite, light brown; shale; limestone Sandstone Dolomite, gray, semi-crystalline Dolomite, gray, slightly calcareous Dolomite, brownish Dolomite, white, calcareous Dolomite, white; shale, gray; sandstone Dolomite, white, calcareous, flinty Dolomite, white, calcareous Dolomite, brown to buff Dolomite and sandstone New Richmond formation (136 feet): Sandstone; colorless quartz grains Sandstone and dolomite Sandstone; colorless quartz grains Sandstone and dolomite Sandstone, coarse, colorless Sandstone and dolomite Dolomite and sandstone Sandstone, fine; little limestone Oneota formation (335 feet) : Dolomite Sandstone Feet 30 00 40 1,280 1.380 1,420 782 882 922 10 75 10 15 1,430 1,505 1,515 1,530 932 1,007 1,017 1,032 120 12 4 38 5 4 25 8 37 5 6 24 18 4 37 9 16 14 46 8 3 3 215 120 Actual depth Feet 1,488 1,500 1,504 1,542 1,547 1,551 1,576 1,584 1,621 1,626 1,632 1,656 1,674 1,678 1,715 1,724 1,740 1,754 1,800 1,808 1,811 1,814 2,180 2,300 Total thickness Feet 1,152 1,164 1,168 1,206 1,211 1,215 1,240 1,248 1,285 1,290 1,296 1,320 1,338 1,342 1,379 1,388 1,404 1,418 1,464 1,472 1,475 1,478 1,693 1,813 Cambrian System Cambrian rocks have not yet been reached by the drill in this area, but probably underlie the known rocks, inasmuch as wells at Ottawa and near Marseilles, towns in the eastern part of La Salle County, reach Cambrian sandstone. The full thickness of the Cambrian strata in Illinois has not been determined but at least 1,000 feet is probably present in La Salle County. The Cambrian formations of Wisconsin range in age from middle to upper STRATIGRAPHY 33 Cambrian aud the rocks underlying Illinois are probably of similar age. In Illinois as elsewhere the Paleozoic sedimentary systems lie upon the meta- morphic and igneous rocks of the pre-Cambrian systems. Oedoyiciax System Ordovician rocks underlie the entire area of both quadrangles. East of the La Salle anticline they include all strata from the lower limestone encount- ered in the wells at Utica to the youngest outcropping rocks underlying the Pennsylvanian strata. West of the anticline they extend upward to the base of the heavy Xiagaran limestone encountered in the deep drilling. The maxi- mum thickness of Ordovician rocks encountered in the quadrangle is 1,330 feet in the Avell at St. Bede College.^ The divisions of the Ordovician represented in the area are, in ascending order, as follows : Oneota, Xew Richmond and Shakopee formations of the Prairie du Chien group and the lower Ordovician series, St. Peter, Platte- ville, and Galena formations of the Middle Ordovician series; and Maquoketa formation of the Upper Ordovician series. Lower Ordovician Series PRAIRIE Dr CHIEX GROUP The name Prairie du Chien was applied by Bain and Grant in 1906 to the group of formations to which Owen in 1840 had given the name Lower Magnesian in contrast to the Upper Magnesian, a series under which were included all beds between the top of the Niagaran dolomite and the base of the Galena dolomite.- The Prairie du Chien group includes the strata encountered in the wells at Utica and the dolomitic limestones outcropping along Illinois and Little Vermilion rivers and Pecumsaugan Creek beneath the St. Peter sandstone. Several Avells in the vicinity of Utica and Deer Park and one previously mentioned at St. Bede College between Peru and Spring Valley have penetrated the Prairie du Chien formations. A maximum thickness of 650 feet seems to have been encountered, though the record is not very satisfactory. ONEOTA FORMATION The Oneota formation in this region is known only from the records of S few wells in La Salle County, several of which are outside the quadrangles. The upper part is a dolomite about 200 to 225 feet thick. Below it are various ^ Udden, J. A., Some deep borings in Illinois, 111. State Geol. Survey Bull. 24, p. 47, 1914. 2 Bain, H. F., U. S. Geol. Survey Bull. 294, p. 17, 1906. Grant, U. S., Wis. Geol. & Nat. Hist. Survey Bull. 14, p. 25, 1906. Owen, D. D., Ex. Doc. 239, 26th Cong. 1st sess., p. 17, 1840. —3 GB 34 HEXNEPIN AND LA SALLE QUADRANGLES thicknesses of dolomite, shale, and sandstone, but the dolomitie members pre- dominate for about 300 feet, after which the arenaceous members are in excess. It is thought that the division between the Ordovician and Cambrian systems occurs near the upper part of the lower arenaceous series. The well at St. Bede College penetrates a sandstone below the upper dolomite member of the Oneota formation. NEW RICHMOND FORMATION The New liichmond of the area is known only from drill records. It is an important water-bearing horizon. In thickness it is known to vary from Fig. 9. — Shakopee dolomite outcropping along the north bluff of Illinois River between Split Rock and Utica. Reproduced by permission of the University of Chi- cago Press. 80 feet as reported in the well at St. Bede College to 188 feet as reported in a well at Deer Park. The sandstone resembles the St. Peter sandstone and the two formations are frequently confused by drillers. Examination of drill cuttings from the Illinois Zinc Company's well at Peru indicates that the grains of the St. Peter sandstone are smaller than those of the New Richmond, about two-thirds of the grains from the upper sandstone being less than .473 mm. (.0132 inch) in diameter, whereas only about one-fourth of the grains from the older sandstone are that small. The formation contains some lime- STRATIGRAPHY 35 stone as shown in tlie compiled section of tlie pre-Pennsylvanian rocks shown above. SHAKOPEE DOLOMITE Disfribufioii and phijs'wgrapliic expressioii. — The Sliakopee dolomite is the only member of tlie Prairie dn Chien gronp that outcrops in Illinois. Within the La Salle quadrangle it is known from outcrop and from drill records. It varies in thickness from 170 to 232 feet. The areas underlain by the Sliakopee dolomite are confined almost entirely to that part of La Salle quadrangle lying north of Illinois River and east of the axis of the La Salle anticline. The single exception is a small area along the south bank of the Illinois in sees. 19 and 20, Deer Park Township, (T. 33 X., E. 2 E.). The areal geology maps indicate the position of the outcrops. Fig. 10. — Shakopee dolomite, showing the brecciated structure. Wherever the dolomite is present along streams, gorges have been cut into the rock resulting in cliffs that in places reach a height of 60 to 75 feet (fig. 9). In the valley of the Illinois near Utica and on the upland west of Pecumsaugan Creek the surface of the Shakopee dolomite is very irregular and gives rise to a characteristic topography marked by small knolls inter- spersed with shallow basins, resembling somewhat knoh-and-kettle topography of a terminal moraine. Fossils and correlation. — The fossils in the Shakopee dolomite are rare. Along Little Vermilion Eiver large globular masses of concentric structure 36 HENNEPIN AND LA SALLE QUADRANGLES resembling cryptozoa^ are quite comnion. The knoAvn fossils consist almost entirely of gastropods in a very poor state of preservation. Because of this paucity of fossils, correlation of faunas has not been attempted, dependence being placed rather on similarity of lithology and stratigraphic sequence in this area and in Wisconsin. Lithologic charader. — The Shakopee dolomite varies in color from light gray to buff, and in texture from fine grained and laminated to coarse grained and porous. Certain physical characters of the Shakopee point to deposition in agitated water, though not necessarily near shore. There is considerable sand which locally occurs in layers that may be ripple marked. There are numerous beds of brecciated dolomite (fig. 10), made up of fragments of Fig. 11. — The contact of St. Peter sandstone and Lower Magnesian limestone near Split Rock (vertical scale about 16 feet to 1 inch). dolomite rarely over an inch in diameter embedded in a sandy matrix. Two beds of thinly laminated, fine-grained dolomite, thought to have been deposited in protected lagoons as mud derived from wave-scoured ridges, are found near the middle of the formation. The upper part of the formation contains con- siderable oolite. Finally, the lithologic variation in the beds indicates fluctu- ation of conditions of sedimentation characteristic of shallow-water deposition. Stratigraphic relations. — ^The base of the Shakopee dolomite is not exposed in this region, so that this stratigraphic relation with the underlying New Richmond sandstone is unknown. The contact with the St. Peter sandstone above is exposed in a number of places and in each is unconformable (fig. 11). STRATIGRAPHY 37 MIDDLE ORDO.VIOIAN SERIES ST. PETER SANDSTONE CHARACTER AND DISTRIBUTION Upon the Shakopee dolomite lies the St. Peter sandstone, so named from its occurrence along the St. Peter (now known as Minnesota) Eiver in Minnesota. The following table shows the thickness of the St. Peter sandstone in nine wells where accurate measurements were made : Thickness of the St. Peter sandstone in nine wells of the Hennepin and La Salle quadrangles Thickness Feet St. Bede College 125 Cedar Point 139.8 Princeton 160 Matthiessen & Hegeler Zinc Co 132 Deer Park (Central farm) 125 Deer Park " A " (Strata inclined) 197 Deer Park "B" (70 feet exposed) 120 Deer Park (Northeast farm) 148 Deer Park (Dairy farm) 142 Pig. 12. — Unconformity between St. Peter sandstone and Platteville-Galena dolo- mite at Deer Park. Reproduced by permission of the University of Chicago Press. The variation in the thickness of the St. Peter sandstone is due in the main to irregularities in the surface of the underlying Shakopee dolomite. The top of the sandstone may also have been somewhat eroded prior to the deposition of succeeding Platteville limestone, as is indicated by the uncon- formable relations between the two formations at Deer Park. (See Fig. 12). The base of the formation is a foot or two of blue shale containing oolitic flints and fragments of porous silica weathered out of the underlying Shakopee 38 HENNEPIN AND LA SALLE QUADRANGLES STRATIGRAPHY 39 dolomite, and thouglit to represent the residual soil. Above this shale the lower 3 feet or so of the sandstone usually contains a large number of oolitic flints. The following section observed along Pecumsaugan Creek includes the zone of contact with the limestone. Character of strata at the base of the St. Peter sandstone and the top of the Shakopee dolomite along Pecumsaugan Creek Formation and bed Thickness Depth St. Peter sandstone: Sandstone or sand Clay, soft, blue Sandstone with flints Sand, brown to white Clay, flints and yellow sand Flints and clay, yellow, weathered Sandstone, flinty Clay, blue, oolitic Clay, blue, fine; hard at base Sandstone, flinty, white; oolitic interbedded with hard blue clay not oolitic Flint Sand, oolitic, and blue clay; in four alternating beds Limestone, white; some hard white to blue clay and flints Chert layer, oolitic Clay, oolitic, bluish to buff; lenticular, few inches Shakopee dolomite: Limestone; surface irregular; rock has weathered appearance. . Feet Inches Feet Inches 9 3 7 8 9 11 Along Tomahawk Creek near the NW. cor. sec. 35, Dimmick Township (T. 34 'N., E. 1 E.), an outcrop at the contact of the two formations shows a zone of about II/2 feet composed of yellow sand and very oolitic blue clay throughout which oolitic flints are scattered. Above is the usual Avhite sand- stone. Above the basal flint-bearing portion, the St. Peter formation is practic- ally pure white, quartz sandstone, the proportion of silica being very high, at some places exceeding 99 per cent. In places the sand is so poorly cemented that the rock crumbles readily; but commonly the sandstone is marked by steep clifl^s along sides of valleys (fig. 13). STRATI GRAPHIC RELATIONS The stratigraphic relation of the St. Peter sandstone Avith the underlying formation has been described. The contact with the overlying Platteville formation varies from an erosional unconformity to an apparent conformity. Usually the contact between the two formations is marked by a thin bed of shale, not over a few inches thick in exposures, but reaching four or five feet, as recorded by drillers. In the vicinity of Troy Grove, however, this shale 40 HENNEPIN AND I A SALLE QUADRANGLES is absent, and there is a gradual transition within about a foot from very sandy limestone resting on sandstone to the typical lower buff beds of the Platteville. There is, however, no gradual change from sandstone to lime- stone, the transition being confined to the overlying stratum. A contact at Deer Park, well shown on the west side of the canyon at the end of the bridge at the lower falls (fig. 12), indicates an erosional uncon- formity between the two formations. The surface of the sandstone has a relief of about 2 feet, the limestone filling the hollows and overriding the ridges. FOSSILS AND CORRELATION The St. Peter sandstone can be regarded as practically a nonfossiliferous formation, though a few casts, mainly of pelecypods, have been reported from it in other regions. This formation is widespread in the central Mississippi Valley and is commonly correlated on the basis of its uniform lithological character, and its stratigraphic relations. PLATTEVILLE AND GALENA FORMATIONS NAMES AND RELATIONS Upon the St. Peter sandstone rests the Platteville formation of dolomite and limestone, which in this region is commonly spoken of as the "Trenton." It is thought to be the equivalent of the Platteville formation of southwestern Wisconsin, which is well exposed in the vicinity of Platteville. Overlying the Platteville and generally conformable upon it is the Galena dolomite. This latter formation was named from the lead ore (galena) which it contains in certain localities and from its typical exposure at Galena, Illinois. Because of the similarity in the two formations and the difficulty of separating them in this region they have been mapped and will be described as one formation. THICKNESS The total thickness of the Platteville and Galena formations where drill- ing has penetrated the entire thickness is shown in the accompanying table. Thickness of the Platteville and Galena formations in the Hennepin and La Salle quadrangles Thickness Feet St. Bede College 325 Matthiessen & Hegeler Zinc Co 370 Chicago Portland Cement Co 385 Illinois Zinc Co 350 City of Peru 360 Cedar Point. 390 Princeton 335 The variations in thickness shown in the table probably arise from the occurrence of limestone in the lower part of the overlying Maquoketa forma- STRATIGRAPHY 41 tion whicli is difficult to distinguish from the Galena in drilling. There are also unconformities between underlying formations. DESCRIPTION AND SUBDIVISIONS The Platteville-Galena formations are known from drill records, well samples, and outcrops. Along and east of the anticline the formations are known from outcrop and from drill records while west of the fold the rocks are known only from drilling. A section of the Platteville-Galena formations recorded from driller's samples is presented in the compiled section given near the beginning of this chapter. The Platteville-Galena formations in this region in some places show lithologic characteristics similar to those in the sections observed in Wisconsin. The formations in Wisconsin are generally divisable into three parts, a massive chert-bearing dolomite above and below, and an intermediate thinner series of thin-bedded compact and fossiliferous limestone. The limestone begins near the contact of the two formations, usually slightly above it, and extends down- ward. In the Oglesby well the hard, bluish-gray limestone found at a depth of 1,425 to 1,430 (see compiled section noted above), resembles very much the bluish limestone commonly found near the junction of the formations in Wisconsin. Similar lithologic changes were not noted in the sample from a well at Cedar Point, but a well at Depue shows rather definitely the three-fold division. As is indicated on the areal geology map, these formations are exposed in limited areas only, and the rock displays such varied lithologic character from place to place that no typical section can be given. In general, however, the two dolomitic and one intermediate limestone series are recog- nizable. Along Little Vermilion Eiver in the vicinity of Troy Grove and north one-half to three-quarters of a mile the rock is a flat-lying, buff, flinty dolo- mite, part of the lower buff beds of the Platteville formation. No more than 15 feet of the formation are exposed. The beds immediately overlie the St. Peter sandstone, the latter formation being found in contact with the dolo- mite in the NW. % SE. % sec. 35, Troy Grove Township (T. 35 N., E. 1 E.), on the east side of Vermilion River. In the IST. 1/2 sec. 27 and the SW. % sec. 22, Dimmick Township (T. 34 N., R. 1 E.), the Platteville is exposed lying between the ^^Coal Measures'' and the St. Peter sandstone. The limestone is also exposed along the anticline at Split Eock and along the east bank of the Vermilion and at the mouths of the tributary valleys near Deer Park in sees. 30 and 31, Deer Park Township (T. 33 N., E. 2 E.), in the bed of Vermilion Eiver and along the ravine in the N. 1/2 sec. 5, Vermilion Township (T. 32 X.^ r. 2 E.), and along Ver- milion Eiver in the E, % sec. 8 and in sec. 9, of the same township, near Lowell. Dolomite or magnesian limestone similar to that found at Troy Grove 42 HENNEPIN AND LA SALLE QUADRANGLES is found along Deer Park Glen east of the axis of the fold. The rock found above the sandstone in the clay pits south of Utica though probably contain- ing a high percentage of magnesia is not buff in color nor porous in texture as the dolomite is elsewhere. At the mouth of Deer Park Canyon near the contact with the St. Peter sandstone there is an old quarry showing about 10 feet of heavy-bedded and flint-bearing rock that is somewhat dolomitic. Along Vermilion Eiver in the NE. cor. sec. 31, Deer Park Township (T. 33 'N., E. 2 E.), north of the mouth of Deer Park Canyon, thin-bedded, bluish- gray, hard limestone is exposed dipping into the stream at an angle of 33 Fig. 14. — Galena-Platteville thin-bedded limestone exposed in Clayton's Ravine in the NE. 14 sec. 31, Deer Park Township. degrees (fig. 14). The dip decreases to the east, the heavy flint-bearing beds noted above dipping 22 degrees west. Along Hollinger Branch in the ^. 1/2 sec. 5, Vermilion Township (T. 32 N., E. 2 E.), the Platteville-Galena outcrops for nearly one-half mile. The strata vary from a thin-bedded, mottled limestone at the west and above to bluish, dense, hard, limestone breaking with conchoidal fracture to the east and below. These latter beds resemble beds in the upper part of the Platte- ville in the Wisconsin region. The dip of the beds along this valley varies STRATIGRAPHY 43 from 2 degrees at the east to 10 degrees at the west. The thickness exposed at any one phiee does not exceed 10 feet, and the total thickness is not over 35 to 40 feet. The most extensive exposure of the formation occurs along the banks and in the channel of Vermilion Eiver near Lowell. Here the limestone dips S. 35° W. at an angle of 7 degrees, and outcrops continuously from between one-half and three-quarters of a mile above Lowell bridge to about one-quarter of a mile below the bridge. The rock is brown to gray, mottled limestone and dolomite and resembles the strata exposed at Split Rock and Deer Park. FOSSILS AND CORRELATION Small collections of fossils have been made from various outcrops of the Platteville and Galena limestones and dolomites. These have been identified by Prof. T. E. Savage. Fossils from the PlatteviUe-Galena formations Receptaculites oweni Hall Lingula near cobourgensis Orthis tricenaria Conrad Platystrophia biforata Schlotheim Dalmanella subaequata var Dalmanella testudinaria Dalman Plectambonites sericea Sowerby Rafinesquina alternata Conrad Rafinesquina minnesotensis N. H. Winchell Strophomena incurvata Shepard Strophomena cf trentonensis Ctenodonta alta Hall Ctenodonta gibberula Salter Endodesma sp Cyclospira bisulcata Emmons Scenella cf . affinis Scenella cf . compressa Bellerophon sp Tetranota sp Hormotoma gracilis Hall Lophospira sp Trochonema cf . beachi Trochonema sp Raphistoma lenticulare Emmons Illaenus sp X 1. From dense bluish limestone near base of section in ravine in N. 3^ sec. 5, Vermilion Township (T. 32 N., R. 2 E.) 2. Clayton's Ravine in the NE. i^ NE. }4 sec. 31, Vermilion Township (T. 32 N., R. 2 E.) 3. Neai Lowell bridge over the Big Vermilion. 4. Gully in the NE. i^ NW. H sec. 27, Dimmick Township (T. 34 N.. R. 1 E.) R, 5. Along Little Vermilion River in the SE. ^ NE. i^sec. 27, Dimmick Township (T. 34 N., 1 E.) 44 HEXNEPIX AND LA SALLE QUADRANGLES The fossils listed above are common to strata of middle Ordovician age, and such horizons as contain Receptaculites can be definitely assigned to the Galena. Other forms common in the Galena dolomite of Wisconsin are Ortliis tric&naria, Dalmanella testudinaria, Plectamhonites sericea, certain varieties of Rafinesquma alternata^ and 8trop]iomena incurvata. Xo definite fannal evidence for the correlation of any of the exposures with the Platteville can be cited. It is thought, however, that the beds over- lying the St. Peter sandstone, as at Deer Park, including the heavy, flinty beds at the base and the thin-bedded, fossiliferous beds above are probably of Platteville age. The upper and middle parts of the Platteville are commonly correlated with the Lowville formation of New York and the lower part, known in places as the "Lower Buff Beds,^' are thought to be of the same age as the upper iMMiiiiiii 1 vmpjfiMi «/ ^m :^^^'^H ' *!^^HH^^BH^^^^HHHli^' ' * §^^^ -1>. V.:- .. ^ ^■■■' y^\ - 1 rT ■ :& jw*^^^ Fig. 15. — The irregular surface of Galena-Platteville limestone upon which was deposited the Pottsville clay, as exposed in the bottom of an abandoned clay pit in section 15, Vermilion Township. part of the Stones Eiver group of Tennessee. The Galena formation is about the same age as the Trenton limestone of New York, but includes at the base beds of upper Black Eiver age. STRATIGRAPHIC RELATIONS The stratigraphic relations with the underlying St. Peter sandstone have already been described. The unconformity at the top of the Galena dolomite is, in the area of exposed rocks, unquestionable. Wherever the Galena is overlain by younger rocks, the exposed record indicates a great lapse of time and in most places angular superposition. The Pennsylvanian sediments either lie upon the STRATIGRAPHY 45 roughly truncated edges of the Galena-Platteville as at Split Eock, Deer Park, and Lowell, or fill in the hollows of the eroded irregular surface of the flat- lying strata as at the clay pits south of Utica (fig. 15). West of the fold, where the Galena-Platteville group is known only from drilling and Avhere the Galena is overlain by probably the next youngest Ordovician strata in northern Illinois, namely the Maquoketa, the nature of the contact is unknown. That this contact may be marked by a discordance is suggested from the relationships observed elsewhere in the State and by variations in the thickness of the Galena-Platteville noted in the deep wells of this area. UPPER OrvDOVICIAX SERIES MAQUOKETA FORMATIOX NAME AND DISTRIBUTION Upon the Galena dolomite in the area west of the fold lies a shale, or shale and limestone, formation thought to be the Maquoketa formation, a name derived from the Little Maquoketa Eiver in Iowa on which these strata are typically exposed. The same formation has been known in the older Wisconsin and Illinois reports as the Cincinnati shale. In these quadrangles the formation is known only from drilling samples and from records of deep wells. It is thought to be everywhere overlain in this area by the Niagaran dolomite, except possibly in the northwest corner of Westfield Township (T. 17 N., E. 11 E.) and the southwest corner of Troy Grove Township (T. 35 N. R. 1 E.), where a few farm wells pass through a thick blue shale before reach- ing a dolomite, probably Galena. This area is included in that indicated as undeterminable Ordovician on the Structural and Economic geology sheet. (See Plate II in pocket.) THICKNESS AND CHARACTER The records of several deep wells show thicknesses of the Maquoketa as tabulated below. Thickness of Maquoketa formation in various wells in the Hennepin and La Salle quad- rangles Feet Chicago Portland Cement Co. (Oglesby) 190 Western Clock Co. (Peru) 182 Matthiessen & Hegeler Zinc Co. (La Salle) 180 Peru Beer Co. (Peru) 180 Illinois Zinc Co. (Peru) 178 St. Bede College (Spring Valley) 240(?) Cedar Point (Mine No. 5) 145 Mineral Point Zinc Co. (Depue) 170 Princeton 175 46 HENNEPIN AND LA SALLE QUADRANGLES It seems probable that the Maquoketa in this region is from 175 to 190 feet thick. At Cedar Point the lower part of the formation is limestone, and therefore the position of the contact of the Maquoketa and the Galena could not be determined with certainty. If 40 feet of the upper part of the strata assigned to the Galena should be assigned to the Maquoketa, each formation would have about the average thickness characteristic of the area. It seems probable, likewise, that the contact of the two formations in the St. Bede well was placed too low on the basis of the driller's record, since the Maquoketa is thicker and the Galena-Platteville thinner than the average. The Maquoketa varies in its lithologic character from shale to limestone. The shale occurs at the top of the formation and commonly at the bottom. The limestone when found commonly exists as one or two ledges near the middle of the formation. Probably in some places limestone is found at the base of the Maquoketa overlying the Galena dolomite. Descriptions of the Maquoketa strata encount- ered in one well in the quadrangle based on the examination of driller's samples are presented in the compiled section given near the beginning of this chapter. CORREL,ATION AND STRATIGRAPHIC RELATIONS Except for its stratigraphic position between the underlying Galena and the overlying Niagaran formations there are no data u]^on which to base the identification of the Maquoketa shale in this region. But as this is the relation that exists generally wherever the Maquoketa outcrops in northern Illinois the correlation is thought to be correct. The Maquoketa formation of northern Illinois and neighboring parts of Wisconsin aiul Iowa is of liichmoud age, having been deposited in the upper part of the C^incinnatian or Upper Ordovician epoch. So far as known there are no de])osits of Lower or Middle Gincinnatian time in this region. An unconformity is widespi'cad below the T\ichniond in the Mississi])pi Valley. Upon the Maquoketa shale and limestone lies a dolomite thought to be the Niagaran. The contact between the two formations has been shown^ to be unconformable in Kankakee and Will counties to the east of the anticline. Data are lacking to the west, but the general prevalence of this unconformity in Illinois, Wisconsin, and Iowa makes it probable that it also exists in this area. SiLurjiAN AND Devonian (?) Systems THICKNESS AND LITHOLOGIC CIIARACTEK Between the rocks believed to be of Garboniferous age and those of Ordo- vician age is a thick series of dolomite and greenish-gray, siliceous, dolomitic limestone, or 'hard calcareous shale 400 to 490 feet thick, the lower part of 1 Savage, T. E., Stratigraphy and paleontology of the Alexandrian series in Illinois and Missouri: Illinois State Czeological Survey Bull. 23, p. 92. 1013. STRATIGRAPHY 47 which is probably of iViagaran age. These rocks are known only from drilling records and driller's samples. The following table shows the thickness of the limestone and dolomite lying between the Maquoketa shale and the strata of probable Pennsylvanian age, as shown by drilling at varions places in the quadrangles : Thickness of strata thought to be Silurian or Devonian in the Hennepin and La Salle quad- rangles Thickness Feet Western Clock Co. (Peru) 393 Peru Beer Co. (Peru) 400 Matthiessen & Hegeler Zinc Co. (La Salle) 365 Cedar Point 305 Chicago Portland Cement Co 405 St. Bede College (Spring Valley) 460 Depue 479 Princeton 335 The differences in the thickness of these strata are believed to be due to pre-Pennsylvanian erosion. From the various drilling records and drilling samples a two-fold litho- logic division of this heavy series of dolomite is apparent. The lower part of the series is a M'hite or straw-colored dolomite that is very hard and resembles the Niagaran dolomite outcropping in the vicinity of Joliet and Chicago. This part of the section has a thickness of about 225 feet, not vary- ing from this more than 25 feet. The upper part of the section is composed of a dense, siliceous, impure limestone, possibly a hard shale, of a greenish- gray color and, apparently at least in some of the wells, containing beds of grayish shale, resembling the fire clay below the La Salle (M'o. 2) coal at Utica or Lowell. These strata have been variously identified as of Niagaran, Devonian, and Pennsylvanian age, but so far as known they offer no evidence for accurate determination, and lithologically they differ, except for the asso- ciated shales, from the outcropping rocks of any of the systems witli which they have been identified. Carboniferous System pennsylvanian series GENERAL STATEMENT Pennsylvanian sedimentary rocks rest unconformably upon the lime- stones, shales, and sandstones of possible Devonian (?) and of Silurian and Ordovician age. Successively younger rocks underlie the Pennsylvanian strata in passing from east to west across the quadrangles. Where the contacts are exposed the Pennsylvanian rocks overlie Ordovician rocks — either the St. Peter sandstone or Galena-Platteville limestone and dolomite. Information relative to the unexposed contacts has been obtained from deep drilling. The 48 HENNEPIN AND LA SALLE QUADRANGLES Pennsylvanian rocks underlie the glacial drift over the entire area except portions of the La Salle quadrangle on the north side of Illinois Eiver east of a line connecting Dimmick (sec. 4:, T. 34 N., E. 1 E.), and Split Eock (sec. 13, T. 33 'N., E. 1 E.). It is possible that pre-glacial erosion may have cut through the Pennsylvanian strata along the line of the Eock-Illinois Valley in the vicinity of Princeton. SUBDIVISIONS The Illinois Pennsylvanian or "Coal Measures" rocks are divided into three formations all of which are represented in the Hennepin and La Salle quadrangles. In ascending order these are the Pottsville, Carbondale, and McLeansboro formations. In southern Illinois where the section is more com- plete and thicker these formations are more characteristically developed, and the horizons of separation are definite. In this area the Pottsville is distinct as elsewhere, but the boundary between the Carbondale and the McLeansboro is not so definite. These formations, correspond, in part at least, to the Potts- ville, the Allegheny and Conemaugh formations, respectively, of Pennsylvania. POTTSVILLE Thickness and litliology of the Pottsville hetween Bureau and Ogleshy. — There is some difficulty in determining the boundary between the Pennsyl- vanian and pre-Pennsylvanian rocks in this part of the area l^ecause of the character of the underlying rocks, which have been included in the description of the strata of the Silurian and Devonian systems. As has been said this part of the section has been variously interpreted as of Pottsville, Devonian, and Silurian age and there seems to be no means of definitely determining its age at present. It is thought that possibly the Pottsville sediments extend down only to the top of a series of strata which are predominantly dense, somewhat calcareous and siliceous, greenish-gray and reddish shale or impure limestone. The nature of the strata is exceedingly difficult to determine with confidence because the samples of the harder rock are commonly mixed with fragments of soft shale resembling fire clay of Pennsylvanian age. On the basis of these identifications the following thicknesses of the Pottsville have been noted : Thickness of the Pottsville formation in wells in the Hennepin and La Salle quadrangles Thickness Feet Chicago Portland Cement Co. (Oglesby) 75 Matthiessen & Hegeler Zinc Co. (La Salle) 70 Western Clock Co. (Peru) 64 Peru Beer Co. (Peru) 78 Mine No. 5 (Cedar Point) 185 St. Bede College (Spring Valley) about 50±(?) Mineral Point Zinc Co. (Depue) between 106 and 78 STRATIGRAPHY 49 The following descriptions of the drilling samples show the character of the Pottsville formation in the vicinity of La Salle : Description of the drill cuttings from the base of the La Salle (Ao. 2) coal into the known Niagaran dolomite in the well of the Matthiessen & Hegeler Zinc Co., at La Salle Surface elevation 585 feet Description Thickness Depth Elevation of the La Salle (No. 2) coal about 90 feet above sea level. Shale, dark gray, rather dirty Shale, dark gray, and pieces of hard, greenish shale Shale, dark gray, slightly calcareous Shale, light gray, siliceous; pieces of hard, dark shale Shale, gray; quartz sand; sandstone; black "slate" Sandstone, fine, light gray Sandstone; gray shale; gray fire clay; greenish shale Coal; sandy gray shale, dark carbonaceous clay, like fire clay Shale, gray, siliceous; dark, brownish-gray clay, like fire clay, with smooth surfaces Clay, brownish gray, structureless; fine sandstone or siliceous shale. . . . Same as above; clay more siliceous Coal, large fragment; black carbonaceous shale; clay, dark gray Shale, black, carbonaceous; fine sandstone or siliceous shale, light gray; floor clay -like shale Shale or earthy dolomite ; light greenish gray Same Same Dolomite, grayish, dense Dolomite, as above; pieces of dark clay shale breaking with smooth, irregular, sub-lustrous surfaces on broken floor clay of No. 2 coal. . . . Dolomite, grayish green, dense, and a large lump of clay resembling floor clay, somewhat carbonaceous Same as above but in fine pieces. . Dolomite or shale, light greenish; shale, dark, earthy; little bnwn fissile shale Same as above; with fragments of dark to black massive clay re- sembling floor clay Shale, dark, fire clay-like shale with some hard, greenish-gray rock, possibly represents a bed of fire clay Samples lost. Possibly base of Pennsylvanian between 615 and 630 feet. Dolomite, light, grayish green; clear rock with no shale differing in this respect from the preceding Dolomite, grayish green with specks of pyrite Dolomite, greenish gray; chips somewhat smaller than those above. . . . Dolomite, like that above; with streaks of fine-grained, reddish sandstone Dolomite, fine-grained, light greenish Same Same, with a few black fragments Samples 695 to 720 lost Typical Niagaran probably begins between 695 and 720 feet Dolomite, white and greenish, fine-grained Dolomite, light greenish to cream colored Sample lost Dolomite, light cream colored Same Dolomite, very fine powder Niagaran continued to Feet Feet 5 500 5 505 5 510 5 515 5 520 6 526 6 532 6 538 5 543 7 550 5 555 5 560 5 565 5 570 5 575 5 580 5 585 5 590 5 595 5 600 5 605 5 010 5 615 5 635 5 640 5 645 5 650 5 655 35 690 5 695 720 5 725 5 730 10 740 10 750 10 760 10 770 930 4GB 50 HENNEPIN AND LA SALLE QUADRANGLEvS A comparison of the different available records of the Pottsville west of the anticline indicates the presence in most of the artesian wells of two beds of coal, probably thin, the upper one 50 to 60 feet below the La Salle (No. 2) coal, and the other 20 to 30 feet lower. The rest of the section is mostly dark grayish, structureless clay resembling the Lowell pottery clay and the Utica clay of the Pottsville east of the anticline. There is commonly a sandy layer near the top and another near the bottom of the formation. Pottsville formation along and east of the La Salle anticline. — The Potts- ville formation on the east side of the anticline is of interest and of economic importance as a source of pottery clay and fire clay. It is best known from its outcrops in the La Salle quadrangle and in the Ottawa quadrangle to the east. The exposures of the Pottsville are numerous both along and east of the fold, as shown on the geological map. StratigrapMc sequence. — The stratigraphic sequence of the formation can be apparently correctly interpreted only from the succession found in the vicinity of Deer Park. The following drilling record of a well located at the power house in Deer Park gives the most complete section of the Pottsville available and shows the relation of the formation to the underlying and overlying strata. Driller's record of a well at the power house in Deer Park, in the SE. 3^ SE. % sec. 30, Deer Park Township Surface elevation about 625 feet above sea level. Description of strata Thickness Depth 1 . Surface clay 2. Clay and limestone 3. Clay or shale, green 4. Clay, reddish 5. Clay, blue and black 6. "Slate" and black clay 7. Clay, hard, dark 8. Clay, dark, hard, mixed with chocolate-colored clay 9. Coal, not over 2^ feet 10. Coal and clay 11. Clay 12. Coal 13. Limestone 14. Clay 15. Clay, limestone and sulphur 16. Clay, white 17. Clay and sand 18. Limestone 19. White sandstone STRATIGRAPHY 51 The sandstone, Xo. 19, in the preceding section is the St. Peter sand- stone; the overlying limestone is the Platteville. The succession from No. 17 to the clay below the coal in No. 10 belongs to the Pottsville formation. The remainder of the section up to the surface clay belongs to the Carbondale formation. The surface clay is glacial or post-glacial. Commonly there is found at Deer Park a sandstone occupying the position of part of or all of the clays, number 14, 15, and 16. Limestone No. 13 where seen in outcrop com- monly has a pisolitic structure and is not uncommonly underlain by sandstone possibly the "limestone" in Xo. 16. :^% to 5 inches 17. Shale, black 16. Shale, dark gray 15. Shale, greenish to dark gray 14. Limestone, nodular 13. Clay, yellowish green, structureless 12. Shale, nodular, greenish gray 11 . Shale, reddish 10. Shale, dark gray 9. Shale, greenish gray; with chunks of nodular limestone. . . . 8. Limestone, bluish green to gray, very argillaceous; fossils 2K to 4 feet 7. Shale, gray to green; fossils 6. Shale, green to black; fossils 5. Shale, green with a thin streak of coal near the top 4. Shale, carbonaceous 3. Shale, dark to light gray 2. Shale, concretionary, red to gray, 8^ to 1034 feet 1 . Limestone Feet 11 Inches 10 3 1 4 5 2 1 Feet 11 11 14 14 18 18 19 25 26 26 31 36 36 38 40 43 44 45 48 49 54 64 66 Inches 10 1 2 6 11 1 2 2 8 1 1 7 7 7 7 1 1 10 4 1 4 4 La Salle limestone. — Of the strata comprising the McLeansboro forma- tion the most conspicuous and the most important economically in the quad- rangles is the La Salle limestone which outcrops along the bluffs of the Little Fig. 23. — La Salle limestone in the west quarry of the Chicago Portland Cement Company, near La Salle. Photo by Rhodes. —5 GB 66 HENNEPIN AND LA SALLE QUADRANGLES Vermilion, Big Vermilion, and Illinois rivers in the vicinity of La Salle. The base of the limestone member is arbitrarily taken as the top of a black, fissile shale about 1 foot thick, commonly associated with a seam of coal about 1 inch thick. This black shale forms the floor of the quarry of the La Salle (formerly German- American) Cement Company east of La Salle and appears in the clay pits below the limestone at the quarries of the two cement plants at Oglesby. The limestone terminates below a red, concretionary shale. Fig. 24. — Strata at the horizon of the La Salle limestone in a gulley in Peru, showing the more siliceous phase. The typical La Salle limestone is found only near the west flank of the anticline in a strip not much over a mile wide. As found at the cement quarries it is a succession about 30 feet thick of limestone varying from white, crinoidal and oolitic strata to brecciated, nodular, dense, thin-bedded layers. (See fig. 23). These latter beds are associated with different amounts of argillaceous material either between the beds or forming the matrix in whicli STRATIGRAPHY 67 the limestone nodules are embedded. In all places where the limestone is typical it is highly calcareous, but may contain variable amounts of argil- laceous material in certain beds. Such silica as is present is that found in finely divided state in the shale or, possibly, in silicates. The rock is essentially non-siliceous. West from the Vermilion rivers the La Salle member becomes lithologic- ally different because of the amount of argillaceous material in certain beds so that they become essentially shale, and an increase in the siliceous content in other beds so that they are eventually calcareous sandstones or very siliceous limestones (fig. 24). l^ot all the beds, however^ are affected by these changes, some being very pure Avherever exposed. This applies especially to the coarser white crinoidal beds. The stratigraphic succession of the La Salle limestone member where it is typical has been compiled from observations at various exposures, at no one of which the section is complete. The sequence is about as follows: A general section of the typical La Salle limestone Formation and bed Thickness Depth McLeansboro formation : La Salle limestone : 7. Limestone, very pure, semi-crystalline, gray; weathering to brownish or reddish color, 5 to 9 feet Feet 6 2 12 7 2 2 Feet 6 6. Shale, gray; often entirely absent or incorporated in the under- lying bed . . . 8 5. Limestone, compact and heavy bedded where fresh but develops thin beds where weathered. A two-foot bed at the base is relatively constant over the entire area as a pure semi- crys- talline gray limestone 4. Limestone or shale; south of the Illinois commonly argillaceous, thin bedded; north of the Illinois more argillaceous to nearly a shale 6 to 8 feet 20 27 3. Limestone, hard, gray, crinoidal. Over large areas about 1 foot thick, but near Vermilion rivers as thick as 6 feet Base of the La Salle limestone: 2. Shale, blue; locally absent 29 293^ 1. Shale, black, sheety. 1 foot to 2}4 feet 313^ Gray shale below with a thin bed of coal, about 5 feet. Sections of the siliceous phase of the La Salle limestone are available at many places along the ravines tributary to the Illinois west of La Salle on both sides of the river. The following are samples : 68 HENNEPIN AND LA SALI^ QUADRANGLES Section of the siliceous phase of the La Salle limestone member in the NW. 14 SW. }4 sec. 33, Hall Township, near mine No. 3, Spring Valley Coal Co. Thic oiess Dei Feet Inches Feet 12 12 6 18 1 3 19 2 21 4 25 6 25 4 29 1 30 1 31 9. Limestone, siliceous, argillaceous. . . 8. Shale, light gray above, dark below 7. Limestone, gray, crinoidal 6. Shale, black, fissile 5. Shale, gray 4. Coal 3. Limestone, gray, concretionary. . . . 2. Limestone, hard, crinoidal 1. Shale, nodular, greenish Inchei 9 'No. 7 above is the base of the La Salle limestone member. No. 9 is the Productus-hesLTmg limestone. The following fossils have been collected from the La Salle limestone member. The identifications were made by Prof. T. E. Savage: Fossils collected from the La Salle limestone Fossils Typical La Salle limestone Arenaceous La Salle limestone Lophophylum profundum Milne-Edwards and Haime Orbiculoidea subtrigonalis McChesney Rhipidomella pecosi Marcou Meekella striaticostata Cox Chonetes granulifer Owen Chonetes verneuilianus N and P Productus cora D'Orbigny Productus costatus Sowerby Productus lasallensis Worthen Productus nebraskensis Owen Productus semireticulatus Martin Marginifera splendens N and P Pustula punctata Martin Cryptocantha compacta W and St. J Dielasma bovidens Morton Spirifer cameratus Morton Ambocoelia planiconvexa Shumard . . Squamularia perplexa McChesney Spiriferina kentuckyensis Shumard Composita subtilita Hall Sedgewickia topekensis Shumard Aviculopecten hertzeri (?) Meek Aviculopecten interlineatus M and W Porcellia peoriensis or gillanus Pharkidonotus percarinatus Conrad Murchisonia sp Euconospira turbiniformis M and W Pleurotomaria spironema M and W Platyceras parvum Swallow Temnocheilus latus M and W Metacoceras sangamonensis M and W X STRATIGRAPHY 69 McLeansboro strata above the La Salle limestone.- — Eocks similar to those below the La Salle limestone continue above that member of the McLeansboro formation. The area underlain by these strata is small, being confined to Peru and La Salle townships, to sec. 6, Vermilion Township, and to sec. 1, Eden Township. The strata are exposed along Illinois River, the Vermilion rivers, and Bailey Creek in the valley bluffs. A measured section of this part of the McLeansboro formation is given below. Sections of that part of the McLeansboro formation above the La Salle limestone member. Measured along the road south of La Salle in sees. 22 and 27 , La Salle Township Surface elevation 600 feet above sea level at the top Thickness Depth Pleistocene: Drift, till, red to yellow McLeansboro formation: 11. Shale, bluish gray 10. Shale, grayish yellow 9. Shale, nodular, calcareous 8. Shale, dark, to light gray 7. Limestone, shaly 6. Shale 5. Limestone, dark, impure 4. Shale 3. Coal 2. Shale, gray to blue 1. Shale, red Top of the La Salle (Prodwc^ws-bearing) limestone. Feet Inches Feet 10 3 3 5 8 5 13 10 23 2 25 2 27 1 3 28 4 28 8 29 10 39 20 59 Inches Certain strata represented in the foregoing section are of especial interest. The shales numbered 10 and 11 and a still higher one are used in the manu- facture of pressed brick at Peru. The nodular calcareous shale, No. 9 in the preceding section is locally a limestone. In the ravine in the NW. 1/4 sec. 27, La Salle Township (T. 33 N., R. 1 E.), there is at this horizon at the head of the gully a semi- crystalline, very fossiliferous, white limestone. The fauna is very different from that of the strata below, such as the La Salle limestone, being conspicuous for the great numbers of gastropods and lamellibranchs and the numerous sharks^ teeth. Also embedded in the limestone are small pebbles of greenish, igneous rocks and quartz. Tertiaky (?) System Neither Mesozoic nor Tertiary rocks are found in place in the quadrangles, but the basal part of the glacial drift in several places contains many well- rounded pebbles of quartz. Here and there are found small deposits of quartz and chert gravel containing some pebbles of igneous rock, all colored a dark 70 . HENNEPIN AND LA SALLE QUADRANGLES red by iron oxide. Such a deposit is found under the more recent gravels at Spring Valley in the SW. l^ sec. 35, Hall Township. At this place there are 6 to 10 feet of greatly oxidized quartz pebbles lying above the "Coal Measures'^ strata and underlying a bed of silt or loess. A fragment of a leg bone of a species of elephant was found in this gravel. The deposit here is thicker than it has been found elsewhere, but traces of its presence have been observed in the gullies between Peru and Spring Valley, especially those in sec. 19, Peru Township, and in several of the ravines on the south side of the Illinois west of Cedar Creek. A deposit resembling shingle was noted in the ravine south of Spring Valley in the E. % sec. 27, Granville Township, lying at the base of the drift and composed of fragments of the siliceous La Salle limestone. Whereas these deposits at the base of the drift are possibly of Pleistocene age, they indicate the previous wide distribution of quartz gravels over this area. Fragments of similar conglomerate are found widely distributed over the southern part of the Mississippi Valley and as far north as the quartzite ridges at Devils Lake, Wisconsin. They are commonly regarded as being the product of an extended erosion period during the Lafayette epoch of the late Tertiary. Quaternary System pleistocene series The Pleistocene deposits are believed to represent seven of the stages of Pleistocene time in North America. It is probable that the lowest drift exposed within the area and encountered in numerous test holes drilled for coal belongs to a pre-Illinoian stage. This drift has overlying it in places a bed of silt and organic material resembling a forest soil thought to be possibly of Yarmouth age. Overlying the oldest drift or the Yarmouth soil is a till believed to be Illinoian, and this in turn is overlain by the Sangamon soil. This latter deposit is not uncommonly associated with a body of loess or silt possibly of Sangamon, lowan, or Peorian age. The uppermost till of the area is thought to be the Early Wisconsin till, and except for areas where it has been eroded this drift underlies almost the entire area. Certain of the terrace deposits bordering the larger valleys may be of late Wisconsin age. Covering almost the entire surface of the area is a thin layer of pebbleless silt somewhat similar to loess which seems to have been deposited soon after the Wisconsin ice melted away. Although all these deposits are exposed at the surface it is thought best because of the difficulty in many places of separating the two older drifts to show on the surficial geology sheet only the distribution of the two upper till sheets. STRATIGRAPHY 71 PRE-ILLINOIAX DRIFT AND YARMOUTH (?) SOIL The exposures of pre-Illinoian till are found almost entirely in the numer- ous ravines which cut into the bluff of the Illinois Valley on the west side of the La Salle and the east side of the Hennepin quadrangles. The greater mass of this earliest drift is found below an elevation of 500 feet and in the pre-glacial valleys. Eather large bodies of drift, possibly pre-Illinoian, occupy the lower part of the pre-glacial Eock-Illinois Valley which runs through the central part of the Hennepin quadrangle. The tributary valleys of this early stream were also in part filled with the pre-Illinoian drift. So far as has been discovered the Illinois Valley east of the bend at Hennepin does not contain any of the oldest till. The oldest till is best exposed where the present drainage has cut deeply into the filling of the pre-glacial valleys. A noteworthy example is in the ravine in the W. 1/2 sec. 28, Granville Township (T. 33 K., E. 1. W.). The till here exposed at an altitude of about 530 feet, is dark gray, fine grained, and tenaceous, and contains organic matter such as partly decayed sticks. In one place this till is overlain by. a bed of silt, the lower part of which is bluish in color and the upper part black with organic matter. This silt is overlain by Illinoian drift which is about 25 to 30 feet thick and which is in turn overlain by the Wisconsin till at an altitude of 600 to 620 feet. In the Hennepin quadrangle the older drift seems to be largely sand and gravel and to lie at a lower level than it does on the La Salle quadrangle. Exposures of till as definitely separated as those in the La Salle quadrangle have not been observed in outcrop, though certain exposures of dark-gray, adhesive boulder clay at elevations below 500 feet have been suspected of being older than Illinoian. Knowledge of the oldest drift is obtained almost entirely from drilling. Its determination is based upon the existence at the top of a muck or soil bed which is thought to separate the Illinoian from the older drift. This soil bed is found in many of the drill holes at an elevation of from 500 to 540 feet above sea level. A tabulation of the character of the material composing the drift on the Hennepin quadrangle below an elevation of about 500 feet, which is about the elevation of the soil bed marking the top of the pre-Illinoian drift on this quadrangle, shows a great preponderance of sand and gravel over other kinds of drift. Although it is impossible to separate gravel of two drift sheets from the data recorded in the logs, yet where two gravel beds each of considerable thickness are separated by a forest soil bed or a silt at the same elevation over a considerable area, it is believed that a considerable difference in age between the gravel beds is indicated. The correlation of the oldest drift on this area with deposits in other regions cannot be made with certainty because of the meagerness of the infor- 72 HENNEPIN AND LA SALLE QUADRANGLES mation concerning it. If this dark-gray, indurated till found below or at the base of the Illinoian drift in this area is of either Kansan or Nebraskan age, the more widespread distribution of the Kansan drift makes its presence in this region more probable than that of the older drift. PRE-ILLINOIAN LOESS AND SILT Associated with the Yarmouth or pre-Illinoian soil bed, and where it is so associated commonly overlying the soil bed, is a variable thickness of fine silt or loess of a yellow to blue color. This silt is found from place to place at elevations varying from 480 to 520 feet above sea level along the streams •issuing from the north and south bluffs of the Illinois west of the anticline and east of the mouth of Bureau Creek. The silt is a very fine, even-grained material, compact where moist and dust-like where dry. When dry it is more commonly yellow than blue. It is in places stratified and elsewhere unstratified. Not uncommonly it contains numerous shells of land and fresh water gastropods. This deposit, though evidently of greater age, resembles in many particulars the silt and loess found between the Illinoian and Wisconsin drifts about 100 feet higher in the section. There are numerous localities where this pre-Illinoian silt is exposed. The most continuous and best exposures are found along the valley of Spring Greek and in the tributary valleys that enter from the west in sec. 26, Hall Township. Along this valley unlaminated and structureless brown, yellow, or blue silt or loess lies at the bottom of the deposit. This material contains shells in considerable numbers. It is from 10 to 20 feet thick at least, the base of the silt being nowhere exposed. Overlying the unlaminated silt are about 10 feet of laminated silt which contains an increasing number of small, water-worn pebbles toward the top, and grades without apparent break into a pebbly, yellowish till, apparently the Illinoian drift. In the various drilling records that have been given, sections of this silt bed are shown at the base of the Illinoian till. Such a deposit is found in many of the deep drift wells and coal borings of the area especially in the Hennepin quadrangle. In places it probably acts as a reservoir for natural gas originating in the drift. ILLINOIAN DRIFT The greatest mass of drift in the area is probably of Illinoian age. It is found at the surface in most places where Wisconsin drift has been eroded. In general it is more widely distributed in the pre-glacial valleys that were cut below 600 feet above sea level. Eock surfaces that are higher than 600 feet do not commonly have any cover older than the Wisconsin drift, except possibly in the vicinity of Granville. The Illinoian drift is thickest along the west side of the area particularly STRATIG15APHY 73 along Bureau Creek west and south of Princeton where 150 feet are probably present. The average thickness is about 100 feet, which distance represents about the average interval between the forest soil beds at the base and at the top of the Illinoian drift. The altitudes at which the lower soil bed is found vary from 500 to 540 feet above sea level, possibly to 560 feet, and the upper soil bed is found between 600 and 640 feet. On the La Salle quadrangle the Illinoian drift is thin, varying from as little as 5 feet along Bailey Creek to possibly as much as 70 feet. There are no known outcrops of the Illinoian till north of the Illinois and east of the Little Vermilion within this area. The Illinoian drift in this region includes sand and gravel as well as till. The till is rather variable in character. On the Hennepin quadrangle it is of a brownish to reddish color where slightly weathered, as in a stream cut that Illinoian till Fig. 25. — Buried peat of the Sangamon interglacial epoch, lying above Illinoian till and below" loess along East Bureau Creek near the center of N. l^ sec. 8, Selby Township. has dried at the surface, but where fresh it is gray to yellowish to buff colored. It is generally blocky and too hard to be easily penetrated by a trowel. Not uncommonly it is cut by cracks which run vertically through the till and along which the material is oxidized for the width of an inch or two. Toward the east the Illinoian till has a yellowish rather than a reddish color. Along Cedar Creek, the till varies in color from gray to buff and yellow. The same is true of the till along the valleys of the Little Vermilion and Spring Creek. In the vicinity of Spring Valley there is a considerable thick- ness of a dark, dirty, grayish till, hard when fresh, which is thought to be Illinoian in age, but which may be older. Not uncommonly the upper 10 feet or so of the Illinoian till shows evi- dence of having been leached and oxidized before the deposition of the over- 74 HENNEPIN AND LA SALLE QUADRANGLES lying Wisconsin drift. The leached zone of the older till is, in general, thicker than that of the Wisconsin till. Gravel deposits of Illinoian age are not separable lithologically from the later gravels. Exposures of the Illinoian till are very numerous. Along the north and east bluff of Bureau Creek the Illinoian till makes up practically the whole section from where the valley opens into Illinois Valley as far north as the center of Dover Township (T. 17 K., E. 9 E.). Good exposures have also been observed along East Bureau Creek, Little Vermilion Eiver, Vermilion River, and Cedar Creek. Two measured sections are given below. Section of the drift north of the Illinois and Mississippi Canal in the NW.}4 sec. 9, Arispie Township Measurements approximate. Observations by U. S. Grant. Surface elevation 565 to 575 feet± Depth Loess, yellow Sand and gravel Till, light red to yellow Sand and gravel Till, light reddish to gray . . Sand, fine; not well exposed Feet 5 8 23 26 33 58 Section of Illinoian drift in an old gravel pit in the SW. }/i SE. }4 sec. 8, Leepertown Township, near Bureau Observations by U. S. Grant Thickness Loess-like material, yellow, pebbleless Illinoian till, weathered passing into Till, red, freely effervescing, with small irregular patches of gravel scattered throughout Gravel and sand, not well exposed A few rods southwest of the latter and lower down Gravel (the same as the last) cemented into rough, craggy forms . . Till (pre-Illinoian) dark gray, with the upper 1 to 2 feet changed to yellow or yellow-brown material by oxidation. Not leached . . Depth Feet 4 1 Inches Feet 4 5 9 25 14 39 25 25 10 35 Inches Not uncommonly within the main body of the Illinoian till well below the top, there are found thin lenses of yellowish silt or possibly loess resembl- ing the silt found between the Illinoian and Wisconsin tills. These silts are not associated with soil beds in any known instance, at least, and the till above and below them is of the same character. STRATIGKAPHY 75 SANGAMON INTEE-GLACIAL DEPOSITS The interglacial stage following the Illinoian stage is called the Sangamon stage because of the widespread development of a soil and peat bed above the Illinoian drift in Sangamon County, Illinois. The Sangamon soil is well exposed in the Hennepin and the La Salle quadrangles (see fig. 25), and it has been noted in many wells and coal borings. It is probably persistent over many square miles especially within the Hennepin quadrangle along lines of interglacial drainage. Where the present valleys have cut down to the level of the floors of the older drainage lines the soil is often exposed. The deposits in this area thought to have been formed during the Sanga- mon interglacial stage are silt and beds of peat or soil. Furthermore, a leach- ing of the upper few feet of the Illinoian till is evident in many places. The Forest bed Fig. 26. of figure 25. 'Forest bed" below loess near the same location as the buried peat beds of silt are similar to those found beneath the Illinoian drift; that is, structureless to laminated, blue and bufl-colored silt or loess of very fine tex- ture, carrying numerous shells of land and fresh water gastropods. The beds of peat are made up of greater or less amounts of vegetable matter and silt such as is found in the silt beds. In places the bed is largely organic matter, being composed of partly decayed or even well-preserved, closely matted pieces of wood, such as the twigs and branches of trees. At no place where a peat bed was observed was there decisive indication of its having been formed in place. Eoots extending into the underlying drift were not seen and logs and twigs lie horizontally. The deposits seem to be drift wood of local origin. 76 HENNEPIN AND LA SALLE QUADRANGLES The oxidized zone of the upper part of the Illinoian drift may extend as much as 7 feet below the top of the till. Where the peat or forest bed is present the oxidized zone lies below. At other places there may be yellow or blue silt above the oxidized zone or even the unoxidized pinkish Wisconsin till. Localities along the contact of the two tills where the Sangamon soil may be observed are indicated on the map showing the preglacial surface (Plate V), by a small cross and figures indicating the approximate elevation of the soil bed. These figures will be observed to range from about 600 feet to about 635 feet and to be confined for the most part to the lower parts Fig. 27. — Loess above gravel of probable Illinoian age, east of Princeton. of the basins of East Bureau and Negro creeks, with a few localities along the south bluff of the Illinois and in the tributary ravines. LOESS AND SILT BETWEEN THE ILLINOIAN AND WISCONSIN TILL SHEETS As elsewhere in Illinois the Sangamon soil bed is associated with a deposit of silt and loess (fig. 26). The material composing this deposit is a friable, uniformly fine-grained, dust-like material, which may or may not be lami- nated. It usually contains more or less lime carbonate, and is not uncommonly fossiliferous. The deposit resembles the silt lying below the Illinoian till though it is possibly less commonly of a bluish color where exposed. STRATIGRAPHY 77 The thickness of the silt or loess between the Wisconsin and Illinoian tills is variable but where it is most characteristically developed in association with the underlying forest bed it commonly has a thickness of about 10 feet. In those parts of the area where the Wisconsin drift is not present and the Illinoian is the underlying till, this loess may appear at the surface. Loess and silt 6 to 9 feet thick is found at the top of the bluffs from place to place north of Bureau Creek in the west half of Arispie Township. Fig. 27 shows this loess above gravel east of Princeton along Bureau Creek. Thick surface loess is found in the hilly area east of Hennepin in sees. 6 and 7, Granville Township, and in sees. 1, 11, and 12, Hennepin Township. N'o outcrops of till have been observed along the gullies and roads in this region above an elevation of 640 feet, though doubtless in the cores of the hills it rises to a higher elevation. EARLY WISCONSIN DRIFT THICKNESS AND DISTRIBUTION Drift of Early Wisconsin age probably originally covered the entire sur- face of the quadrangles, but has been removed over considerable areas bordering the larger streams, especially along Illinois Eiver on the east side of the La Salle quadrangle. Many of the streams have cut through the Wisconsin drift into one of the older drifts or into the rock and exposed sections of the till or gravel along the valley walls. The drift of Wisconsin age is remarkably thin over much of the area especially toward Illinois Valley and in the southwestern part of Hennepin quadrangle. The thinness of the deposit is in part due to erosion but prin- cipally to meagerness of deposition. Between Bureau Creek and East Bureau Creek south of Princeton the drift of the Wisconsin epoch is confined to a slightly elevated ridge with morainic expression running north from sec. 5, Arispie Township, toward Princeton. On the south side of Bureau Creek Valley the Wisconsin drift seems to be present only toward the crest of the divide running through sees. 19, 20, 23, 24, 28, and 29, Arispie Township, which forms the watershed for Bureau Creek and streams to the south. The divide follows a line of irregular hills with morainic expression thought to have originally been continuous with moraine running northeast through Princeton. The disorganized character of the upland topography indicates that much of the Wisconsin till could not have been removed, and therefore that it was never of great thickness. On La Salle quadrangle there is an area of about 2 square miles lying west of Pecumsaugan Creek in sec. 6, Utica Township, and extending southwest to the outcrop of the La Salle limestone in the NW. corner sec. 13, La Salle Township^ where there is very little drift other than boulders 4 feet or less in diameter scattered here and there over the surface, but not in great pro- 78 HENNEPIN AND LA SALLE QUADRANGLES fusion. This apparently is an area from which all but the heaviest and coarsest parts of the latest drift have been washed. A similar area from which at least part of the upper till has probably been washed, borders the Illinois Valley from La Salle to Spring Valley through sees. 16, 17, and 18, Peru Township, and sees. 35 and 36, Hall Township, and also west of Spring Valley in sees. 33, 34, and 35 of the same township. There is, so far as is known, no till present in the inner Illinois Valley, though gravel and sand, possibly in part of glacial origin, are found there in considerable quantities. There are numerous terraces bordering the streams which were formed after the invasion of this area by the ice and which are not primarily of glacial origin. In the northeast part of the Hennepin quadrangle where the Wisconsin drift seems to be thickest, as much as 141 feet of what is probably this drift has been encountered in one of the drill holes. From this extreme thickness it decreases in amount southward toward the Illinois Valley as described in the preceding paragraphs. It seems therefore that although over much of the area this drift is thin, it nevertheless attains a thickness in places greater than that known to be reached bv either of the other drift sheets. The Wisconsin till where it is typically developed is of a drab color with a rather conspicuous tinge of pink. In the exposures along Vermilion Kiver, especially the pink color of the upper till in contrast to the yellowish color of the lower is unmistakable. Toward the west side of the Hennepin quad- rangle the older till has a brownish or reddish tinge, is therefore easily con- fused with the pink of the upper till, and the separation of the tills on the basis of color is impracticable. There is a slight difference in color, however, which can be seen when the two tills occur in the same exposure, as along East Bureau Creek in sec. 8, Selby Township. Over a large part of the La Salle quadrangle the lower till is yellowish or gray and readily separable from the upper where the two occur in the same section. Although the difference in color is useful as a means of separating the two tills it is not a criterion of wide application. It is serviceable in restricted areas and especially where colors can be readily compared without trusting to memory. The relative age of the tills in each region of numerous exposures may be first determined by the usual criteria after which the color is useful in tracing the drifts from place to place. The AVisconsin till is notably unconsolidated as compared with the more or less jointed Illinoian. It is rather friable, easily penetrated with pick or trowel, and is always calcareous nearly to the top. Not uncommonly it is very stony, almost a gravel, and then it is in general of a yellow color. The pebbles and larger fragments in the till are dominantly limestone or dolomite with a STRATIGRAPHY 79 relatively small percentage of flints and quartz. The larger boulders are predominantly of dark diabase which weathers to a grayish exterior. Boulders of red quartz porphyry are not uncommonly found in the till or apparently weathered out of it. The character of the Wisconsin till in various parts of the area is suffici- ently well shown in the numerous sections already given in the description of the earlier drifts to make unnecessary the introduction of additional drilling records or measured sections. SAND AND GRAVEL Between the sand and gravel of Early Wisconsin age and that of Late Wisconsin or even of Illinoian age the writer has found no lithologic difference. ^,,,,^^^ ^^ mJUSt ■HP ^H| Pp|[|^^H ^^^^s f^^M ^^^(fr*^ K JiMfc ^ Jf 1 1 'WfS^^^^^B^^^^^^^BI '9 if - '^^^^^^^fl^H^B ^^^ Fig. 28. — Gravel pits east of Spring Valley. Certain gravel deposits are known to be of Illinoian age because they lie within bodies of Illinoian till^ but they are not markedly different from other gravels underlying or overlying Wisconsin till. In general the gravel deposits of Wisconsin age in the area may be regarded in part as Early Wisconsin in age and in part as younger than Early Wisconsin. The earlier gravel lies without the valleys or forms the valley walls in places, and the younger gravel is found within the valleys principally as valley trains or terrace deposits. Of the Early Wisconsin gravel there may be two kinds, the gravel deposited on the surface formed during the Sangamon (Illinoian-Wisconsin) 80 HENNEPIN AND LA SALLE QUADRANGLES interglacial stage as the Wisconsin ice sheet advanced, and hence underlying the Wisconsin till, and that formed while the ice was moving back and forth over the region and hence deposited within or above the till sheet. Several large bodies of gravel underlying Wisconsin till and practically unweathered are thought to be of Wisconsin age. The age is unquestionable where they overlie the soil and silt beds of the Sangamon stage where thev in turn rest upon Illinoian drift. It is open to question where they overlie unweathered Illinoian till, the Pennsylvanian or older rocks, or silt and buried peat resting upon pre-Pleistocene strata. Of the gravel of probable Early Wisconsin age there are numerous out- crops along the bluif of Illinois Eiver from one side of the area to the other. Heavy deposits of gravel are found near Utica, La Salle, Spring Valley, between Depue and Bureau, and south of Bureau on the west bluff of the Illinois, all of which are thought to have originated with the ice advance of the Early Wisconsin stage. This does not include the Hennepin gravels which lie within the Illinois Valley and are of a later origin. As the gravel at Spring Valley (fig. 28), has been studied in some detail and as it is typical of the group a description of the characteristics displayed at that locality will suffice for all the exposures. The gravel in the pits at Spring Valley has a thickness of about 60 feet. The section is about as follows: Section of the gravel in the pits near Spring Valley on the east side of the valley of Spring Creek . SW. }4 sec. 25, Hall Township Thickness Dei Feet Inches Feet 2 2 2 4 1 3 5 1 6 6 40 46 5 51 10 61 7. Clayey surface soil 6. Sand, coarse, like torpedo sand 5. Sand, fine, well sorted 4. Loess, yellow 3. Sand and gravel, coarse, dipping 25° W.; upper surface and contact with loess is level 2. Loess, yellow, in part laminated, about 1. Gravel, many quartz and flint pebbles, much oxidized Pennsylvanian series. Inches The gravel of bed No. 3 in the section given above is poorly sorted but at the same time is well stratified in layers dipping west at a high angle. The boulders contained in the gravel vary greatly in size up to 3 or 4 feet in diameter. The limestones are very rarely decayed but the boulders of igneous rock are commonly so soft that they can be easily penetrated with a trowel or pick. There are also many fresh boulders of igneous rock. Numerous boulders of till are also scattered throughout the deposit. These may be as STRATIGRAPHY g] large as 1VL> feet in diameter but are commonly smaller. They are generally coated with small pebbles and broken surfaces show the characteristics com- mon to the tills of the region. Till balls of at least three different colors have been observed, reddish, gray, and yellowish gray, mainly the tills characteristic of the older drifts. The till is commonly indurated and characterized by the presence of small quartz and flint pebbles. MORAINES The various halts of the ice sheet that deposited the Early Wisconsin drift in this area are marked by lobate recessional moraines. One of these moraines crosses the La Salle quadrangle east of La Salle (fig. 29), and another composite moraine is found on Hennepin quadrangle. They have been previously described and named in Chapter III on the topography of the quadrangles. The moraine in the Hennepin quadrangle is the Arling- Fig. 29. — Farm Ridge moraine north of Illinois Valley. ton moraine and comprises an outer or west branch, the Dover moraine, and an inner or east branch, the Coal Hollow moraine. The moraine in the La Salle quadrangle is known as the Grand Eidge or Farm Eidge moraine. The moraines are composed almost entirely of till, but lenses of gravel are found which increase in thickness toward the west. In some cases, as on the north side of the Illinois along Tomahawk Creek, the gravel beds slope sufficiently to give an artesian head to the water contained in them. POST EARLY WISCONSIN DEPOSITS GRAVEL The later Wisconsin gravel of this area is in the main that found within the valleys and is of two sorts, that found in the terraces and that forming part of the valley filling. The former is thought to be of glacial origin in part at least and to have been formed when the ice edge was not far to the east of this region. The gravel forming part of the valley filling is possibly of more recent origin, having been deposited after the Illinois Valley ceased to be the outlet of glacial Lake Chicago. —6 GB 82 HENNEPIN AND LA SALLE QUADRANGLES The terrace gravel composes the terraces bordering the valleys, the surface elevation of which varies from about 480 to 575 feet above sea level. The most notable example is the Hennepin gravel underlying the Hennepin terrace. Other gravel deposits of the same age as the Hennepin gravel are found along streams tributary to the Illinois but apparently not elsewhere along the Illinois Valley within this area. The Hennepin gravel i.s most extensively exposed along All forks Creek in sees. 25 and 36, Hennepin Township, (T. 33 N., E. 2 AV.), where it is being excavated at large pits. The gravel composing the Hennepin terrace extends down to at least 480 feet above sea level in one place (SW. % NW. 1/4 sec. 35, Hennepin Township), in the central part of the terrace and at Hennepin to an elevation of about 400 feet. In general as compared with the earlier gravel, this deposit is finer, better sorted and apparently lacks the till balls commonly found in earlier gravels. Other terrace gravel probably of the same age is found in isolated patches along the larger streams. The Hennepin gravel was deposited after the ice withdrew from this region and before the valley was the outlet of glacial lake Chicago, and prob- ably represents a valley train extending west past the Marseilles moraine. Terraces of the same age are found along the Illinois south of this area as an examiuation of the topographic sheets of the Lacon, Dunlap, Metamora, and Peoria quadrangles will show. East of the bend in the Illinois at Bureau no deposits of this age have been identified in this area within the valley. Their absence is thought to be due to their removal, if they were ever deposited, by the waters of the outlet river from glacial lake Chicago, removal being more complete in the narrow rock-bound ^'alley above the bend than iu the wider valley below. Gravel underlyiug terraces of a later date than the Hennepin terrace is relatively unimportant. It is common along all streams but especially along Bureau and East Bureau creeks. Such terraces are difficult, probably impos- sible, to correlate with each other or with the different events of the physio- graphic history of the region following the deposition of the Hennepin terraces. Even the Hennepin terraces in the upper part of the streams where relief is less are indistinguishable from later ones. The surface of the area except for small areas on the upland and the most recent flood plains of the streams is covered with loess or silt similar to loess varying in thickness from a few inches up to 15 feet or more. In those parts of the region where there is no Wisconsin till in the section the upper part of the loess is probably of Wisconsin or later age, whereas the lower part is possibly of late Sangamon age, and represents the outcrop of the loess found STRATIGRAPHY 83 here and there between the Illinoian and Wisconsin drifts. This thick loess has previously been described as pre- Wisconsin loess. The latest loess, which lies at the surface, is distributed alike over moraines and till plains, except that in certain poorly drained areas which until very recently have been swamps the loess, if any is present, is obscured by the presence of large quantities of humus rendering the soil black. Over such parts of the area as have been more or less modified by water as especially the various terraces and high-level benches, the loess seems to be of secondary origin being mixed to a slight degree with small stones and pebbles. Such secondary loess is found on the high benches along the bluff of Illinois Eiver and along similar benches on the smaller streams, such as Vermilion Eiver and Bureau Creek. RECENT SERIES DUNE SAND Deposits of dune sand are found only on the Hennepin terrace. Dunes are distributed along the crest of the terrace and the sand of which they are built is apparently derived from the slope to the west and possibly also from the river flood plain. Some of the dunes are somewhat crescentic in shape with the horns pointing with the wind, others are of no definite form. The west side of the hills is almost invariably the steeper. The height of these dunes on the upper terrace is apparently as much as 15 feet, though the height is often difficult to determine because the slope of the west side of the dune is a continuation of the slope of the hill to the west. These are actively shifting dunes. Another area of shifting sand forms the top of the bench or terrace running north from Hennepin at an elevation between 500 and 520 feet above sea level. A third area of dune sand lies at the base of the drift hills at the east side of the Hennepin terrace especially in sec. 11, Hennepin Township. ALJLUVIUM Deposits of alluvium are present along most of the streams, but most notably along Illinois Eiver. Of the smaller streams those in the Hennepin quadrangle have the widest flood plains. Except for N"egro Creek these valleys have not reached the rock, but are cut entirely in drift. In the La Salle quadrangle the flood plains are narrow and insignificant along the streams where the elevation is less than 560 feet above sea level, but are commonly well developed above that elevation. The preservation of the allu- vium in the upper part of the valleys is due, along some streams at least, to the establishment of a temporary base level at the elevation of a resistant rock ledge through which the stream cuts with relative slowness. This relationship is well illustrated by the distribution of alluvium along Cedar Creek and 84 HENNEPIN AND LA SALLE QUADRANGLES Bailey Creek^ the flood plains essentially terminating where the outcrop of the La Salle limestone crosses the valleys. In other cases it is possible that the wider upper flood plain along some streams may represent practically the level reached while the Illinois was depositing the Hennepin terraces. Along Illinois Eiver the amount of alluvium in the valley varies greatly. As indicated in Chapter III on topography the rock floor of the valley is believed to represent the channel of the river which was the outlet of glacial lake Chicago, at least as far west as the bend at Bureau. West of the anticline the old channel has become filled with alluvium to a thickness of about 100 feet in one place at least and generally along the middle of the valley to a thickness of about 60 to 90 feet. East of the anticline for about II/2 miles, in places indicated by the swampy areas on the topographic map, are deposits of alluvium and peat of unknown thickness. Much of the east half of sec. 18, Utica Township, and thence east to the boundary of the quadrangle is underlain by rock with only a soil or with no covering. This surface is very irregular due to the presence of elongated hills of dolomite separated by swampy depressions probably containing more or less alluvium. South of the bend at Bureau the thickness of alluvium is difficult to estimate as the gravel in the channel is essentially like the gravel in the drift below and the position of contact impossible to determine in the one available drill record. In the area between the anticline and the bend at Bureau there has been extensive drilling, so that in the vicinity of La Salle and Peru at least the character and depth of valley filling is well known, and is composed of surface alluvium, blue or yellow silt, and gravel. The blue silt which is found 20 to 25 feet below the surface is generally present on the east side of the highway in sees. 15 and 22, and extends east- ward at least beyond the La Salle city wells in the SW. % SW. i/4 sec. 14, La Salle Township. It is due doubtless to the presence of this blue clay, which resembles the blue silts associated with the till sheets of the region, that the water in the lower gravels is not contaminated by the surface and river water. WnERSTnOFIlililtlSLtBRfc. OCT X ** :^«^^. CHAPTEK A^--STEUCTUEE Defiititiox The term structure as used in geology commonh^ refers to the attitude or "lay^^ of the rock layers^ that is, whether they are fiat lying, inclined, folded, or broken by faults. Structure of this kind can be represented by photographs and sketches, by diagrammatic cross-sections and block drawings, but most accurately by meaus of structure contours. Structure or the Quadraxgles The structure of the pre-Pennsylvanian rocks underlying the area is shown in Plate III, by contours based upon the elevation of the surface of the St. Peter sandstone. Structure Coxtour Map A structure contour map is similar to a surface contour map but differs from it in showing the elevation of the top or bottom of a selected stratum rather than the elevation of the surface of the ground. The detail of the structure contour map is much less than that of the surface map, because the elevations of all points on the surface can be readily determined, whereas the elevation of a buried stratum is known only at outcrops and where it has been reached by the drill or mine shafts. Such data is obviously neither system- matically arranged or evenly spaced over large areas. Uses of Structure Coxtours The primary use of the structure map is to show the structural features. The key rock slopes or dips from the contour lines of higher elevation to those of lower, and in this way the map shows the "lay^' of the strata. The map can also be used to determine the approximate depth of the coal bed used as a key rock and of other beds of known interval above or below the key stratum. It can thus be used for locating -coal, clay, limestone, and water-bearing rocks. One of the special services of the coal structure map in Illinois has been to determine the possible areas of oil and gas accumulation. • It lias' been found as a rule that structural features affecting the "Coal Measures^^ affect also the underlying rock to a considerable depth in the same way, though possibly to a greater or less degree. K relationship of areas of accumulation to anti- clinal folds and domes is known to exist, and the fact that, at least in some 85 86 HENNEPIN AND LA SALLE QUADRANGLES places, domes in the coal strata indicate conditions favorable for oil and gas has given added value to structure contour maps of the coal beds. Accuracy of Structuee Contours The accuracy of the structure contours is determined first, by the number and distribution of the observations; second, by the reliability of geologic data, v^^hich involves the accuracy of statement by those from whom the infor- mation is sought, the accuracy of the location of the drill holes, and correct- ness of correlation of strata at outcrops and in drill holes; and third, by the method of determining the elevation of the bed to be contoured. In the accompanying structure maps of the Hennepin and La Salle quad- rangles (see Plate II in pocket, and fig. 28), the reference strata used are the La Salle (Third Vein or No. 2) coal, which is the most widespread and extensively worked coal bed in the area, and the St. Peter sandstone. Both have numerous natural exposures along and east of the La Salle anticline. Elsewhere the sandstone is widespread beneath the older rocks and the coal has nearly as general a distribution. Several artesian wells reach the sand- stone and the coal has been penetrated by mine shafts and borings. These represent about the only source of information concerning the structure west of the anticline and away from the Illinois Valley. They are numerous except in the east half of the Hennepin quadrangle especially south of Bureau Creek. In this part of the area the structure of the rocks is not known. Near the Illinois and in a few tributary valleys outcrops substantiate the interpretation of structure as determined by other methods. The drilling records are based upon diamond-drill borings under com- petent supervision and are judged to be accurate. Generally it was possible to locate accurately the exact position of the drill hole, and the consistency of the structure as delineated precludes the probability of great error from inaccurate location. The correlation of the recorded strata at the different drill holes is open to doubt only in a limited area in the vicinity of Bureau where the La Salle coal is thinner than common and liable to confusion with one or two probably lower thin coals of restricted distribution. The structure of the St. Peter sandstone, as shown in Plate III, is gen- eralized since it is less accurately known than the structure of the coal. Except for a single boring for oil, the artesian wells aiford the only information concerning the altitude of the sandstone, and most of these are located along or south of the Illinois. The elevation and location of these wells were deter- mined with a possible error of not more than 5 feet, and the depths were probably reported correctly since the top of the sandstone is an easily recog- nized horizon. Three methods are used by the Survey to determine elevations: (1) by B S/SA b BIAA 87 c^,..-^[me from ich marks Id (3) by Tor about y->^ -A n 3r^ 3.t 17 3t • • the sand- \^ 3 are local cannot be vals, how- elevation ^-_ t the same f the area. ^ ire correct sandstone : 3^11). 1" jT) ^ '^-^tj^«^Ui ^5' tt_^^' !^s ^9--'^}^^ d rock are , B La Salle } \ since the ' "he fold all ! strata dip at about a ,. east of the apparently so that the I structural the "Lower strata little le anticline :. 3 St. Peter. •: >'>lays minor ^-^^Peter sand- 19 ^^ — j^ y VJ 1 yx:t ']" Dv;\7 V- he La Salle t about 100 t some have he anticline which above d Little and 86 places, d( has given The and disti data, whi mation is ness of c( method o Intl rangles ( the La S; extensive! have nun.Tfd" Elsewhere *" r has neart stone and represent of the ant. i : in the eas I •_ In this p ' ; Illinois a] < of structn ' ; The : petent su I H t E 3 .*r^ «0 A E- ;p 3^ ^d 0^^{63^oX \? ! 30 >■» I '^^^c^;'',f '''; to locate of thesti':!--^ ^^;^ inaccurate —^ drill hole,' where the one or tw( The eralized si for a sin^ '■-. concernin, ;i ,;. •■ \ ^- or south c -JL^ 1^ ^^ : \ mined wi probably : ,' nized hori ' Thre< \ If : 'V: i \e ^^QLOA^ I 5^ .^3 JLLOA \ lA Q^OA^ ^'^ :>5\ BiirrEii^ 510* 3^' LrvxE in STRUCTURE 87 accurate instruments, with a negligible error; (2) by hand-level line from adjacent bench marks, with a possible error in this region, where bench marks are numerous, of rarely more than 5 and usually less than 3 feet; and (3) by estimation from the contour map direct in the field with a possible error about the same as the 20-foot contour interval. The surface of the coal, especially, and probably also that of the sand- stone is somewhat more irregular than the contours indicate. There are local variations in the elevation of the coal as great as 10 or 15 feet that cannot be shown because of the largeness of the contour interval. The intervals, how- ever, are sufficiently large to minimize the effect of slight errors in elevation to which the map is liable, as noted in preceding paragraphs, and at the same time it is sufficiently adequate to show the main structural features of the area. It is believed that the structure lines for the surface of the coal are correct within a contour interval, that is 25 feet, and for the surface of the sandstone within one-half a contour interval or 50 feet. (See Plates II and III). Structure of the Quadrangles In the Hennepin and La Salle quadrangles the layers of hard rock are not horizontal, for an important structural feature, known as the La Salle anticline, is pronounced. This is more correctly called a monocline, since the fold is step-like, with the depression on the west side. West of the fold all the strata of the bed rock slope toward it. To the east, however, the strata dip slightly east from the crest of the fold to the State line. The altitude of the top of the sandstone has been determined at about a dozen Avells scattered over the area at numerous outcrops along and east of the anticline. The overlying Ordovician and Silurian rocks are apparently approximately parallel to the surface of the St. Peter sandstone, so that the contours, drawn at intervals of 100 feet vertically, show the main structural features of all the formations between the Pennsylvanian series and the "Lower Magnesian^^ formations. Concerning the structure of the older strata little is known. Data based upon the records of three wells west of the anticline indicate a general parallelism of the "Lower Magnesian^' with the St. Peter. East of the fold where it outcrops the Shakopee dolomite displays minor undulations which are not known to continue above into the St. Peter sand- stone. In the Hennepin and La Salle quadrangles the altitude of the La Salle coal west of the outcrop along the anticline has been ascertained at about 100 places. Most of the data are derived from mines and coal tests, but some have been obtained from well borings. Several determinations near the anticline were calculated from the outcrops of black slate, the distance of which above the coal is known. Along the anticline in the valleys of Illinois and Little and ss HEXXEPIX AND LA SALLE QUADRANGLES Big A'ermilion rivers, and west of the anticline along Illinois Valley there are many exposures of the coal to which measurement has been made direct. The contour lines on the structure map (PI. II in pocket), are based upon these figures and they show the elevation of the coal in feet above sea level, ])eing drawn at intervals of 25 feet vertically.^ The La Salle anticline crosses the area in a direction varying from N. 4^)° \V. at Arlington to X. 27° AV. where it is crossed by Illinois Valley, IVo 1^ Graj/ Shale Caved - WESCi-'/^l Scale in feet "'■"■"=-"Sr^S::S^§ 2 4 6 "^'"■;?>-?^ Scale in feet NEBEL'ia] Fig. 30. — Normal faults in Black Hollow mine. miles east of La Salle, to nearly north and southeast of Bailey Falls and west of Lowell. The steeply dipping west limb of the anticline has a width of ^ In Plate II the closed contour in sees. 20 and 29, Berlin Township, should be desig- nated 275 feet, that in sees. 25, 26, 27, 34, and 35 of the same township also 275, and that IV2 miles north of La Salle, 100. STRUCTURE 89 one-half to three-quarters of a mile along Illinois Eiver and at Deer Park, but the strata continue to rise for about one-half mile east of the steep inclination. To the northwest and to the south the dip of the west limb apparently becomes less abrupt than it is along the Illinois, but exact values are wanting. N'ear Deer Park the La Salle coal is known to dip in places as much as 52 degrees, but the usual exposed dip of the PennsylYanian series rarely exceeds 15 degrees. At such places the underlying Ordovician rocks dip 30 to 35 degrees, and it is not known whether this dip increases toward the trough of the fold. Fig. 31. — Thrust fault in the Oglesby mine. ?-^:r — : Gray '^^^^hale^ Scale in 1 2 ^-NXN ^■^ ^\\\ <;v feet 3 4 ^mcPiished Shale ^^^^t^-=^3-^G!ra7/ Shale- Fig. -Step fault in Dalzell shaft, Spring Valley Coal Company. Because of this structure the La Salle coal, which east of the anticline has an altitude of 610 feet or slightly more, west of the anticline in the trough parallel to the fold has an altitude of only about 100 feet above sea level. All the Pennsylvanian strata west of the fold dip eastward or southeast- ward at a low rate averaging about 12% feet to the mile from the probable outcrojD northeast of Princeton to the bottom of the trough near La Salle. The actual dip is greater to the west than near the anticline. Considerable differences in the altitude of the coal have be?n noted within short distances 90 HENNEPIN AND LA SALLE QUADRANGLES of Bureau, the elevations being constantly greater to the east as is shown by map. There is a gradual rise of the floor of the trough parallel to the anti- cline both to the northwest and to the south which accompanies the decrease in the strength or height of the anticline in the same directions. The rise to the south from Oglesby is at the rate of about 2 feet per mile and to the north- west from La Salle probably as much as 10 to 12 feet to the mile. East of the crest of the anticline the coal dips very gently to the east, possibly about 5 feet to the mile. Minor irregularities in the structure of the "Coal Measures" strata, especially of the coal beds not brought out by the map are not uncommon. The coal in any mine does not lie perfectly horizontal or dip uniformly in any direction, but small, broad basins rarely over 20 feet in depth are found here and there throughout the area (see Plate IV). These are not, however, of sufficient importance to affect mining materially. In Black Hollow and Oglesby mines small faults have been observed affecting the coal and adjacent strata. In no case does the throw amount to more than the thickness of the coal bed. Figs. 30, 31, and 32 illustrate observed faults in these two mines. Nearly vertical slickensided planes tra- verse the roof shale. In addition to these faults, slipping has also occurred along bedding planes, with the result that commonly the upper layer of coal fingers off into the shale and is bent back in the direction of movement. In poorly consolidated beds, such as the roof shale of La Salle (No. 2) coal, irregular fracturing has been produced, so that upon undermining the coal the roof shale falls away in irregular masses, the sides of which are slickensided surfaces. Small faults have also been observed in the mine at Dalzell. In general faults are unimportant in the area, and those observed are confined to the strata associated with La Salle (No. 2) coal. None has been observed in the pre-Pennsylvanian strata. UHlVERSITVOf/UiBo/SL/BBMy OCT i -i . W STAITT ,W ■.fm mTKMiJja > --■ '^J ,'■'■ I/a/'). \\\ Jy. .•)ai?na .ioiWtSriO.eO^ .•^, v?rj VERSITY OF ILLIBO/S LIBB^ :.-=»?> ^ ^f>^J nl iBoo V^ .^y ^.-Jjh:_ '^If^fi'm.^ ^Ii/cim. . V-i 3TA.T1 CHAPTER VI— GEOLOGIC HISTORY Imperfection of the Record The record of the geologic history of this region is read in part in the outcropping rocks and in part in the rocks known only from drilling. There are long gaps in the record, some of which can be filled by facts inferred from the results of studies in other areas in the province, but of other gaps the story cannot be supplied with satisfaction even by inference. The data are most complete for several periods of the Paleozoic era and for the latter part of the Quaternary era, but this represents but a small fraction of geologic time. Of the long history before the Upper Cambrian epoch we have no knowledge except as inferred from the history of adjacent regions. The main outlines of this history were presented in the introductory chapter on the history of the province. Erosion has erased the record of events during Mesozoic and Ter- tiary eras so that this part of the story is exceedingly fragmentary in the Glaciated Plains province. Paleozoic Era CAMBRIAN PERIOD At the opening of the Paleozoic era the surface of Illinois had probably been above the sea for a long time and had been eroded until it was nearly flat. Early in the era the region was gradually submerged and sandy deposits were laid down in the encroaching sea. The submergence probably took place in Middle Cambrian time and lasted at least until the close of Cambrian time. This upper Cambrian formation is not exposed or known from drilling in this region but it has been reached by the drill at Ottawa, Illinois, and at other places farther east and probably underlies this entire area. After the deposition of this great formation which reached a maximum thickness of 1,000 feet, there appears, from the record elsewhere, to have been a withdrawal of the sea from the Central Mississippi area, if not from the whole continent, accompanied by uplift and erosion. Ordovician" Period LOWER AND MIDDLE ORDOVICIAN EPOCHS In the early Ordovician time the sea again advanced over this region and a great series of strata, the Prairie du Chien group of formations, was 91 92 HENNEPIN AND LA SALLE QUADRANGLES deposited. During this period of deposition conditions mnst have fluctuated sufficiently to allow the deposition at times^ of massive dolomite and at other times of sandstones. Toward the end of the earlier epoch when the Shakopee dolomite was being deposited conditions of relatively shallow water prevailed as is indicated by the evidences of shallow water deposition found in the strata. Eipple marks, oolite, brecciated limestone, thought to be possibly largely the result of chemical precipitation point to shallow water origin. Probably sometime after the deposition of the Shakopee dolomite and before the deposition of the overlying St. Peter sandstone, slight deformation of the Shakopee seems to have taken place. At the close of the period in which the Shakopee dolomite was deposited the interior sea was withdrawn and widespread erosion followed. Possibly gentle folding of the Shakopee formation took place at this time and produced the gentle undulations in the dolomite as exposed between La Salle and Utica. During this erosion interval the Shakopee member suffered considerable loss of material. The irregular upper surface of the dolomite exposed over the area is indicative of considerable relief due to erosion and to deep surface weather- ing. As a result of the latter process a mantle of regolith two or three feet thick and composed of residual clay interspersed with undissolved flint and siliceous nodules is found lying between the ^'Lower Magnesian" and the St. Peter formations. At the end of this period of uplift and erosion the sea gradually advanced on the land and this area formed part of a broad shallow basin where wave and wind action was of such character as to form great deposits of pure white sand. Mr. E. 0. Ulrich regards the St. Peter sandstone as a well-developed beach formed of wind-transported quartz grains from the north, and hence is in part a land deposit. With the close of St. Peter time there was a period of non-deposition which is less important than the one at the base of the St. Peter and in many places is unrecognizable structurally. Following the emergence was a trans- gression of the sea in Platteville time. The area was covered by waters that probably were warm and free from much sediment, and the limestone composed of the remains of various marine animals was deposited immediately overlying the sandstone with relatively slight transition. A relatively unimportant emergence separated Platteville from Galena time. The Galena is remarkably free from shale and sand imiDurities, indi- cating a constancy of level. This epoch closed with widespread emergence. At some period following the deposition of the Galena and before the depo- sition of the Pennsylvanian rocks further folding occurred along the axis of the La Salle anticline. GEOLOGIC HISTORY 93 UPPER ORDOVICIAN EPOCH The next series of rocks, the Maquoketa limestones and shales, represents a period of finctnating conditions and shallow water. At certain times great quantities of mnd were brought into the sea from lands adjoining this district on the north. At other times the sea became clearer and possibly deeper and allowed the deposition of limestone in more or less lenticular beds. SILURIAN PERIOD The Maquoketa closes the marine Ordovician record in this region. The region remained emergent for a long time after the last Ordovician sediments were deposited. The Silurian period like the Ordovician was characterized by constant oscillations of the strand line, and the system is made up of many formations representing various epochs in the history. As our knowledge of the Silurian in this region is based entirely upon drilling records and drill cuttings from a number of wells, it is impossible to determine what members of the system are present except by inference. The widespread distribution of the Magaran dolomite in Illinois makes it probable that much of the heavy white dolomite found in the artesian wells was deposited in the great epi-continental sea which covered large parts of the interior in middle Silurian or Niagaran time. Whether the earlier or later incursions of the Silurian sea are represented in the section is not known. DEVONIAN PERIOD If, as may be possible, part of the limestone found in some of the deep wells underlying the Pottsville formation is of Devonian age, it was probably deposited following a lapse in sedimentation which was general in the province between Silurian and Devonian periods. The sea which advanced over parts of western Illinois in Middle Devonian time possibly reached this region, the incursion being recorded in some if not all the strata included in the upper part of the limestone and dolomite series between the Pottsville and the Maquoketa. Late Devonian deposition may have taken place in this region and the rocks later eroded, since shale overlying probable Devonian limestone is reported by Udden^ in a well as near Henry, in Marshall County, Illinois. CARBONIFEROUS PERIOD MISSISSIPPIAN EPOCH Although Mississippian rocks are found in considerable thickness in the vicinity of Peoria there is no evidence that the seas of that period extended 1 Udden, J. A., Some deep borings in Illinois: State Geol. Survey Bull. 24, p. 56, 1914. 94 HENNEPIN AND LA SALLE QUADRANGLES as far north as the Hennepin and La Salle quadrangles. Through all this long epoch, therefore, this region, so far as we know, was a land surface sub- jected to the processes of weathering and erosion. PENNSYLVANIAN EPOCH Pottsville erosion and deposition.- — Preceding the deposition of any Penn- sylvanian rocks in this region was extensive erosion of the older rocks. If rocks younger than the Galena extended across the anticline these were entirely removed along the crest and for some distance down the flanks of the fold. The erosion surface also truncated the Galena-Platteville formations and extended into, if not through, the St. Peter sandstone. The resulting surface did not have great relief but along the west side of the anticline the surface was possibly 50 to 100 feet lower than to the east of the fold. The dip of the older rocks along the west limb of the anticline was about one-half of what it is at present, and to the east they lay nearly flat; thus the fold had even greater resemblance to a monocline or step fold than it has at present. The pre-Pennsylvanian erosion lasted possibly through the Mississippian epoch and certainly during the early part of the Pottsville stage. x\s coarse elastics are lacking in any Pottsville beds that have been observed it seems probable that the relief was not great and hence the stream erosion was here not rapid in early Pennsylvanian time. Denudation by solution was probably more important than erosion by streams. Considerable thicknesses of residual clay accumulated above the limestones from which it was formed, some of which was washed by the streams into the depressions such as that lying parallel to the anticline. North of the present position of the Illinois Valley, the pre-Pottsville surface extended across the Platteville limestone and upon the St. Peter sandstone. From this sandstone surface was derived the sand which lies interbedded with the clays or forms most of the Pottsville section as the case may be, the sandstone increasing in relative amount toward the north where the St. Peter underlies the "Coal Measures." It is believed that the Pottsville period terminated with the formation of an essentially level surface of clay or sand extending Avide-spread over the area. Carhondale deposition. — During the deposition of the Carbondale sedi- ments the region was at times covered by the sea and received deposits of mud, both aluminous and calcareous, and of sand, and at other times the sea was essentially banished for longer or shorter periods, when the surface commonly stayed so low and so level that brackish or fresh water marshes covered large areas, in which were accumulated beds of vegetal matter that were afterward transformed into coal. A large part of the sand may have been deposited on land, but the limestone and some of the shale formed during that time contain well-preserved remains of marine animals, showing con- clusively that the rocks in which they are found were deposited in the sea. GEOLOGIC HISTORY 95 There appear to have been also times of both local and general emergence and erosion, though the erosion was not nearty so extensive as that at the beginning of the preceding Pottsville epoch. Conditions generally were more quiet than in that epoch. Sand beds, though in some places coarse and thick, were not so extensive, and clay was the predominant material laid down. Peat made up a larger part of the deposits formed in this epoch than in any other. McLeanshoro deposition. — Between McLeansboro and Carbondale times there seems to have been a cessation of deposition in this area at a time when marsh conditions existed farther south and west toward the interior of the Illinois basin represented by the Herrin (Belleville or No. 6) coal. Erosion of the Carbondale formation seems to have locally exposed to surface condi- tions the Springfield (No. 5) coal and in places even to have entirely removed it. With the subsequent advance of the sea coarse sand and siliceous mud filled the irregularities of the surface. Where the sand overlay the weathered Springfield (No. 5) coal it penetrated the joints and bedding places loosened by weathering. The resulting rock is the peculiar mixture of sand and coal observed in mines and known as "white top.^^ Eventually the sea was again filled and marsh conditions on a large scale prevailed for the third time in this region. In this period the peat forming Streator (No. 7) coal was accumulated. There followed further subsidence accompanied by the deposition of 50 to 60 feet of arenaceous deposits. The remaining Pennsylvanian strata record a continuation of shallow- water deposition upon a gradually subsiding sea floor. The conditions were probably more continuously marine than they were earlier in the epoch as is indicated by the greater number of limestones carrying marine fossils. POST-CARBONIFEROUS DEFORMATION Carboniferous deposition was closed by widespread movements, which resulted in the uplift of the Appalachian Mountains on the east and the Ouachita and Ozark mountains to the west. These movements permanently banished the sea from the region. At this time or later there took place further movement along the line of the Ordovician fold. An additional 500 feet of throw was added to the displacement resulting in a total difference in altitude of 1,500 feet between strata on the two sides of the fold. Several small ridges and basins found in the ^Toal Measures'' probably originated at this time. MKSOZOIC EEA Following the withdrawal of the sea and the deformation near the close of the Paleozoic era the processes of erosion and weathering became active on the new surface. In the time that followed during the Mesozoic era the level of the sea fluctuated greatly, but it is not known to have advanced over central 96 HE>^EPIX AND LA SALLE QUADRANGLES or northern Illinois. Local warpings possibly took place accompanying the greater movements in the bordering mountain regions with a resulting acceler- ation of erosion from time to time, but the records of such C3xles have been erased by subsequent erosion. In southernmost and in northernmost Illinois the tops of certain hills of resistant rock apparently constitute remnants of peneplains older than any recorded in the Hennepin and La Salle quadrangles; hence a cycle of uplift and erosion sems to have taken place before the oldest surface in the quadi-angles was formed. CEXOZOIC ETJA TERTIARY PERIOD DEVELOPMENT OF RELIEF The processes that prevailed during Mesozoic time continued until the close of the Cenozoic. During the latter part of this era the land was essenti- ally a plain, as the surface upon which the glacial deposits rest is very even except that here and there narrow valleys were cut. The present average elevation of this ])lain beneath the drift is about 600 feet above sea level in this region. AVhen it was fornu'd it was probably more nearly at sea level. Near the close of the Tertiary period there was apparently a general uplift of the land, which accelerated erosion and caused the streams to deepen their valleys. Wells within the area show that several such valleys crossed these quadrangles, the rock floor below whiclu at one ])lace at least, was about 250 feet beloAv the surface of tlie plain into which the valley was cut. An elevation of only 350 feet above sea level of the rock floor is indicated by drilling near Bureau. From the data a Horded by 500 to 600 wells and from coal borings in the area it is possible to reconstruct the genei'al form of the bed-rock sur- face before glaciation. (See Plate V). The Tertiary drainage lines differed considerably from those existing at present. A large and deeji valley ran south across the west side of the Henne- pin quadrangle. The center of the valley lay about at the position of Prince- ton, where it turned slightly southeast toward Bureau and then continued southward slightly east of the center of the present Illinois Valley. This valley is thought to have been occupied by a large stream coming possibly from the direction of Pock River. As a large river valley crossed the area it is apparent that the Tertiary plain into which it was cut would be less well preserved within these quadrangles than in other areas at a greater distance from this valley. Lateral valleys of considerable size extending from the east half of the La Salle quadrangle were tributary to the large valley. The two most import- ant of these lay one to the north and one to the south of the present valley of the Illinois east of the bend. Characteristics of the valley south of the 3 W 9^. ■■K \\ M,M I Q V [I . 1^ .V ' ' ■ las lis er of \ '^ ^ .-I s# J )jS£fD allfiR £J bn; ii.,-^^--^^'Y^n .^^♦. n i|S ^J2..*l :3 he lal re- of VI 5^ ■ hud to ^buHHn sdi baa ©OBliixa iBlDBia-Q 96 ^Pl^ YaVEU8 greater i^ atioii of ^' erased b} tlie tops c: peneplain '' ^ie]ice a c^ surface in^ T '■::d<- : / i loC, ^ JAOIOOJOHD KTAT8 glOVi -^15 J AQ ''^ ^ mij ^Jsjiti J5" 3 V ai Tl 61 \ The p^^ close of the aJJy a plain ,,{^ except that elevation of' this region. ^, ^ear tl] of the land, | valleys. We" ■ quadrangles, r - y feetbelowth o ^ : • n i% J* N-^^ r^ I -V ■ 1 ^eet Inches SS 42 u 42 )9 66 59 48 )6 42 )0 42 >2 42 )8 42 \^ : Ibb^' i 3 I 36 '^iv no-lb 0a, ^ .3 H H'S H ) a small lie value. \icLeans- le coal is opposite, sec. 29, ■^in thick- of coal 8 La Salle sec. 27, 106 west side of the Matt this bed was not wor Coal Co./ in the SP,. therefore, that therf,,- Valley and Cherry The physical dj under the Carbon c'/ 35 are based upon, ■ "7"--<^^^e t^- ci 5C ^t ^3 j^ •^ V 5 The Streai former years it i^ LO 1 f .1 FEET Fig. r beds in H of lUinc ably wi decreas this be been unde] k In. '!jp\r'ri Illin the out^^ bet )%/ Gs ej 9//./ Sat^ orfa O oi c3 o o o o o CO g; (N 'O lO lO O CO O O O (M 00 to O ^* C3 r-I ' lO lO O c5 o f^i ' t>- O 00 O 00 c5 o f^i ^C: t>^ co' o im" 05C^ aiooO"*co oc^i• I T-l X (M 1 CO r-l ■ lO CO O ■*! CO 1-H CD -t^ t-^ -^ 1-1 C:5 OC' 1-1 CD O (M* t^ iQ Tj? 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