Digitized by the Internet Archive in 2018 with funding from University of Illinois Urbana-Champaign https://archive.org/details/stratigraphygeolOOthwa STATE OF ILLINOIS DEPARTMENT OF REGISTRATION AND EDUCATION A. M. SHELTON, Director % DIVISION OF THE STATE GEOLOGICAL SURVEY M. M. LEIGHTON. Chief REPORT OF INVESTIGATIONS—NO. 13 STRATIGRAPHY AND GEOLOGIC STRUCTURE OF NORTHERN ILLINOIS WITH SPECIAL REFERENCE TO UNDERGROUND WATER SUPPLIES PRINTED BY AUTHORITY OF THE STATE OF ILLINOIS URBANA, ILLINOIS 1927 ILLINOIS STATE LIBRARY STATE OF ILLINOIS DEPARTMENT OF REGISTRATION AND EDUCATION A. M. SHELTON. Director DIVISION OF THE STATE GEOLOGICAL SURVEY M. M. LEIGHTON. Chief Committee of the Board of Natural Resources and Conservation A. M. Sheltox, Chairman Director of Registration and Education Charles M. Thompson' Representing the President of the Uni¬ versity of Illinois Epson S. Rastin Geologist — •uta [COUNCIL * Jeffersons Printing & Stationery Co. Springfield, Illinois 1927 The accompanying report on Stratigraphy and Geologic Structure of Northern Illinois with Special Reference to Underground Water Supplies has been in the process of preparation for some time. Most of the buried rock formations of northern Illinois have been studied in surface exposures in Wisconsin by the author, Mr. F. T. Thwaites of the Wisconsin Geological and Natural History Survey, and the Illinois State Geological Survey has been fortunate in securing his interest in the study of the well cuttings of a large number of wells in that part of the State. Mr. Thwaites has also given much attention to the ap¬ plication of geologic knowledge to well drilling for water recovery. It is therefore believed that this report will be helpful to those seeking information regarding the geologic conditions of northern Illinois from the standpoint of underground water supplies. The report has had the benefit of a careful review by Mr. G. C. Habermeyer, Engineer of the Illinois State Water Survey, which both the author and the Illinois State Geological Survey gratefully ac¬ knowledge. M. M. Leighton, Chief, State Geological Survey Division. STRATIGRAPHY AND GEOLOGIC STRUCTURE OF NORTHERN ILLINOIS WITH SPECIAL REFERENCE TO UNDERGROUND WATER SUPPLIES By F. T. Thwaites OUTLINE Page Introduction . 9 Geologic formations and their water supplies. 9 General statement . 9 Pennsylvanian system . 10 McLeansboro, Carbondale, and Pottsville formations. 10 Description . 10 Water supplies . 11 Drilling conditions . 11 Devonian system . 11 Sweetland Creek shale, Cedar Valley limestone, and Wapsipinicon limestone . 11 Silurian system . 12 Niagaran dolomite, Kankakee limestone, and Edgewood formation (Niagaran and Alexandrian series). 12 Description . 12 Water supplies . 14 Drilling conditions . 14 Ordovician system . 15 Maquoketa shale (Richmond group). 15 Description . 15 Drilling conditions . 16 Galena and Platteville formations (Trenton and Black River groups).. 16 Description . 16 Water supplies . 18 Drilling conditions . 18 St. Peter Sandstone . 19 Description . 19 Water supplies . 20 Drilling conditions . 20 Shakopee, “New Richmond,” and Oneota formations (Prairie du Chien group) . 21 Description . 21 Water supplies . 24 Drilling conditions. 24 Cambrian system . 24 Jordan and Trempealeau formations . 24 Description . 24 Water supplies . 25 Mazomanie and Franconia formations. 25 Description . 25 Correlation . 27 Water supplies . 28 Drilling conditions. 28 Page Dresbach sandstone . # . 28 Description . 28 Water supplies . 31 Eau Claire formation . 31 Description . 31 Water supplies . 33 Drilling conditions . 34 Mt. Simon sandstone . 34 Description . 34 Water supplies . 35 Drilling conditions . 36 Structure . 36 Introduction . 36 Structural Features . 37 Cause of deformation . 43 Water quality and problems. 43 General statement . 43 Nature of soft waters. 43 Method of testing quality of water. 44 Contamination of deep wells. 45 Well construction . 45 Future of underground waters. 47 Well logs . 48 ILLUSTRATIONS Figure 1. Sketch map of aerial geology of northern Illinois. 8 2. Diagrammatic columnar section of northeastern Illinois showing revised correlation of strata . 29 Plate I. Structure map of northern Illinois.In pocket II. Section from Clinton, Iowa, to Chicago, Illinois.In pocket TABLES 1. Correlation table of geologic names used in Illinois and adjacent states.... 9 2. Generalized section of Pennsylvanian rocks of La Salle County. 10 3. Well log illustrating character of Devonian rocks. 12 4. Well logs illustrating character of Silurian rocks. 12 5. Well logs illustrating character of the Maquoketa shale. 15 6. Well logs illustrating character of the Galena and Platteville formations.. 17 7. Well logs illustrating character of the St. Peter sandstone. 19 8. Well logs illustrating character of the Prairie du Chien group. 23 9. Well logs illustrating character of the Jordan and Trempealeau formations 25 10. Well logs illustrating character of the Mazomanie and Franconia formations 26 11. Well logs illustrating character of the Dresbach sandstone. 30 12. Well logs illustrating character of the Eau Claire formation. 32 13. Well logs illustrating character of the Mt. Simon formation. 34 14. Well data used in the construction of the structure map (PI. I). 38 8 STRATIGRAPHY AND GEOLOGIC STRUCTURE OF NORTHERN ILLINOIS Scale 1 inch = 28 miles Sketch map of aerial geology of northern Illinois. INTRODUCTION Potable underground waters can be obtained in quantity through¬ out northern Illinois. They form an important resource which is utilized by many industries, cities, villages, and individuals. Very few cities of the size of Chicago have as large supplies of potable underground water. Rockford is the largest city in Northern Illinois to use well water for its public supply. The subject of underground waters in northern Illinois has been in¬ vestigated by a number of geologists and engineers. 1 The attention of the writer was drawn to the district in connection with studies in Wis¬ consin and the correlation of the deeply buried formations was an¬ nounced in 1923. 2 To simplify forecasting of depths to the Cambrian sandstones a structure map was made (PI. I). The present paper is designed primarily as an aid to engineers and well drillers. GEOLOGICAL LORMATIONS AND THEIR WATER SUPPLIES General Statement North of the 41st parallel of latitude Illinois is underlain beneath the soil and loose surface material by rocks of the Pennsylvanian, De¬ vonian, Silurian, and Ordovician systems. (See fig. 1.) Cambrian rocks may underlie parts of the Rock River valley but are elsewhere concealed under younger formations. In the far northwestern part of the State the mantle rock or “surface” was mainly formed by weathering of the bed rocks, in part was deposited by the wind, and in the valleys was laid down by streams. In the remainder of the district the mantle rock was deposited by glaciers with associated streams and lakes; these deposits are collectively known as “drift”. The bed rocks alone are discussed in this report. The names of the different formations are those used in 1926 by the Illinois State Geological Survey. For the convenience iStone, Leander, The artesian wells of Chicago: Chicago Acad. Sci. Bull. 1, pp. 93- 102, 1886. Rolfe, C. W., Artesian water from the drift in eastern Illinois: Am. Geologist, vol. 6, pp. 32-35, 1800. Mead, D. W., Notes on the hydrology of Illinois in relation to its water supplies: Illi¬ nois Soc. Engr. Rept., vol. 8, pp. 48-68, 1893. Mead, D. W., The hydro-geology of the upper Mississippi Valley and of some of the adjoining territory: Assoc. Engrs. Soc. Jour., vol. 13, pp. 329-396, 1894. Leverett, Frank, The water resources of Illinois: U. S. Geol. Survey Seventeenth Ann. Rept., pt. 2. pp. 695-828, 1896. Shufeldt, G. A., Jr., History of the Chicago artesian well, Chicago, 1865, Religio-Phil- osophical Publishing Assoc., Chicago, 1897. Udden. J. A., A new well at Rock Island. Illinois: Am. Geologist, vol. 21, pp. 199-200, 1898. Rolfe, C. W., The geology of Illinois as related to> its water supply: Univ. of Illinois, Chem. Survey of the Waters of Illinois, Rept. 1S97-1902, pp. 41-56, 1903. Udden, J. A., Geological classification of the waters of Illinois: Illinois State Geol. Survey Bull. 10, pp. 8-21, 1909, and Illinois State Water Survey Bull. 4, pp. 8-21, 1909. Anderson, C. B., The artesian waters of northeastern Illinois: Illinois State Geol. Survey Bull. 34, 1919. McClenahan, W. T., Predicting the results of deep well borings: Illinois Soc. Engrs., Proc., vol. 38, pp. 37-47, 1923. Habermeyer, G. C., Public ground-water supplies in Illinois: Illinois State Water Survey Bull. 21, 1925. 2Thwaites, F. T., The Paleozoic rocks found in deep wells in Wisconsin and north¬ ern Illinois: Jour. Geology, vol. 31, pp. 529-555, 1923. ( 9 ) 10 STRATIGRAPHY ANI) GEOLOGIC STRUCTURE OF NORTHERN ILLINOIS of those who have not followed all of the changes in usage of geologic names, a table (Table 1) has been prepared, in which the use of for¬ mation names in different reports on Illinois and adjoining states is compared. The description of formations in this report is confined to data from well logs and does not include a discussion of their paleon¬ tology and exact correlation. Each formation is illustrated by typical well records from the region here discussed. In quoting records by others some of the wording has been slightly changed to correspond with the usage in the data collected by the writer. Pennsylvanian System MCLEANSBORO, CARBONDALE, AND POTTSVILLE FORMATIONS DESCRIPTION Rocks of the Pennsylvanian system are known as the '‘Coal Measures” or the Upper Carboniferous. Formations of this age under- Table 2. —Generalized section of Pennsylvanian rocks of La Salle County a Description of strata Thickness Depth McLeansboro formation Feet Feet Shale, and clay, red, yellow, and blue, with limestone nodules . 24 24 Coal . 1 25 Shale, and clay, olive, yellow, and black, with gray, shaly limestone . 19 44 Coal and fire clay. 1 45 Shale, blue and brown. 25 70 Limestone, gray with layers of blue and gray shale. 29 99 Limestone, blue . 5 104 Shale, hard, black, with thin coal at base. 7 111 Shale, blue, with layers of blue limestone. 24 135 Coal and fire clay. 1 136 Shale, blue, with layers of gray limestone. 69 205 Shale, brownish red to brown, with thin limestone seams... 17 222 Sandstone. 18 240 Shale, black, slaty and in part sandy. 36 276 Coal and fire clay. 11 287 Shale, dark brown. 16 303 Sandstone. 34 337 Carbondale formation Shale, black . 10 347 Coal and fire clay. 10 357 Shale, sandy . 30 387 Sandstone, varies from 15 feet to.••. 35 422 Shale, brown, olive, blue, gray, and black, with thin layers of shaly limestone, and shaly, calcareous sandstone.... 83 505 Coal and fire clay. 5 510 Pottsville formation Sandstone. 6 516 Shale, gray and dark gray. 124 640 Sandstone, white . 40 680 Shale light gray. 36 716 Sandstone, fine to very fine grained, gray. 20 736 Shale, blue to green, sandy. 25 761 Total Pennsylvanian 761 feet ^Adapted from Cady, G. H., Coal resources of district I (Longwall) : Illinois Coal Mining Investigations Bull. 10, pp. 124-126, 1015. System Northern Illinois 1927 Noi Iowa, 1912 e Minnesota, 1911/ Minnesota 19000 Devonian Sweetland Creek Cedar Valley Wapsipinicon Sweetland Creek Cedar Valley Wapsipinicon Hamilton Marcellus? Corniferous Silurian Niagaran Alexandrian Niag Alex, Niagara Ordovician Richmond group Maquoketa Maqi Maquoketa Hudson River Galena Decorah Platteville Galei Platt Galena Decorah Platteville Galena Trenton St. Peter St. E St. Peter St. Peter Prairie du Chien group Shakopee “New Richmond” Oneota Praii Sh Ne Or Shakopee New Richmond Oneota Shakopee New Richmond Lower Magnesian Cambrian Jordan/ Jordan Jordan Trempealeau* St. Lawrence St. Lawrence Mazomanie Franconia Dresbach Jord Dresbach and undifferentiated Cambrian Dresbach Eau Claire St. Mt. Simon Dre Hinckley B., The artesian waters of northefr resources of Iowa: U. 29-1186, 1912. aAnderson, C. II, 1919. ftDoes not show new geologic systems proposed by Ulrich, E. O., Notes on new names in table ( Paleozoic systems in Wisconsin: Wisconsin Acad. Sc S. Geol. Survey Water-Supply Paper 293, ground waters of southern Minnesota: U. S. Geol, Survey Water- —_ _ _ ~ vo * - 6 ’ ^ a P °* s ^ e * 1901* Thwaites^F." T\*, ThePaTeozoic rocks found" in del by Ulrich; it is the opinion of the writer that it is equivalent to vol. 31, pp. 529-555, 1923. 1 * St.Lawrence.” « tt a n t cs cWeidman, Samuel, and Schultz, A. It., The undefoved by the Board of Geologic Names of the U. S. Geol. Survey. Geol. and Nat. Hist. Survey Bull. 35, PI. II, 1915. P art of Illinois. dChamberlin. T. C., Geology of Wisconsin, vol. Table 1 . —Correlation table of geologic names used in Illinois and adjacent states System Northern Illinois 1927 Northeastern Illinois 1919a Wisconsin 1923 & Wisconsin 1915c Wisconsin to 1915 d Iowa, 1912® Minnesota, 1911/ Minnesota 19000 Devonian Sweetland Creek Cedar Valley Wapsipinicon Milwaukee Milwaukee (Hamilton) Hamilton Sweetland Creek Cedar Valley Wapsipinicon Hamilton Marcellus? Corniferous Silurian Niagaran Alexandrian Niagaran Alexandrian Waubakee Niagaran series—Guelph, Ra¬ cine, Waukesha, Byron, Mayville Waubakee Niagaran series Salina Niagara Niagara Ordovician Richmond group Maquoketa Maquoketa Richmond group Maquoketa Richmond group Maquoketa Cincinnati Maquoketa Hudson River Galena Decorah Platteville Galena Platteville Galena-Black River groups Galena Decorah Platteville, Beloit Galena Decorah Platteville Galena Trenton Galena Decorah Platteville Galena Trenton St. Peter St. Peter St- Peter St. Peter St. Peter St. Peter St. Peter Prairie du Chien group Shakopee “New Richmond” Oneota Prairie du Chien group Shakopee New Richmond Oneota Lower Magnesian group Shakopee Oneota Lower Magnesian group Shakopee Oneota Lower Magnesian Shakopee New Richmond Oneota Shakopee New Richmond Lower Magnesian Cambrian Jordan/ Madison Mendota, Devils Lake^ Jordan Madison (Jordan) Madison Jordan Jordan Trempealeau* Mazomanie Franconia Trempealeau* Mendota (St.Lawrence) Mendota St. Lawrence St. Lawrence Mazomanie Franconia Franconia Potsdam Dresbach Jordan/ Dresbach Dresbach Dresbach and undifferentiated Cambrian Dresbach Eau Claire St. Lawrence Eau Claire Eau Claire Mt. Simon Dresbach Mt. Simon Mt. Simon Hinckley oAnderson, C. B., The artesian waters of northeastern Illinois: Illinois State Geol. Survey Bull. 34, Plate n, i9i9. fcDoes not show new geologic systems proposed by Ulrich. Ulrich, E. O., Notes on new names in table of formations and on physical evidence of breaks between Paleozoic systems in Wisconsin: Wisconsin Acad. Sci. Trans., vol. 21, pp. 71-107, 1924. Thwaites, F. T., The Paleozoic rocks found in deep wells in Wisconsin and northern Illinois: Jour. Geology, vol. 31, pp. 529-555, 1923. cWeidman, Samuel, and Schultz, A. R., The underground and surface water supplies of Wisconsin: Wisconsin Geol. and Nat. Hist. Survey Bull. 35, PI. II, 1915. dChamberlin. T. C., Geology of Wisconsin, vol. 1, pp. 119-212, 1883. eNorton, W. H., et al, Underground water resources of Iowa: U. S. Geol. Survey Water-Supply Paper 293, PI. II, 1912 ; Iowa Geol. Survey, vol. 21, pp. 29-1186, 1912. /Hall, C. W., et al, Geology and underground waters of southern Minnesota: U. S. Geol, Survey Water- Supply Paper 256, PI. VI, 1911. ^Winchell, N. H., Geology of Minnesota, vol. 6, Map of state, 1901. AThe Mendota is assigned to this position by Ulrich ; it is the opinion of the writer that it is equivalent to the lower part of the Trempealeau, the original “St. Lawrence.” iThe name Trempealeau has not been approved by the Board of Geologic Names of the U. S. Geol. Survey. /Formation present only in northwestern part of Illinois. STRATIGRAPHY AND GEOLOGIC STRUCTURE OF NORTHERN ILLINOIS 11 lie only the southern part of the area here described. (See fig. 1.) They have been divided, in descending order, into the McLeansboro, Carbon- dale, and Pottsville formations which are all composed of: gray to black shale which is in part calcareous, and much of which is very pyritic; im¬ pure, shaly, brownish-gray, pyritic, calcareous sandstone; gray, calcitic limestones; and coal. The relative proportions of these rocks vary much within short distances. The thickness reaches a known maximum of about 760 feet in this region but is much less in most of the area. The generalized section in Table 2 shows the character of the Pennsylvanian formations. WATER SUPPLIES The greater part of the Pennsylvanian rocks are impervious or yield little water the quality of which is notoriously poor. In all but a few places near the outcrop of a water-bearing layer, salt and hydrogen sul¬ phide make the water unfit for most uses. This condition is apparently explained by (1) the discontinuity of the porous strata which has pre¬ vented thorough flushing of the soluble minerals, and (2) the presence of pyrite and carbon. Inasmuch as the Pennsylvanian rocks rest upon several of the underlying formations, the water in the latter is contam¬ inated in many localities which are adjacent to such contacts. Explora¬ tion for potable water in the Pennsylvanian rocks and in formations which are in contact with water-bearing layers in them cannot be recom¬ mended where other sources are possible. DRILLING CONDITIONS Most of the Pennsylvanian rocks offer no unusual difficulty to well drilling so far as the experience of the writer has shown. Some of the shales, particularly the fire clays, swell or disintegrate to clay when wet but most of the rocks do not cave to an unusual extent. Locally old caved mine workings may be encountered. On account of the character of the water it is absolutely necessary to case off all of the Pennsyl¬ vanian formations. Devonian System SWEETLAND CREEK SHALE, CEDAR VALLEY LIMESTONE, AND WAPSIPINICON LIMESTONE Devonian rocks outcrop only in the vicinity of Rock Island. They consist of brown and black shale, and white, gray, and blue, more or less shaly limestones and dolomites. The total thickness does not exceed 300 feet. These rocks contain some water near the base especially in Mercer County. They offer no unusual drilling difficulty. The log in Table 3 may include some of the underlying Silurian. 12 STRATIGRAPHY AND GEOLOGIC STRUCTURE OF NORTHERN ILLINOIS Table 3. —JVell log illustrating character of Devonian rocks Partial log of city well No. 2, Aledo, Mercer County Thickness Depth Feet Feet 135 Shale, brownish gray, fossiliferous. 130 265 Dolomite, light gray, dark spots, pyritic, in part oolitic, fos¬ siliferous . 20 285 Limestone, light gray . 65 350 Shale, light gray . 5 355 Limestone, light gray . 10 365 Dolomite, light gray to white. 63 428 Shale, gray, dolomitic, logged as “black slate”. 2 430 Total Devonian 295 feet Silurian System NIAGARAN DOLOMITE, KANKAKEE LIMESTONE, AND EDGE WOOD FORMATION (NIAGARAN AND ALEXANDRIAN SERIES) DESCRIPTION The rocks of the Niagaran and Alexandrian series of northern Illi¬ nois consist mainly of light gray, more or less cherty dolomite. Toward the base, in the Alexandrian series, there are interstratified beds of green, pink, red, and blue dolomitic shales. These lower beds have been divided by Savage 4 into the Kankakee limestone and the Edgewood formation. From the standpoint of the well driller it is hardly worth while to at¬ tempt to distinguish these thin formations but the base of the Silurian may be fixed at the bottom of the light-colored dolomites. The total thickness of the Silurian nowhere exceeds 500 feet and in the vicinity of Chicago is much less than that. The well logs in Table 4 indicate the character of the formations in'various parts of the district. Table 4. —IVell logs illustrating character of Silurian rocks (a) Partial log of city well No. 2, Aledo, Mercer County Thickness Depth Feet Feet 430 Dolomite, light gray. . 25 455 No sample, log shows same as above. . 10 465 Dolomite, bluish-gray, vesicular. . 10 475 Dolomite, light gray. . 85 560 Total Niagaran 130 feet 4Savage, T. E., Alexandrian rocks of northeastern Illinois and eastern Wisconsin: Bull. Geol. Soc. America, vol. 27, pp. 305-324, 1916. STRATIGRAPHY AND GEOLOGIC STRUCTURE OF NORTHERN ILLINOIS 13 (b) Partial log of well No. 2, Mineral Point Zinc Company, Depue, Bureau County a Thickness Depth Feet Feet 388 Dolomite, white . 10 398 Dolomite, dense, light gray. 17 415 Dolomite, white, pyritic . 7 422 Dolomite, light gray, some white chert. ........... 13 435 Dolomite, white, porous, white chert and quartz. 35 470 Dolomite, white, soft, fine-grained, some quartz and white chert 50 520 Dolomite, gray and greenish gray, soft, some green clay. 20 540 Dolomite, white, soft, fine-grained, white chert. 15 555 Dolomite, light gray, soft, white chert. 47 602 Dolomite, coarse-grained, buff . 6 608 Dolomite, coarse-grained, buff, some dark gray and pyritic.... 7 615 Dolomite, dense, light yellow, with some white rock, fossiliferous 17 632 Dolomite, light gray, porous, coarse-grained. 18 650 Dolomite, fine-grained, white . 23 673 Dolomite, buff, with some gray staining. 27 700 Dolomite, dense, white . 20 720 Dolomite, buff, coarse-grained, porous. 17 737 Dolomite, gray, with some green and gray shale. 28 765 Dolomite, gray to white, some white chert. 77 842 Dolomite, gray, pyritic . 8 850 Total Silurian (possibly including some Devonian) 479 feet aUdden, J. A., Some deep borings in Illinois: Illinois State Geol. Survey Bull. 24. p. 49, 1914. (c) Partial log of well of Abbott Laboratories, North Chicago, Lake County 100 Dolomite, light gray, porous . 10 110 Dolomite, white . 80 190 Dolomite, gray, some dark gray clay. 20 210 Shale, gray, very dolomitic. 20 230 Dolomite, light gray . 25 255 Dolomite, pink to pinkish gray. 25 280 Dolomite, light gray . 65 345 Dolomite, light and dark gray. 5 350 Total Niagaran 250 feet (d) Partial log of A. D. Lasker well, Everett, Lake County 154 Dolomite, white to gray . 39 193 Dolomite, white to light gray, with white chert. 35 228 Dolomite, light gray . 20 248 Dolomite, light gray, with white chert. 5 253 Dolomite, light gray and greenish gray. 5 258 Dolomite, light gray with brown spots; white chert. 10 268 Dolomite, white to light gray. 90 358 Dolomite, dark gray to white. 5 363 Dolomite, light gray, pyritic. 15 378 Shale, green, dolomitic; dolomite, light gray. 5 383 Dolomite, light gray . 5 388 Dolomite, light gray and light greenish gray. 10 398 Total Niagaran and Alexandrian 244 feet 14 STRATIGRAPHY AND GEOLOGIC STRUCTURE OF NORTHERN' ILLINOIS WATER SUPPLIES The Silurian dolomites yield water mainly from cracks and to some extent from openings caused by solution. Quantities of as much as 1,100 gallons per minute have been reported in some localities but these are exceptional. The amount of water depends upon the local amount of fracturing of the formation. Near Chicago the water of many wells contains hydrogen sulphide. 5 In some localities there are strong show¬ ings of oil whose presence renders the water undrinkable. Exploration for oil was once carried on in and near Chicago without securing com¬ mercial production. 6 In later years a little oil and gas were found near Lake Forest. 7 . In view of the lack of an impervious capping over the dolomite and the universal presence of fresh water which implies thor¬ ough flushing of the formation, it is extremely unlikely that any large quantity of either oil or gas will ever be discovered. According to an average of 31 analyses 8 the Xiagaran waters of Cook County average less than 0.7 pound of encrusting solids per 1000 gallons and are there¬ fore decidedly softer than the water of Lake Michigan which carries slightly more than one pound in the same quantity. Near La Salle, water with as little encrusting solid but considerable salt is ascribed to the same formation 9 but such soft waters are not typical. Most Xiagaran waters contain more than 3 pounds of encrusting solids per 1000 gallons. Anderson 10 mentions several wells which were contaminated by the effluent from gas works, so that sewage contamination is possible in all wells where the glacial drift is thin or pervious. DRILLING CONDITIONS No unusual drilling conditions are likely to exist in the Silurian dolomites except incline fissures which may cause a crooked hole. When an intercepting tunnel was constructed at Argo to cut a number of wells at a depth of 357 feet the holes were all found to be out of vertical by amounts ranging from 6 inches to 11 feet. 11 It is recom¬ mended that all wells in large cities where the drift is thin be cased through the Silurian rocks in order to eliminate the danger of contam¬ ination as well as the objectionable hydrogen sulphide. 5 Wei land, H. J.. and Bartow. Edward. Hydrogen sulphide in the well waters of Chi¬ cago and vicinity: Illinois State Water Survey Bull. 13, pp. 359-368. 1916. sShufeldt, G. A., On an oil-well boring at Chicago: Am. Jour. Sci., zd ser., vol 40, pp. 388-389, 1865. Hunt, T. S., On the oil-bearing limestone of Chicago: Canadian Naturalist, vol. 6, pp. 54-59, 1871. Grant, U S., Possible horizons for oil and gas in northeastern Illinois: Illinois State Acad. Sci. Trans., vol. 15, pp. 389-392, 1922. Alden, W. C., U. S. Geol. Survey Geol. atlas, Chicago folio (No. 81), p. 13, 1902. 7Anderson, C. B.. The artesian waters of northeastern Illinois: Illinois State Geol. Survey Bull. 34, pp. 185-186, 1919. sidem, p. 96. 9Cady, G. H., Geology and mineral resources of the Hennepin and Ea Salle quad¬ rangle: Illinois State Geol. Survey Bull. 37, p. 126, 1919. lOAnderson, C. B.. op. cit., p. 103. nAnonymous, Corn products water supply and underground pumps: Engr. News- Record, vol. 93, pp. 501-503. 1924. STRATIGRAPHY AND GEOLOGIC STRUCTURE OF NORTHERN ILLINOIS 15 Ordovician System MAOUOKETA SHALE (RICHMOND GROUP) DESCRIPTION The Maquoketa formation consists of bluish gray to greenish gray, rarely brown dolomitic shale with subordinate amounts of white to dark gray or blue dolomite, some of which is very shaly. In far northwestern Illinois there are some layers of calcitic limestone. The thickness varies from about 100 feet to nearly 200 feet. No attempt has yet been made to subdivide the formation. The amount of water is negligible. The well logs in Table 5 indicate the character of the Maquoketa. Table 5. —Well logs illustrating character of the Maquoketa shale (a) Partial log of village well, Grays Lake, Lake County Thickness Depth Feet Feet 400 Shale, blue, dolomitic . 30 430 Dolomite, dark gray, and shale, blue, dolomitic. 10 440 Shale, blue, dolomitic . 30 470 Dolomite, dark gray with light gray spots. 20 490 Shale, blue, dolomitic. 60 550 Total Maquoketa 150 feet (b) Partial log of A. D. Lasker well, Everett, Lake County 398 Shale, blue, dolomitic . 25 423 Dolomite, gray and dark blue, pyritic. 20 443 Shale, dark blue, hard, dolomitic; dolomite, gray. 5 448 Shale, blue, dolomitic . 80 528 Total Maquoketa 130 feet (c) Partial log of well drilled for Chicago Portland Cement Company Oglesby, La Salle County rt Shale, gray, calcareous . Limestone, gray, somewhat shaly . Limestone, gray, subcrystalline . Limestone, gray to bluish gray. Shale, gray, rather soft . Dolomite, gray, subcrystalline, some shale Dolomite, gray, subcrystalline . Limestone, gray, subcrystalline . Shale, gray, slightly calcareous . Shale, gray, calcareous . Total Maquoketa 165 feet 5 985 990 10 1000 20 1020 20 1040 20 1060 25 1085 25 1110 15 1125 15 1140 10 1150 aCady, G. H., Coal resources of district I (Longwall): Illinois Coal Mining- Investiga¬ tions Bull. 10, p. 127, 1015. This log is based upon study of samples by T. E. Savage; another log, stated slightly differently, is given in a later publication, Cady, G. H., Geol¬ ogy and mineral resources of the Hennepin and La Salle quadrangles: Illinois State Geol. Survey Bull. 37, p. 30, 1919. 16 STRATIGRAPHY ANI) GEOLOGIC STRUCTURE OF NORTHERN ILLINOIS (d) Partial log of city well No. 2, Aledo, Mercer County Thickness Depth Feet Feet 560 Shale, blue, dolomitic. 73 633 Dolomite, gray and blue, pyritic. 15 648 Shale, blue, dolomitic . 50 698 Shale, light brown, dolomitic . 35 733 Shale, brown, very dolomitic, hard. 10 743 Total Maquoketa 183 feet DRILLING CONDITIONS The Maquoketa shales are very soft and drill easily but at times it is difficult to pick up the mud with the bailer. Sand or gravel poured into the hole often aids drilling. The collection of accurate samples is difficult because some of the cuttings form a thin mud which will not settle. Material stuck to the bit is not reliable, for it may have been scraped from the side of the hole while the tools were coming up. Masses of mud often stick to the side of the hole and later fall off. Washed samples are misleading since the dolomite and limestone beds are all interbedded with more or less shale. The entire formation should be cased off and it is best to place the casing before the well is drilled any deeper. A water-tight joint or “shut-off” must be made at the bot¬ tom of this string of pipe. In most wells this may be accomplished by the use of a shoe, for enough mud will settle around the pipe to seal it. The shut-off may be tested by bailing down the Avater inside the pipe to a level that is lower than it was before casing and watching the level over night to see that it is substantially stationary. Failures to effect a shut-off have been known to be due either to a crooked hole or to the fractures in the rock on which the bottom of the casing rested. The difficulty can be overcome by either drilling deeper to form a new shoul¬ der or feeding mud or cement down the outside of the pipe while the water inside is bailed down. GALENA AND PLATTEVILLE FORMATIONS (TRENTON AND BLACK RIVER GROUPS) DESCRIPTION The Trenton group is represented in northern Illinois by the Galena dolomite. This formation consists of gray, cherty, coarse-grained dolo¬ mite which weathers at the surface to a yellow sand-like consistency. Below the Galena in northwestern Illinois is a few feet of shale which is equivalent to the Decorah shale of Minnesota and Wisconsin. Beneath this shale, which is generally bituminous, is the Platteville formation of the Black River group. In the west this formation is calcitic limestone; to the east it gives way to dolomite of gray and blue colors, the equiv¬ alent of the Beloit formation of Wisconsin. In the northwestern and STRATIGRAPHY AND GEOLOGIC STRUCTURE OF NORTHERN ILLINOIS 17 central parts of the State, the base of the Platteville is marked by a brown and green shale and sandstone, the Glenwood member; 12 farther east it is marked by coarse- to medium-grained, dolomitic sand¬ stone. Locally a layer of very dolomitic sandstone with a maximum thickness of 40 feet is found in the basal Platteville. This “stray sand” is separated from the St. Peter sandstone by 10 feet or more of some what sandy, gray dolomite. The basal Platteville is very pyritic. The thickness of the Galena and Platteville formations combined varies from a little less than 300 feet to as much as 450 feet. The well logs in Table 6 show the character of these formations which are not everywhere easily separated. Table 6. —Well logs illustrating character of Galena and Platteville formations (a) Partial log of well of Clinton Brewing Company, Clinton, Iowa® Galena formation Dolomite, gray, crystalline, in part cherty. Dolomite, gray, cherty . Dolomite, gray or light buff; crystalline; in part vesicular.. Dolomite or magnesian limestone, brown, crystalline; with fossiliferous bituminous shale. Decorah (?) Shale, brown, highly bituminous and fossiliferous. Platteville formation Limestone, magnesian, brown . Limestone, magnesian, dark gray, subcrystalline, in part cherty . Limestone, bluish gray, dense, fossiliferous, thin bedded... Limestone, light yellowish gray, soft, fossiliferous. Limestone, light bluish gray, fossiliferous. Shale, brown, bituminous; and limestone, dense. Shale, bluish green, pyritic, flaky. Thickness Depth Feet Feet 330 170 500 10 510 55 565 5 570 5 575 12 587 33 620 25 645 5 650 5 655 10 665 5 670 Total Galena-Platteville 340 feet aNorton, W. H., et al, Underground water resources of Iowa: U. S. Geol. Survey Water-Supply Paper 293, p. 391, 1912. (b) Partial log of city well No. 2, Aledo, Mercer County Dolomite, light brown- to gray. Dolomite, light gray . Dolomite, light gray with some white chert Dolomite, white, white chert, pyritic. Dolomite, brown and gray, pyritic. Shale, greenish gray, dolomitic. Limestone, brown, blue, and white. Dolomite, gray, blue spots. Total Galena-Platteville 322 feet 743 20 763 102 865 85 950 15 965 30 995 5 1000 10 1010 55 1065 i2Bevan, Arthur, The Glenwood as a horizon marker at the base of the Platteville limestone: Illinois State Geol. Survey Rept. of Investigations No. 9, p. 6, 1926. 18 STRATIGRAPHY AM) GEOLOGIC STRUCTURE OF NORTHERN' ILLINOIS (c) Partial log of well drilled for Chicago Portland Cement Company, Oglesby, La Salle County^ 1150 Dolomite, gray, fine grained. 35 1185 Limestone, magnesian, gray . 40 1225 Limestone, gray, fine grained, some magnesium. 85 1310 Limestone, gray, fine grained . 55 1365 Limestone, gray, very fine grained. 120 1485 Limestone, dark gray to light gray, very fine grained. 10 1495 Limestone, gray, fine grained . 25 1520 Total Galena-Platteville 370 feet ibCady, G. H., Coal resources of district I (Longwall): Illinois Coal Mining Investi¬ gations Bull. 10, pp. 127-128, 1915. Log based on examination of samples by T. E. Savage; a different log was published by Cady at a later date. (d) Partial log of village well, Grays Lake, Lake County Thickness Depth Feet Feet 550 Dolomite, gray . 100 650 Dolomite, gray, with a few blue spots. 40 690 Dolomite, gray and buff, some blue spots; white chert. 10 700 Dolomite, gray, with blue spots; white chert. 40 740 Dolomite, gray, with some blue layers. 10 750 Dolomite, dark gray, with blue spots. 50 800 Dolomite, light gray, dark gray, blue and buff. 30 830 Dolomite, dark gray, with blue spots. 10 840 Sandstone, medium grained, white, calcareous. 10 850 Sandstone, fine grained, white, calcareous; thin layers of gray, calcareous shale . 30 880 Dolomite, light bluish, gray, very pyritic. 10 890 Sandstone, very coarse, white; dolomite layers like above. 10 900 Total Galena-Platteville 350 feet WATER SUPPLIES Except on the outcrop, and along the La Salle anticline where there has been much Assuring, the Galena and Platteville formations yield little water. On account of the presence of pyrite, the Avater contains considerable amounts of sulphates ; this fact explains the very hard scale formed when waters from these formations are used in boilers. Because of the comparative thinness of the formations, it is rarely advisable to stop drilling before the underlying St. Peter sandstone is reached. Shows of oil are not at all uncommon; a notable instance was reported at the Crotian Orphanage east of Des Plaines. There is a remote possibility of commercial production of oil should domes be discovered on the crest of the anticline north and northwest of Des Plaines, but it is probable that almost all of the oil that was once present has been washed out by fresh water. DRILLING CONDITIONS Unless it is desired to shut off some of the lower sandstones, the hole through the Galena and Platteville formations may be left uncased. STRATIGRAPHY AND GEOLOGIC STRUCTURE OF NORTHERN ILLINOIS 19 Presence of sand in the cuttings gives warning of the near approach to the St. Peter sandstone. The “stray sand" should not be mistaken for the St. Peter. It is coarser grained and more dolomitic than the St. Peter, breaks in chips, and contains no chert or red non-dolomitic shale. In disturbed zones, as near Des Plaines and La Salle, caving rock may be found in these formations. ST. PETER SANDSTONE DESCRIPTION As found in wells, St. Peter is a light gray, less commonly pink or yellow, fine- to medium-grained, more or less dolomitic sandstone. A layer a few inches thick which is cemented by marcasite, a variety of iron sulphide, is found at the top in many places. Beneath the sand¬ stone, which was originally defined as the St. Peter formation, is a variable thickness of red and green shale with pebbles of white and yel¬ low chert, sandstone with chert pebbles, and some pink and gray dolo¬ mite. In some places only the chert-sandstone conglomerate is found. These basal beds were formerly referred to the Prairie du Chien forma¬ tion but as they are material formed by its weathering and then re¬ worked by water it seems preferable to assign them to the younger formation. Including these beds, the thickness of the enlarged St. Peter varies from 50 to nearly 500 feet. 13 In Wisconsin the absence of the St. Peter at many places is determined from rock outcrops and well records based on the study of samples. It is possible that it is absent in parts of Illinois since it is very thin in the Sears Roebuck well at Chicago. Table 7 illustrates the character of the St. Peter. Table 7.— Well logs illustrating character of the St. Peter sandstone (a) Partial log of city well, Galena, Jo Daviess County Thickness Depth Feet Feet 155 Sandstone, medium grained, very light gray, pyritic. 50 205 Sandstone, medium grained, -light gray and pink. 15 220 Sandstone, fine grained, light yellow. 15 235 Sandstone, medium grained, pink, yellow, and white, no sample 265-275 . 50 285 Sandstone, pink; with red shale. 30 315 Sandstone, brownish red; shale, green and purple; chert and some dolomite . 20 335 Shale, purple and green layers; micaceous, hard; some sand.... 10 345 Total St. Peter 190 feet. i3lf the record of an old well at Dixon given by Tiffany is correct this figure may reach 715 feet. Tiffany, A. S., Record of deep well at Dixon, Illinois: Am. Geologist, vol. 5» p. 124. 1890. 20 STRATIGRAPHY AND GEOLOGIC STRUCTURE OF NORTHERN ILLINOIS (b) Partial log of Chicago and North Western Railway well, Malta, De Kalb County Sandstone, medium to fine grained, gray, dolomitic; some green shale . Sandstone, medium to fine grained, gray to white. Sandstone, medium to fine grained, yellow to gray. Shale, red; pebbles of white chert. Sandstone, medium to coarse grained, pink; pebbles of white chert . Sandstone, fine to medium grained, light gray; some light yel¬ low quartzitic layers and pebbles of white chert.. . Shale, pink, very sandy; many pebbles of white chert. Sandstone, fine grained to medium grained, gray - -. Conglomerate; coarse to fine-grained sandstone with pebbles of white chert . Sandstone, coarse to medium grained, pink, shaly. Conglomerate, coarse to medium grained; white and pink sand¬ stone with pebbles of white chert. Total St. Peter 460 feet. 490 • 55 545 55 600 260 860 5 865 10 875 25 900 10 910 5 915 5 920 25 945 5 950 (c) Partial log of well of North Shore Country Club, Glenview, Cook County Thickness Depth Feet Feet 840 Sandstone, fine grained, gray, dolomitic . 20 860 Sandstone, fine grained, gray . 100 960 Conglomerate, pebbles of white chert in coarse to fine grained, gray sandstone; some gray, dolomitic shale. 20 980 Conglomerate like above mixed with gray dolomite and shale... 10 990 Total St. Peter 150 feet. WATER SUPPLIES The St. Peter sandstone is an important water-bearing formation near its outcrop in the north-central portion of the State but no longer yields large quantities of water in the Chicago district because of (a) its irregular thickness and (b) overdraft by the numerous wells in Chicago. As a result of oxidation of the pyrite and marcasite at the top of the formation and in the adjacent dolomites, the water contains considerable sulphate and rarely are less than 3 pounds of encrusting solids found in 1000 gallons. Where adjacent to water-bearing layers in the Penn¬ sylvanian, as east of La Salle, the water carries some hydrogen sulphide. In localities where the St. Peter is thick and wells are widely separated, yields of 200 gallons per minute or more may be obtained. DRILLING CONDITIONS The top and bottom of the St. Peter are levels at which an unusual amount of trouble may be encountered in drilling. Many strings of tools have been lost soon after the St. Peter has been entered. Drilling pro¬ ceeds without apparent trouble until the attempt is made to withdraw the tools which are then found to be stuck. Drillers ascribe this diffi¬ culty to the fact that the water level is in many places lower in the St. STRATIGRAPHY AND GEOLOGIC STRUCTURE OF NORTHERN ILLINOIS 21 Peter than in higher formations and the water runs down the hole carry¬ ing fine cuttings which settle around the tools or mud up the walls of the hole. Insertion of casing from the surface to the bottom of the Maquoketa shale with care to secure a good shut-off is said to remedy this difficulty. It is also advisable to make short runs, clean the hole thoroughly, and not drill continuously when the St. Peter is reached. Should the tools be lost, it is almost hopeless to attempt to withdraw them without clearing away as much of the debris as possible by drilling past the tools; this cannot be accomplished in holes much smaller than 8 inches in diameter since there is not room enough to permit the tools to swing sideways. An objection to holes less than 6 inches in diameter is that efficient fishing tools are not made for smaller sizes. The only way to fish a five-inch cable drill is to use a friction socket which does not take a firm hold. These difficulties do not apply to flowing wells where the water carries away the cuttings. The reason many old wells are of very small diameter is that pole tools were used and fishing was easier than with cable tools. Pole tools have, however, been abandoned on account of their many disadvantages. Another danger zone is the shales and conglomerates at the base of the St. Peter. These can generally be drilled through without much caving, but unless the hole is cased at once either large chunks fall in or swelling reduces the size of the hole enough to cause trouble. This horizon is known to well drillers as “The Cave”. Examination of rocks that have fallen into wells shows that the formations are filled with cracks and small faults. It is difficult to determine in some places the point at which all of this kind of material has been passed through, since shale beds may be found in the underlying dolomites. In some wells casing has ap¬ parently not been inserted at once, for samples from underlying forma¬ tions are contaminated with caved shale and chert. Most, if not all, of the chert ascribed to the Cambrian formations by some students of well records is unquestionably of such an origin. It is best to insert casing which rests on a shoulder as soon as it is apparent that the caving zone is safely passed. The work must be planned to allow for a reduction in the size of the hole and foresee the contingency that one reduction may not be enough. Underreaming in these formations is by all means to be avoided as it is very slow and troublesome. SHAKOPEE, “NEW RICHMOND,” AND ONEOTA FORMATIONS (PRAIRIE DU CHIEN GROUP) DESCRIPTION The Prairie du Chien or Lower Magnesian group is divided in de¬ scending order into the Shakopee dolomite, the “New Richmond” sand- 22 STRATIGRAPHY AND GEOLOGIC STRUCTURE OF NORTHERN ILLINOIS stone, and the Oneota dolomite. These subdivisions cannot as yet be made in all parts of the State since the sandstone which separates the dolomites is not found at all points. In part the irregularity in the pres¬ ence of the formations is accounted for by pre-St. Peter 14 erosion which locally removed the entire group and appears to have destroyed the Shakopee formation over nearly all of the eastern part of northern Illinois. The dolomites are mainly light gray in color but red, pink, or buff dolomite is found in some localities. White, yellow, and pink chert, both oolitic and dense, is common near the outcrop but is much less abundant at depth. Thin layers and specks of green shale are wide¬ spread and locally some red shale is found. Some layers are sandy and quartz-lined cavities are common. Some of the dolomite breaks under the drill into small regular chips which have been mistaken for sand¬ stone by inexperienced drillers. It is an open question whether the medium-grained dolomitic sandstone near the middle of the group in the western part of the State is really the same as the original New Richmond sandstone of Minnesota. Near Dixon this sandstone con¬ tains red and green shale together with chert. It is difficult to fix the base of the group. At Glenview there is a sandstone which may be the Madison formation of Wisconsin. Some of the purple-spotted non- chertv dolomite of the basal Prairie du Chien of northeastern Illinois may be equivalent to the Mendota dolomite of Wisconsin but there ap¬ pears to be no way to confirm the correlation. If it is correct, the Madi¬ son sandstone is absent in most of Illinois. In this report no attempt has been made to separate these beds of questionable age from the main body of the Prairie du Chien. In former reports (Table 1) this group was extended downward to include the sandy glauconitic strata here placed in the Mazomanie and Franconia formations. The strata of the Shakopee are in many places irregularly inclined, closely folded, and complexly faulted. The disturbances which caused the present attitude of the beds evidently antedate the deposition of the overlying St. Peter. 15 The thickness of the Prairie du Chien group as defined in this report reaches a maximum of about 400 feet. In certain places, as at Lom- i4According to Sardeson (Sardeson. F. W., Shakopee dolomite and its cone domes: Pan-Amer. Geologist, vol. 45, pp. 29-48, 1926) the period of erosion was between the deposition of the Oneota and the Shakopee formations. He urges that the Shakopee con¬ sists of domes of dolomite separated by sandstone and shale, all of which are conform¬ ably overlain by what he calls the “Peter” sandstone. It is a fact that the Shakopee strata have never been seen to be truncated by the St. Peter beds but instead dip parallel to the irregular contact. The fact is indisputable that neither Shakopee nor Oneota dolo¬ mite is present beneath the St. Peter in many localities in Wisconsin and that the basal St. Peter gives indubitable evidence of long weathering of the underlying formations. In Wisconsin the St. Peter is known to rest upon all formations from the Shakopee to the Dresbach, a fact supported by outcrops and by well logs. Sardeson’s claim that the Prairie du Chien is not absent at Galena because he has collected Shakopee fossils nearby is of no weight since the pinching out of the dolomites takes place in very short distances. loCady, G. H., The structure of the La Salle anticline: Illinois State Geol. Survey Bull. 36, pp. 109-112, 1920. STRATIGRAPHY AND GEOLOGIC STRUCTURE OF NORTHERN ILLINOIS 23 bard, Grays Lake, Rondout, and possibly part of Dixon, the entire group of dolomites is absent and the St. Peter rests upon the Cambrian. At Des Plaines no “third” dolomite is reported in the city well but its absence may be the result of faulting since the rocks are very much broken. The well logs in Table 8 illustrate the character of this group. Table 8. — Well logs illustrating character of the Prairie du Chien group (a) Partial log of well of Clinton Brewing Company, Clinton, Iowa a Thickness Depth Feet Feet 730 Shakopee formation Dolomite, gray, sandy . 23 753 Dolomite, gray; with layers of hard, dark gray shale. 22 775 Dolomite, gray to bluish gray, pyritic. 18 793 Sandstone, medium-grained, gray, with gray dolomite. 7 800 Dolomite, light gray, dense, cherty and shaly. 10 810 Dolomite, light gray, shaly, sandy . 15 825 Dolomite, light to dark gray, coarse grained, porous, cherty 70 895 New Richmond (?) formation Dolomite, light gray, sandy . 16 911 Sandstone, light gray, dolomitic, hard, fine grained. 9 920 Oneota formation Dolomite, light gray, buff, and pink; white chert. 70 990 Dolomite, buff, very fine, sandy. 10 1000 Dolomite, light gray to bluish gray; cherty and sandy in . some layers . 75 1075 Total Prairie du Chien 345 feet. ctNorton, W. H., et al, Underground water resources of Iowa: U. S. Geol. Survey Water-Supply Paper 293, p. 391, 1912. (b) Partial log of well No. 2, Dixon Epileptic Colony, near Dixon, Lee County Dolomite, fine grained, red and gray; top part caves. Dolomite, fine grained, gray . Dolomite, fine grained, gray, with soft, red, caving shale. Dolomite, gray, some pink; sandy gray pink, and yellow chert in part oolitic . Dolomite, reddish purple and brownish gray; some white chert. . Sandstone, medium grained, white; shale, green and dark red, caving; chert, oolitic, white and light brown. Dolomite, gray to light brown and pink, sandy; some red and green shale; chert, white and pink with quartz. Thickness Depth Feet Feet 182 8 190 61 251 11 262 143 405 37 442 18 460 140 600 Total Prairie du Chien 418 feet (c) Partial log of well of North Shore Country Club, Glenview, Cook County 990 Dolomite, gray . 80 1070 Sandstone, fine grained, gray, very dolomitic. 20 1090 Dolomite, gray, sandy .. 80 1170 Total Prairie du Chien 180 feet 24 STRATIGRAPHY AND GEOLOGIC STRUCTURE OF NORTHERN ILLINOIS (d) Partial log of Mt. Carmel Cemetery well, Hillside, Cook County 880 Dolomite, light gray, with green shale... 10 890 Dolomite, light gray, with white chert and green shale. 10 900 Dolomite, light gray, with some green shale. 20 920 Dolomite, light gray, with white chert. 10 930 Dolomite, light gray and pink. 20 950 Dolomite, light gray, with green shale. 30 980 Dolomite, very light gray; with white chert, oolitic and dense. . . 20 1000 Dolomite, gray and pink, sandy, green shade; oolitic white chert 30 1030 Dolomite, gray, sandy, pyritic. 20 1050 Dolomite, light gray, with green shale. 60 1110 Dolomite, gray, sandy . 10 1120 Dolomite, light gray . 50 1170 Dolomite, light gray, pyritic . 10 1180 Dolomite, gray, some glauconite. 10 1190 Total Prairie du Chien 310 feet, beds from 1110 to 1190 possibly Mendota WATER SUPPLIES Where deeply buried the Prairie du Chien dolomites carry little water. Wells formerly in use to supply the village of Utica are sup¬ posed to depend for their supply mainly on water from the “New Rich¬ mond” sandstone and furnish water with about 2.7 pounds of encrusting solids per 1000 gallons. 10 Other wells may draw a portion of their sup¬ ply from this formation. DRILLING CONDITIONS The irregular and inclined layers near the top of the Prairie du Chien group where the Shakopee dolomite is present may cause much trouble by deflecting the hole. In some places, as near Dixon, caving layers of shale and broken dolomite have been found but rarely does a “cave" occur which is sufficiently bad to require immediate casing with reduc¬ tion of the size of the hole. Cambrian System JORDAN AND TREMPEALEAU FORMATIONS DESCRIPTION The Jordan and Trempealeau formations are so closely related that they can be discussed together. Formerly the Jordan was defined as sandstone and the Trempealeau as sandy to pure dolomite but recently Ulrich 17 has endeavored to confine the name Jordan to the coarser grained sandstone just beneath the Prairie du Chien dolomite excluding the fine-grained sandstone below. This has been objected to by leCady, G. H., Geology and mineral resources of the Hennepin and La Salle quad¬ rangles: Illinois State Geol. Survey Pull. 37, pp. 124, 126, 1919. i 7 Ulrich, E. O., Notes on new names in table formations and on physical evidence of breaks between Paleozoic systems in Wisconsin: Wisconsin Acad. Sci. Trans., vol. 21, pp. 72-90. 1924. STRATIGRAPHY AND GEOLOGIC STRUCTURE OF NORTHERN ILLINOIS 25 Stauffer 18 on the ground that it conflicts with the original definition of the Jordan. Irrespective of the merits of the controversy the older classification is preferable from the standpoint of the well driller. The Jordan sandstone thus defined is medium- to fine-grained and is wholly white or light gray in color; more or less dolomite is present and to both the south and east of its outcrop in western Wisconsin, northeastern Iowa, and southeastern Minnesota the formation grades laterally into sandy dolomite. The maximum thickness of the Jordan is about 60 feet in the western part of Illinois; no Jordan can be distinguished east of Dixon. The Trempealeau formation consists of nearly 200 feet of more or less sandy dolomite of gray, pink, and brown colors, and of red and gray dolomitic shale. Some glauconite is present. The thickness diminishes to the east and no Trempealeau has been distinguished east of Dixon. The well logs in Table 9 illustrate the character of the Jordan and Trempealeau formations. WATER SUPPLIES Although the Jordan sandstone is very important as a source of water in Iowa, the writer does not know of any large wells in Illinois that derive a significant part of their production from that formation. The nature of the Trempealeau formation indicates that it probably furnishes little water. MAZOMANIE AND FRANCONIA FORMATIONS DESCRIPTION The Mazomanie and Franconia formations consist of fine- to medium-grained sandstone which is for the most part dolomitic and glauconitic. The colors are gray, green, pink, and red. In the far west¬ ern part of the State there is some shale and sandy dolomite. In the Table 9 —IVell logs illustrating character of the Jordan and Trempealeau formations (a) Partial log of well of Clinton Brewing Company, Clinton, Iowa® Thickness Depth Feet Feet 1075 Jordan formation Sandstone, fine grained, light gray, dolomitic, glauconitic, with layers of gray dolomite.• .. 15 1090 Trempealeau formation Dolomite, light gray, fine grained, rounded sand. 30 1120 Sandstone, light gray, very fine grained, dolomitic. 20 1140 Dolomite, buff; no cuttings.recovered. 70 1210 Dolomite, light brown, hard, medium grained. 10 1220 Dolomite, some sand and glauconite, gray. 10 1230 Dolomite, sandy, glauconitic, pink. 10 1240 Dolomite, sandy, gray . 30 1270 Dolomite, light pink, sandy, glauconitic .. •.. 10 1280 Total Jordan and Trempealeau 205 feet. aNorton, W. H., et al, Underground water resources of Iowa: U. S. Geol. Survey Water-Supply Paper 293, pp. 391-392, 1912. isstauffer, C. R., The Jordan sandstone: Jour. Geology, vol. 33, pp. 699-713, 1925. 26 STRATIGRAPHY AND GEOLOGIC STRUCTURE OF NORTHERN ILLINOIS (b) Partial log of city well, Galena, Jo Daviess County Jordan formation Sandstone, medium grained, white to gray, pink at base-... Trempealeau formation Dolomite, fine grained, sandy, brownish gray to light gray and pink ...... 435 55 490 120 610 Total Jordan and Trempealeau 175 feet Chicago district much of the Mazomanie has been logged by drillers as either “lime” or “sandy lime” but examination of cuttings leaves no doubt that the rock is sandstone so well cemented by dolomite that it breaks into chips under the drill. Separation of the Mazomanie and the underlying Franconia is apparently not practicable in well records; the Mazomanie contains more sand and dolomite than the Franconia which is more shaly. There is probably no Franconia in the eastern half of Illinois. The combined formations vary in thickness from about 80 to nearly 150 feet. The logs in Table 10 illustrate the character of these rocks. Table 10. — Well logs illustrating character of the Mazomanie and Franconia formations (a) Partial log of city well, Galena, Jo Daviess County Thickness Depth Feet Feet 610 Shale, light to medium green, slightly gritty, some mica and pyrite . 10 620 Shale, green, calcareous, sandy, glauconitic. 30 650 Sandstone, medium grained, gray, glauconitic, pyritic. 10 660 Shale, light green, slightly calcareous, sandy, pyritic. 20 680 Dolomite, fine grained, brownish gray, sandy glauconitic, pyritic 10 690 Total Mazomanie and Franconia 80 feet (b) Partial log of city well, Clinton, Iowa a Thickness Depth Feet Feet 1355 Dolomite, pink, very fine, sandy, shaly, glauconitic.. 1 1356 Sandstone, exceedingly fine grained, shaly, dolomitic; dolomite, gray, glauconitic . 14 1370 Sandstone, fine grained, glauconitic, with green shale. 30 1400 Shale, greenish gray, very sandy, glauconitic . 40 1440 Sandstone, fine grained, glauconitic, dolomitic; shale, sandy, red 5 1445 Total Mazomanie and Franconia 90 feet aNorton, W. H., et al.. Underground water resources of Iowa: U. S. Geol. Survey Water-Supply Paper 293, p. 385, 1912. STRATIGRAPHY AND GEOLOGIC STRUCTURE OF NORTHERN ILLINOIS 27 (c) Partial log of Chicago and North Western Railway well, West Chicago, Du Page County^ 1150 Sandstone, fine grained, gray to pink, dolomitic, with red dolo- mitic shale . 25 1175 Sandstone, fine grained, light gray, with green dolomitic shale. 50 1225 Sandstone, coarse to medium grained, gray to white, dolomitic, glauconitic, breaks in chips . 60 1285 Total Mazomanie 135 feet. bSamples examined by R. C. Lentz, inspector. (d) Partial log of well at St. Mary’s Academy, Des Plaines, Cook County 860 Dolomite, gray and light pink, sandy, glauconitic, pyritic. 10 870 Sandstone, fine grained, pink, very dolomitic, glauconitic, pyritic, with pink, red, and green dolomitic shale...... . 15 885 Sandstone, fine grained, gray and pink, very dolomitic, glauconitic 25 910 Sandstone, fine to very fine grained, very dolomitic, glauconitic. 35 945 Sandstone, very coarse to fine grained, with some pink dolomite and glauconite . 20 965 Total Mazomanie 105 feet CORRELATION The Jordan sandstone has been correctly recognized in northeastern Iowa and northwestern Illinois for a long time. The underlying forma¬ tion now called Trempealeau was called St. Lawrence by the Iowa geologists. The sandstones beneath, now recognized as Mazomanie and Franconia, were included under the name St. Lawrence. In north¬ eastern Illinois the very dolomitic sandstone here referred to the Mazo¬ manie with confidence was formerly referred to the basal portion of the Prairie du Chien group (fig. 2), although Anderson 19 was some¬ what uncertain about this and thought possibly these strata might correspond with the Madison and Mendota formations of Wisconsin. The formations in question are characteristically glauconitic, and in fact constitute the most prominent glauconitic horizon in the entire section, whereas the Madison and Mendota are both low in glauconite. The Madison and Mendota are everywhere underlain by glauconitic sandstone instead of the pure white sandstone which is beneath the glauconitic beds of northern Illinois. It was suggested that this white sandstone is the Jordan but no place is known where the Jordan sand¬ stone underlies the Mendota dolomite. The succession of a glauconitic- dolomitic sandstone underlain by a white sandstone is strikingly like the proved Mazomanie-Dresbach section of central and northeastern Wisconsin; the only difference is that the Madison sandstone pinches out to the south of Wisconsin and the Jordan sandstone to the east of lOAnderson, C. B., The artesian waters of northeastern Illinois: Illinois State Geol. Survey Bull. 34, pp. 84, 107, 1919. 28 STRATIGRAPHY AND GEOLOGIC STRUCTURE OF NORTHERN ILLINOIS Iowa. This fact is made certain by comparative sections drawn across northern Illinois and from the outcrops in Wisconsin south into Illi¬ nois. 20 In addition, the formations beneath the white sandstone here correlated as Dresbach are entirely unlike those below the proved Jor¬ dan sandstone. WATER SUPPLIES The Mazomanie and Franconia formations are for the most part too dense, fine grained, and dolomitic to furnish much water except where there are crevices. Anderson 21 reports that such are present in the vicinity of Chicago. There is no available information on the quality of the water. DRILLING CONDITIONS No unusual drilling difficulties in the Mazomanie and Franconia formations have come to the attention of the writer. Problems arise when it is desired to set casing to the bottom of the Mazomanie sand¬ stone. The rock at this level is in many places too soft to support the weight of the pipe. In order not to drill too deep to allow of making a shoulder on a firm layer, it is well to define the top of the formation and compute its base from records of thicknesses in adjacent wells. The top of the Mazomanie may be distinguished by the presence of glau¬ conite, a dark green, soft mineral in small grains, and clear quartz sand in amounts making up a considerable portion of the rock. These sub¬ stances may best be found by dissolving the cuttings in hydrochloric (muriatic) acid which is slightly heated in a glass or china dish; this treatment will cause the dolomite to dissolve with effervescence of carbon dioxide and the quartz and glauconite will be left plainly visible. Approach to the bottom of the Mazomanie may be distinguished by the presence of very coarse quartz grains and by reduction in the amount of dolomite which is made manifest by less effervescence with acid. It is the opinion of the writer that a shut-off may best be made when the first coarse sandstone is reached rather than at the true base of the forma¬ tion. It would seem advisable to explore ahead with a hole of smaller diameter and then to ream down to a shoulder on a suitable hard layer. DRESBACH SANDSTONE DESCRIPTION The Dresbach consists of medium-grained, pure white to yellow, or rarely pink sandstone. In few places is there any quantity of dolomite cement; a pink or yellow layer near the middle of the formation is not 2oThwaites, F. T., The Paleozoic rocks found in deep wells in Wisconsin and northern Illinois: Jour. Geology, vol. 31, pp 529-555, 1923. 2iAnderson, C. B., op. cit., pp. 86, 89. STRATIGRAPHY AND GEOLOGIC STRUCTURE OF NORTHERN ILLINOIS 29 SILURIAN ORDOVICIAN CAMBRIAN Fig. 2. correlation 1 Anderson’s correlation 1919 NIAGARAN ALEXANDRIAN MAQUOKETA Galena Platteville 25 w >—* S3 O D Q 09 — cd •—< < od o, St. Peter Shakopee New Richmond Oneota Jordan < 5 03 S < ■< u ci 09 a. a. P St. Lawrence Dresbach 1 HE ■i r ^=3§E- i .J- i r O I I I ■ fl~4l Thwaites’ correlation 1927 NIAGARAN ALEXANDRIAN RICHMOND GROUP MAQUOKETA Galena Decorah Platteville SILURIAN St. Peter Shakopee “New Richmond” Oneota 25 09 S3 u Q 09 >—* cd «— < fu ORDOVICIAN Mazomanie Dresbach Eau Claire Mt. Simon CAMBRIAN LIMESTONE CHERT SHALE SANDSTONE Diagrammatic columnar section of northeastern Illinois showing revised of strata. 30 STRATIGRAPHY AND GEOLOGIC STRUCTURE OF NORTHERN ILLINOIS uncommon. The top of the formation is easily recognized by the change in the color as well as by the softness of the rock. The thick¬ ness varies from 30 feet near Waukegan to nearly 200 feet to the south and west of that point. The logs in Table 11 illustrate the character of the Dresbach. I able 11. — Well logs illustrating character of the Dresbach sandstone (a) Partial log of city well, Galena, Jo Daviess County Thickness Depth Feet Feet 700 Sandstone, medium grained, white. 40 740 Sandstone, rather fine grained, light gray. 100 840 Total Dresbach 140 feet (b) Partial log of city well, Clinton, Iow T a a 1445 Sandstone, coarse grained, white. 5 1450 Sandstone, fine grained, white. 5 1455 Sandstone, coarse, pinkish gray .. 20 1475 Sandstone, fine grained, white. 35 1510 Total Dresbach 65 feet aNorton, W. H., et al. Underground water resources of Iowa: U. S. Geol. Survey Water-Supplj 7 Paper 293, p. 385, 1912. (c) Partial log of Chicago and North Western Railway well, West Chicago, Du Page County 6 1285 Sandstone, coarse to medium grained, white. 20 1305 Sandstone, coarse to fine grained, white and yellowish gray, dolomitic, hard. 100 1405 Sandstone, medium to fine grained, white to light gray, dolomitic 25 1430 Total Dresbach 145 feet ^Samples examined by R. C. Lentz, inspector. (d) Partial log of village well, Grays Lake, Lake County 1200 Sandstone, medium grained, white •.. 60 1260 Sandstone, fine grained, w 7 hite. 50 1310 Total Dresbach 110 feet (e) Partial log of city well, Elmhurst, Du Page County Sandstone, medium to fine grained, gray to white, slightly dolomitic . Sandstone, fine grained, pink, dolomitic... Sandstone, medium to fine grained, yellowish gray, dolomitic at top . 1270 60 1330 10 1340 50 1390 Total Dresbach 120 feet STRATIGRAPHY AND GEOLOGIC STRUCTURE OF NORTHERN ILLINOIS 31 WATER SUPPLIES The Dresbach sandstone is one of the best-known water-bearing formations in northern Illinois and is drawn upon by a very large num¬ ber of deep wells. It is notable for its high porosity and freedom from objectionable substances such as dolomite, calcite, and pyrite. Yields from many of the wells that penetrate no deeper than the Dresbach are more than 500 gallons per minute although a considerable portion of the water is derived from higher formations. It is probably conservative to estimate that the Dresbach alone will furnish at least 200 gallons per minute without an excessive draw-down. 22 Wells at Bensenville which furnish water with only 0.9 pound of encrusting solids per 1000 gallons draw to an undetermined extent on formations below the Dresbach but not above it. Anderson’s 23 average of 57 analyses from wells in the Dresbach gives 5.2 pounds per 1000 gallons, but these wells are mainly uncased and draw to a large but unknown extent on the St. Peter and other formations. The water from the city well at Pawpaw carries less than 1.5 pounds. 24 EAU CLAIRE FORMATION DESCRIPTION The Eau Claire formation consists of fine-grained, dolomitic sand¬ stone of gray and pink colors, calcareous shales of red, gray, and green colors called “marl” by drillers, and more or less sandy and shaly gray to blue dolomite. The sandstones are in many places hard enough to break in chips and are often logged as “lime”. The upper limits of the formation may be distinguished by the change from clean sandstone to either shale or fine-grained dolomitic sandstone. The Eau Claire is noted for its variability and scarcely any two wells show the same suc¬ cession in detail. Sandstone appears to be more abundant near the middle of the formation. The lower limit of the Eau Claire formation is difficult to determine in some logs, for there are local layers of shale as far down as the drill has penetrated; in general it should be placed at the bottom of the shale, dolomite, and fine-grained dolomitic sand¬ stones. With this definition of the upper and lower limits of the forma¬ tion, a thickness of from 300 to nearly 400 feet is obtained. The logs in Table 12 illustrate the character of the Eau Claire. 22The subject of the relative draw-down or specific capacity of wells has not been investigated as the known data are contradictory. The writer believes that few wells in the Chicago district now furnish much more than 5 gallons per minute per foot of lower¬ ing except where some of the water comes from large opening or crevices. 23 Anderson, C. B., The artesian waters of northeastern Illinois: Illinois State Geol. Survey Bull. 34, p. 99, 1919. 24 Habermeyer, G. C., Public ground-water supplies in Illinois: Illinois State Water Survey Bull. 2i, p. 501, 1925. 32 STRATIGRAPHY AND GEOLOGIC STRUCTURE OF NORTHERN ILLINOIS Table 12. —JVell logs illustrating character of the Eau Claire formation (a) Partial log of city well, Galena, Jo Daviess County Thickness Depth Feet Feet 840 Sandstone, medium to fine grained, gray glauconitic. 20 860 Sandstone, like above, with gray sandy shale. 60 920 Sandstone, fine grained, gray, dolomitic, hard, some gray shale and pyrite . 30 950 Shale, gray, sandy, dolomitic, pyritic. 10 960 Sandstone, medium grained, gray, pink and red, dolomitic, glau¬ conitic, with greenish gray dolomitic shale. 190 1150 Total Eau Claire 310 feet (b) Partial log of Chicago and Northwestern Railway well, West Chicago, Du Page County® Thickness Depth Feet Feet 1430 Sandstone, coarse grained, gray, with blue dolomitic shale. 25 1455 Shale, greenish blue, slightly dolomitic, sandy. 25 1480 Shale, grayish blue, dolomitic .20 1500 Sandstone, coarse to very fine grained, light gray. 15 1515 Shale, mixed gray and red, sandy, dolomitic. 10 1525 Sandstone, medium to fine grained, gray to light pink, dolomitic, with gray dolomitic, glauconitic shale. 30 D5^ Sandstone, very fine grained, yellowish gray . 10 1565 Sandstone, medium to fine grained, gray to light pink, with gray dolomitic glauconitic shale . 30 159^ Shale, greenish blue, sandy, pyritic. 10 160^ Dolomite, gray to dark blue, with dark gray glauconitic shale. . . . 125 1730 Dolomite, white to light gray, with dark gray dolomitic and non- dolomitic shale layers. 70 1800 Total Eau Claire 370 feet ctSamples examined by R. C. Lentz, Inspector. (c) Partial log of well at St. Mary’s Academy, Des Plaines, Cook Shale, gray, dolomitic. Sandstone, fine grained, gray, dolomitic. Sandstone, like above with gray shale. No sample .. : . Sandstone, medium grained, gray, very dolomitic, glauconitic. with gray shale . Shale, gray, dolomitic ... Sandstone, fine grained, gray, dolomitic, glauconitic, shaly.... • • Shale, gray, dolomitic ..• •. Sandstone, very fine grained, gray, very dolomitic, hard. Sandstone, like above with gray shale. No sample . Shale, gray, dolomitic .• ... Sandstone, very fine grained, gray, very dolomitic, with some gray shale .. .. /;••••. . Sandstone, fine grained, gray, dolomitic, glauconitic, with some gray dolomitic shale . Cook County 1115 1 1116 24 1140 5 1145 5 1150 15 1165 10 1175 10 1185 15 1200 35 1235 5 1240 10 1250 1 1251 34 1285 65 1350 STRATIGRAPHY AND GEOLOGIC STRUCTURE OF NORTHERN ILLINOIS 33 (c) Partial log of well at St. Mary’s Academy, Des Plaines, Cook County (Concluded) Sandstone, medium to very fine grained, gray and light pink, very dolomitic, some hard layers, some greenish gray dolo- mitic shale, some glauconite. Sandstone, very fine grained, gray, very dolomitic and glauconitic Sandstone, fine to medium grained, gray and pink, dolomitic, hard, glauconitic . Sandstone, fine, gray, dolomitic, glauconitic .. Shale, greenish gray, dolomitic, sandy. Sandstone, fine to medium grained, gray and pink, dolomitic, hard... Sandstone, fine grained, gray, dolomitic .. No sample . Shale, gray, dolomitic . Sandstone, fine grained, gray, dolomitic. Total Eau Claire 375 feet 50 1400 20 1420 5 1425 5 1430 5 1435 15 1450 20 1470 5 1475 10 1485 5 1490 (d) Partial log of Ogden Armour well, Lake Forest, Lake County Thickness Depth Feet Feet 1360 Sandstone, fine grained, pink, domolitic; with shale, red, dolomitic 10 1370 Sandstone, fine grained, gray, dolomitic; with green dolomitic shale. 30 1400 Shale, sandy, pink to gray, slightly glauconitic, dolomitic. 90 1490 Sandstone, medium to fine grained, gray, dolomitic. 80 1570 Dolomite, mainly very sandy, gray; some glauconite. 30 1600 Sandstone, fine grained, gray, very dolomitic, layers of greenish gray dolomitic shale . 70 1670 Total Eau Claire 310 feet WATER SUPPLIES The wells in the Chicago and North Western Railway yards at Proviso described by Anderson 25 are the only ones known to the writer which are so cased that they derive all of their water from the Eau Claire formation. They yield water with less than one pound of encrusting solids per 1000 gallons but at the cost of the excessive draw-down of one foot per gallon per minute. Locomotive engineers complain that this water foams. The foaming is apparently due to the large content of alkalies. At Western Springs 20 water from the Dresbach and Eau Claire formations contains nearly 6 pounds of encrusting solids in 1000 gallons. It is probable that the quality of water in the Eau Claire varies greatly but, as a whole, the formation yields little water. LTnless it is desired to reach the underlying Mt. Simon sandstone it is inadvisable to drill into the Eau Claire farther than to form a settling basin for caved material from the overlying formations in the well. - * . ,_ 1 i f > j / ' 25 Anderson, C. B., The artesian waters of northeastern Illinois; Illinois State Geol. Survey Bull. 34, pp. 116-120, 236, 297-298, 1919. 26 Habermeyer, O. C., Public ground-water supplies in Illinois: Illinois State Water Survey Bull. 2i, p. 682, 1925. 34 STRATIGRAPHY AND GEOLOGIC STRUCTURE OF NORTHERN ILLINOIS DRILLING CONDITIONS The writer has not learned of any particular difficulties that are met in drilling- through the Eau Claire formation. The shales are firm enough in most localities to be left uncased. Some of the more dolomitic rocks drill very slowly. MT. SIMON SANDSTONE DESCRIPTION The Mt. Simon sandstone is fine- to very coarse-grained; its finer portions are mostly gray and some of the coarser beds are pink or red. It is probable that the amount of coarse red sandstone increases with depth below the top of the formation as is indicated by the logs in Table 13. There are some thin layers of red and gray non-dolomitic shale which appear to be erratic in distribution. The Mt. Simon sandstone has never been completely penetrated in Illinois; the farthest that it has been entered, so far as the writer has been able to learn, is 842 feet at Dixon. Table 13 .—JVell logs illustrating character of the Mt. Simon sandstone (a) Partial log of Chicago and North Western Railway well, West Chicago, Du Page County a Thickness Depth Feet Feet 1800 Sandstone, medium grained, white to light gray, pyritic. 10 1810 Sandstone, medium to fine grained, white to light gray and pink; dolomitic in part . 15 1825 Sandstone, coarse to fine grained, white to light gray, some pink 10 1835 Sandstone, coarse to very fine grained, white to light gray 15 1850 Sandstone, coarse to very fine grained, gray to yellowish gray, some pink layers . 30 1880 Sandstone, coarse to very fine grained, pink.• •.. 30 1910 Sandstone, coarse to very fine grained, gray to yellowish gray; with some pink layers . 60 1970 Sandstone, coarse to very fine grained, light pink...... . 50 2020 Sandstone, coarse to very fine grained, gray, yellow, and pink.. 62 2082 Mt. Simon penetrated 282 feet. aSamples examined by R. C. Lentz, inspector. (b) Partial log of city well No. 8, Rockford, Winnebago County Sandstone, fine to medium grained, white. Sandstone, fine to medium grained, gray, shaly. Sandstone, fine to medium grained, white. Sandstone, fine to very coarse grained, gray. Sandstone, fine to medium grained, gray. Sandstone, fine to very coarse grained, gray. Sandstone, very fine grained, gray. Sandstone, fine to very coarse grained, gray. Sandstone, fine to very coarse grained, pink. Sandstone, fine to very coarse grained, gray. Sandstone, fine to very coarse grained, pink. Sandstone, very fine to fine grained gray and yellow.... . Sandstone, very fine grained, dark pink.•.... Sandstone, fine to very coarse grained, angular grains, dark pink Mt. Simon penetrated 710 feet _ 280 790 1070 20 1090 30 1120 35 1155 5 1160 10 1170 20 1190 70 1260 10 1270 20 1290 10 1300 20 1320 90 1410 90 1500 STRATIGRAPHY AND GEOLOGIC STRUCTURE OF NORTHERN ILLINOIS 35 (c) Partial log of wells at Dixon Epileptic Colony, near Dixon, Lee County^ Sandstone medium grained, white, some glauconite and pyrite.. Sandstone, coarse grained, white, some pyrite. Sandstone, coarse to medium grained, gray, pyritic. Sandstone, medium to very coarse grained, white and gray. Sandstone, fine to very coarse grained, yellow to reddish brown Sandstone, fine to medium grained, white - -.. Sandstone, fine to medium grained, light pink and brownish yellow . Sandstone, medium to coarse grained, pink. Sandstone, coarse to exceedingly coarse grained, brownish red, yellow, pink, gray, subangular grains. Sandstone and shale, dark red. Sandstone, coarse to medium, pink . Sandstone, medium to fine grained, dark red. Sandstone, very coarse grained, red . Sandstone, coarse to medium grained, pink. 140 1080 1220 50 1270 50 1320 30 1350 230 1580 44 1624 38 1662 42 1704 76 1780 3 1783 42 1825 38 1863 8 1871 51 1922 Mt. Simon penetrated 842 feet ^Samples described by C. B. Anderson and T. E. Savage. WATER SUPPLIES The coarse layers of the Mt. Simon sandstone furnish large quanti¬ ties of water, sufficient in some localities to mask the character of the waters from higher formations. The very coarse layers are erratic in distribution and are interbedded with fine-grained sandstones which sup¬ ply little water. It follows that adjacent wells may find different con¬ ditions even at the same stratigraphic level. As the Mt. Simon is drawn upon by fewer wells than are the higher formations, the level of the water in it has not yet been lowered to as great an extent. Allowance must be made for the fact that the diameter of many old wells was so small that they either supply little water from the deeper formations or have long since caved or “bridged” at higher levels. For instance, it was noted that the old well of the Chicago and North Western Railway at West Chicago did not interfere with the new well about 25 feet away after the St. Peter had been cased off in the latter; this lack of inter¬ ference demonstrated that the old well had caved below the St. Peter. The waters of the Mt. Simon are in general more highly mineralized than are those from higher formations 27 if total solids alone are considered, but for the most part they contain less encrusting solids than do the shallower waters. The water from the Mt. Simon at Western Springs contains about 3 pounds of encrusting solids per 1000 gallons. Less than 2 pounds in the same quantity is reported at Galena. 2S The depth 27Anderson, C. B., The artesian waters of northeastern Illinois: Illinois State Geol. Survey Bull. 34, p. 102, 1919. 28Habermeyer, G. C., Public ground-water supplies in Illinois: Illinois State Water Survey Bull. 21, pp. 239, 683, 1925. 36 STRATIGRAPHY AND GEOLOGIC STRUCTURE, OF NORTHERN ILLINOIS at which salt water appears varies greatly from place to place but the fact has been given comparatively little attention as wells are now rarely drilled deep enough to strike such water. It appears probable that this condition is due to the lenticular character of the coarse layers so that flushing has extended to different depths in various places. The presence of wells that have artificially accelerated the circulation is also important. As a general rule salt water is to be feared in the Chicago district at any depth greater than 2000 feet although there are several successful wells of greater depths. Contrary to popular ideas the con¬ tact between fresh and salt waters is relatively abrupt. The best way to guard against drilling into salt water is to take samples of water from the bottom of the hole with either the bailer or the sand-pump. If a sam¬ ple runs more than 150 parts per million of chlorine, the well should be stopped. If the water on pumping is too high in salt, then a filling of cement should be placed to a level above the salt-water stratum. DRILLING CONDITIONS Except for local caving layers no particular drilling difficulties in the Mt. Simon sandstone have come to the attention of the writer. STRUCTURE Introduction Although generally thought of as horizontal, the strata of northern Illinois are inclined at angles which range from a few minutes to 50 degrees. Steep dips are found in: (1) the monoclinal fold which passes west of Streator through La Salle and flattens out toward the northwest (the west limb of the La Salle anticline) ; (2) the vicinity of Oregon; and (3) a small district near Des Plaines. The La Salle anticline itself, distinguished from its western slope, is the southward continuation of the broad Wisconsin arch. On it there are several small domes and its flanks are crenulated with minor synclines and anticlines whose axes lie approximately east and west. The position of the different rock formations in an area of compara¬ tively gentle dips is best shown by contours drawn on top of a certain easily identified formation. These contours represent what would be the surface of the land if all overlying formations of rock and soil were stripped off. Except as it controls the position of the outcrops of rock formations which are resistant to weathering and erosion, the structure is independent of the present configuration of the surface. In Illinois much of the present topography is the result of deposition of loose material by glaciers and not of the wearing away of the bed rocks; the northwestern part of the State, however, was not glaciated. It is diffi¬ cult for persons not experienced in geology to grasp the idea that (a) the same rock formation is not found at the same depth at all places, and STRATIGRAPHY AND GEOLOGIC STRUCTURE OF NORTHERN ILLINOIS 37 (b) undulations in the bed rock formations are not necessarily, or indeed commonly, reflected in the shape of hills and valleys. Plate I was con¬ structed on the basis of elevations of the top of the Dresbach sandstone obtained from the wells tabulated in Table 14. As the number of wells is insufficient to permit accurate location of the contours, a study of the interval between the top of the St. Peter and the top of the Dresbach was made. This varies from less than 300 feet to nearly 1000 feet; the increase is from northeast to southwest with only a few minor irregu¬ larities some of which may be due to erroneous well logs. The greatest uncertainty in the determination of this interval is in the western and southern parts of the map. With this interval as a basis, the observed elevations were supplemented by records of wells that go no deeper than the St. Peter and by the structure map prepared by Cady 29 to show the top of the formation. If more records of shallow wells in northern Illinois were available the map would be more satisfactory but this region has not been studied in recent years. Plate II is a section along the line of the Chicago and North West¬ ern Railway from Clinton, Iowa, to Chicago. The vertical scale is ex¬ aggerated about 70 times so that the inclination of the rock is shown at much too great an angle in order to make the scale of the well logs large enough to give the necessary information. Lines between wells simply connect similar formations and do not necessarily indicate the variations in the formations between the points of observation. No at¬ tempt has been made to show the surface either of the ground or of the bed rock. Structual Features Attention has already been called to the La Salle anticline which separates the northeastern region of relatively elevated strata from the deep basin to the southwest. On account of the increase in the St. Peter- Dresbach interval toward the southwest, this basin is much deeper than that in the top of the St. Peter. Little is known of the forms in the deeper strata of the comparatively abrupt domes of the region near Dixon and Oregon. Just north of these, along the Stephenson-Ogle county line, is a marked syncline which extends entirely across the State. Its exact shape is not known in the western part of the area and there may be no such enclosed basin as is shown around Freeport. The ab¬ normal course of Pecatonica River is probably related in part to this flexure. The syncline is best developed in McHenry and Lake counties. South of this syncline is an anticline which can be traced from the vicinity of Savannah, Carroll County, east through the domes at For- reston and Leaf River, Ogle County, to just north of Des Plaines, Cook 29Cady, G. IT., Structure of the Ua Salle anticline: Illinois State Geol. Survey Buil 36, pp. 85-179, 1920. Table 14. — Well data used in the construction of the structure map (PI. 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Q 43 " ~ = " a c * ■a ^ u c 43 X 43 s £4 ’43 X —■ o K O ego ^ 43< Ogle STRATIGRAPHY AND GEOLOGIC STRUCTURE OF NORTHERN ILLINOIS • • -+- + • 0 \ ft\ ON 0 0 0 0 O vo . vo vo O NO VO • to 00 .ft co OO oO CO ft- CM • vo 1—1 O 1—1 ft • LO VO 00 NO NO CO tV NO • CM co co NO tv + O ’ft On ON 00 LO CM . 00 O O vo O CM vo O tv On 00 OO NO ft- 0 VO co co NO O O 1—1 O VO + + + co NNNn r-H 00 VO co ift CM J_ + 1 + + 1—1 00 »—H 1 II II + 1 1 1 1 + O *0 -t O O O O s * • • • rt • • • • • • h • • • 1—1 • • • • Ph • • • • • • flC • • • ID • Ut • • • > * 4 -» iD u > • • • u • • • > * * • Ift • • • ctf • • • M d >s • • • • • • • • >s a • —^ to G (M ft u a • • cn c* • v Go -M 'U • -*-» __T' • fa h .t; 4 -> ft bo O O ft ^ T! .ti u O ^ u . >*£ CQ S at o X o o ft) G JO 'w u* o o « JO In i—i U o _ S3 £ ,+; y y j i ». _—• •- O hn r* ^ o be in 2 "fl bG g, u. o * «-G O CO aJ +-> CO c 3 -H> m yi Q G co CM io 0) ft > 3 g Mm r^sft 0) M o +-> • d •G +-> O to -1 tO r in H m CO o c 3'^ M Sg; g G >>M G OJ I > G G cd «>. ® . be H >G 3 £ G > CO tc G G § ^Ift G G -co G G O O " ^ (ft (ft £e 1-3 00 o| ,r 3 ^ 3pG H S P Sg .73° 0 d-ft gd « §<» O r .” .Wpqd G M 3 O' ^ . G O Hj _ to C c D 1) ft g ftG q be > g — G > 3*0 ft hJMPh^m MM rl M K tMD ; m I- o ID o 41 42 STRATIGRAPHY AND GEOLOGIC STRUCTURE OF NORTHERN ILLINOIS County, and which is an extension of Cady’s “Savanna-Sabula anticline”. At Des Plaines the south limb of this fold is apparently marked by a fault which farther east seems to pass into a monocline and to the south of which is a small basin. The evidence of faulting- is the broken con¬ dition of the rocks in the Des Plaines city well, determined both by ex¬ amination of the samples and from information given by the driller. Great difficulty was experienced in drilling this well and after several liners were side-tracked the Avell was cased to the bottom at 1600 feet. The cuttings from the sandstones were muddy and little water could be obtained. In addition, the Prairie du Chien dolomite appears to be absent so that the evidence favors the view that this well passed through a south-dipping normal fault. About a mile and a half to the east the well at the Crotian Orphanage encountered somewhat similar conditions and very little water was found at any depth. A well at the city hall was reported as finding “shale” to a depth of 1800 feet. 30 Where cuttings have been examined, however, it has been found that the forma¬ tions occur in the usual order, although much broken, and in the Orphan¬ age well with thicknesses about 30 per cent greater than the normal thickness for the vicinity. The latter fact suggests that in places the fault passes into a monocline with the strata dipping at a considerable angle. The St. Peter sandstone is 500 feet deeper at the Orphanage than at St. Mary's Academy, only a trifle more than two miles to the north. Whatever the true explanation of these phenomena may be, the writer advises that well drillers avoid an east-west belt through the southern part of Des Plaines for several miles in each direction. Farther south there is a marked syncline in northern Kendall County which strikes northwest-southeast. This is associated with an offshoot of the Wisconsin anticline which extends southeast from the Oregon dome through the St. Peter outcrops in northeastern La Salle County to Kankakee, and which was named the Kankakee anticline by Cadv. 31 Southwest of the Kankakee anticline a syncline passes through Grundy County and separates the anticline from the true La Salle anti¬ cline east of La Salle and Streator which has a more nearly north-south strike. Although it cannot be expected that the structure map will prove to be absolutely accurate at all points, it serves as a basis for correlating and evaluating the known data so that estimates of the depth to the top of the Dresbach sandstone can be made on a definite and concrete basis. 30 Habermeyer, G. C., Public ground-water supplies in Illinois: Illinois State Water Survey Bull. 21, p. 174, 1925. 3iCady, G. H., The structure of the La Salle anticline: Illinois State Geol. Survey Bull. 36. p. 133, 1920. STRATIGRAPHY AND GEOLOGIC STRUCTURE OF NORTHERN ILLINOIS 43 The intervals shown serve as a check, for when the top of the St. Peter is reached they allow of an estimate of the distance to the top of the Dresbach. Cause of Deformation Judging from the conditions around the partly exhumed pre-Cam¬ brian monadnocks of Wisconsin and modern theories of the origin of folds in the mid-continent oil fields, the structural features of northern Illinois are probably due to irregularities in the pre-Cambrian basement. Cady concluded that the La Salle anticline ‘‘is strongly suggestive of an initial line of weakness . . . possibly due to a concealed fault line which may be present in the deeply buried rocks . . .” 32 WATER QUALITY AND PROBLEMS General Statement It is impossible with present knowledge fully to discuss the quality of the waters in the several formations. It so happens that a number of persons who have been exploring for softer waters than those furnished by ordinary uncased wells are not at present willing to reveal all the in¬ formation that they have collected at considerable expense. Some of the methods of casing wells are regarded as trade secrets as they are not followed by all well drillers. Nevertheless, some facts are so well known that they can profitably be recapitulated. 33 Nature of Soft Waters The waters of northern Illinois generally increase in total solids with depth, but the character of the bases present varies greatly. Near the surface alkaline earths (calcium and magnesium) predominate but with depth alkalies (sodium and potassium) increase in amount. This in¬ crease is for the most part accompanied by larger amounts of chlorides and sulphates. Very deep waters are foaming, corrosive, and undrink¬ able but some intermediate waters contain only a moderate amount, less than one pound per 1,000 gallons, of encrusting solids. The waters also vary in amount of sulphate which when combined with calcium or mag¬ nesium makes the most objectionable form of scale in boilers. The mineral character of the waters is also related to the kinds of rocks from which they come and in general the best water comes from pure quartz sandstones. The cause of the decrease in alkaline earths with depth is not as yet definitely known. It is quite possible that the work of Renick 34 32lclem, p. 179. 33Anderson, C. B., The artesian waters of northeastern Illinois: Illinois State Geol. Survey Bull. 34, pp. 100-101, 116-120, 140, 142, 236; 297-298, 300, 1919. Habermeyer, G. C., Public ground-water supplies in Illinois: Ilinois State Water Survey Bull. 21, 1925. 34Renick, B. C., Base exchange, in ground water by silicates as illustrated in Montana: IJ. S. Geol. Survey Water-Supply Paper 520. pp. 53-72, 1924. 44 STRATIGRAPHY AND GEOLOGIC STRUCTURE OF NORTHERN ILLINOIS in Montana may prove to explain the phenomenon as one analogous to that alteration which takes place in a zeolite water-softener. Methods of Testing Quality of Water The ideal method of testing the quality of the water from different formations is to set casing down to the top of the stratum and then make a pumping test. The expense of such a procedure generally prohibits following it so that it is customary to take samples of water with a bailer or sand-pump. Some of the results thus obtained do not show the true character of the water in the formation which the bottom of the well has reached when the sample is taken. The principal causes of such fail¬ ure are: (a) The head of water is greater in the shallow formations than in the deep ones so that surface waters run down the well and force away the deep-seated waters, (b) The valve in the bottom of the bailer leaks, (c) The bailer is short and allows the mixture of waters at the top while hoisting, (d) The waters diffuse as a result of drilling opera¬ tions or of a long wait after drilling which allows circulation due to thermal differences. The writer has noted diffusion of waters when taking temperatures in a 12-inch hole. The sand-pump is preferable to the baler in that its top is closed although there is danger that a change in rate of lowering may cause it to open before the bottom of the run is reached. Experience has shown that tests of water obtained in this manner are reasonably reliable only when the water lever is constantly rising as the well is drilled deeper. Then they are checked by pumping tests within a reasonable margin of error. Such tests are worthless (a) when the water level falls as the well is deepened and (b) when the lower formations carry little water. In judging of the quality of water which will be obtained on completion of the well it must be realized that (a) bailer tests show nothing as to the yield of the different forma¬ tions, and (b) many wells yield better water after prolonged pumping as a result of the artificial acceleration of the flow of water which washes out the more soluble substances. The following deep wells are reported as yielding water with less than two pounds of encrusting solids per 1,000 gallons: Chicago, Milwaukee and St. Paul Ry., Bensenville. Chicago and North Western Ry., Proviso and West Chicago. St. Mary’s Seminary, Area (Mundelein). St. Mary’s Academy, Des Plaines. Ogden Armour, Lake Forest. Abott Laboratories, North Chicago. Waterworks, Batavia, Bensenville, Hanover, Morrison, Pawpaw, Riverside, St. Charles . 35 The Bensenville railroad wells are cased to the Dresbach, and the Proviso wells to the Eau Claire; the others either have a confidential 3 oHabermeyer, G. C., Public ground-water supplies in Illinois: Illinois State Water Survey Bull. 2i, pp. 57, 274, 436, 501, 551, 582, 1925. STRATIGRAPHY AND GEOLOGIC STRUCTURE OF NORTHERN ILLINOIS 45 casing log or obtain so much water from low formations that the lack of casing is not apparent. The yields from some of these wells is as much as 600 to 800 gallons per minute. It must be realized that the construc¬ tion of a soft water well near an uncased well is nearly impossible be¬ cause waters from several formations may pass through the latter and thus reach the cased well. Contamination of Deep Wells Flowing wells are safe, even if not cased through contaminated upper formations, as above the artesian stratum the flow of water is out¬ ward from the well. In the Chicago district the deeper waters now have a head far below that of the shallow waters. In such circumstances casing with a good shut-off at the bottom is essential to a safe water supply. Two wells are known of where very bad water was obtained at depths greater than 1,000 feet. The tests in each well showed a good shut-off below the Maquoketa shale, but during drilling bad smelling water was observed to come from crevices in the Prairie du Chein for¬ mation. It has been suggested that sewage is being emptied into some abandoned well which is possibly many miles from the locations of these contaminated wells. In one well a packer set in the Mazomanie sand¬ stone removed the trouble, but no information has been received as to the other well although it has been worked on for several years. Well Construction No exhaustive discussion of methods of well construction can be attempted here. 36 Engineers are urged to devote careful attention to the quality of the casing and to the shut-off at the bottom of each string of pipe. It is not enough simply to drop pipe into a hole either with or without casing shoe. The character of the rock in which the shoulder is made must be considered, and brittle, fractured formations avoided. Large crevices are most common near abrupt changes in the character of the rock. It must be realized that holes of different sizes are rarely concentric and that lead seals between different sizes of pipe make joints of indifferent quality. It is far better to follow oil well practice and have the last string of pipe extend to the surface. Shut-offs in water wells may be tested by (a) placing a dye (fluorescein) in the water 3fiMore information on well drilling methods can be obtained from: Bowman, Isaiah, Well-drilling methods: U. S. Geol. Survey Water-Supply Paper 257, 1911. Day, JD. H.. Handbook of the petroleum industry, New York, 1922. Jeffry, W. H., Deep well drilling, Toledo, 1921. Sanderson, R. R , Drill work, methods and costs, Cyclone Drill Co., 1911. Thompson, A. B., Oil field development, pp. 307-409, New York, 1916 Uren, L. C., Textbook of petroleum production engineering, pp. 89-143, 201-301, New York, 1924. Woodworth, R. B., The evolution of drilling rigs: Amer. Inst. Min. Eng. Trans., vol. 54, pp. 210-268, 1915. Ziegler, Victor, Oil well drilling methods, New York, 1923. 46 STRATIGRAPHY AND GEOLOGIC STRUCTURE OF NORTHERN ILLINOIS between the different strings of pipe, and (b) observing the water levels inside and outside a string of pipe when the level is changed in the inner pipe by pumping or bailing. An excellent shut-off may be made by cementing the bottom of each string of pipe. This is best done before the hole is drilled any deeper but many drillers prefer to leave casing until all drilling has been completed for fear that in a crooked hole the wire line will wear through the pipe. It is advisable to use cement thick enough to protect the well from contaminated waters even without any pipe; this should extend from the surface through all contaminated waters. At lower levels many prefer clay instead of cement since that does not prevent the withdrawal of rusted pipe. Cementing of well casing is now standard practice in oil wells and its adoption in water wells is worth careful consideration. 37 Although the use of “genuine wrought iron” pipe is far preferable to that of the mild steel known to the trade as “wrought iron,” it is well to remember that the best pipe will rust out in time since the waters from great depths, with the relief of pressure on being brought to the surface, evolve much carbon dioxide. In wells pumped directly into the mains this evolution of gas often causes foamy water as well as the precipitation of calcium carbon¬ ate. Where the air lift is employed, these processes take place in the tank and some of the dissolved iron is also oxidized and settles out. The freeing of gas is more pronounced in wells which are cased to great depths since all of the water comes from formations where the gas was under high pressure. Some cases of precipitation may be due to mingling of waters from different formations. Engineers should remember that although the methods employed in well drilling and construction appear very simple, their practical application finds every well a different en¬ gineering problem. Although the geological formations may be the same, the number and position of fractures, and variations in hardness and in porosity call for different procedure. In judging costs it should be realized that drilling in holes full of water is necessarily much slower than in oil wells, where water is excluded as far as possible, because the friction on the cable and tools lessens the force of the blows. Another point is that the efficiency of drilling depends to a great extent on the quality of the tool dressing; a poor tool dresser can slow down the work to a remarkable extent. A rule of primary importance is that the hole should be kept large enough to allow for unforeseen caves which entail reduction after the insertion of liners. Many engineers insist on butted joints in drive pipe, but most drillers prefer ordinary joints which may be tightened from time to time by screwing up the pipe. It is said that 37 Tough, F. B., Methods of shutting off water in oil and gas wells: U. S. Bur. Mines ^ jj ^.63 1918 U Kirchoff'er, W. G., Grouting wells in rock formation effective and simple: Engr. News-Record, vol. 81, p. 367, 1918. STRATIGRAPHY AND GEOLOGIC STRUCTURE OF NORTHERN ILLINOIS 47 if drilling is properly done, very hard driving that might injure the threads can be avoided. Liners in the rock should not be driven ; the need to drive them indicates a crooked hole. Attention should be drawn to the fact that crooked holes are more common in large than in small diameters since in the former there is more room for the drill stem and jars to swing to one side of the hole. As a general thing engineers should be slow to criticise the methods of experienced drillers for such methods are the result of years of practice rather than of theory. Well drilling is an art that demands experience and ingenuity; it is an opera¬ tion in which, many times, the personal factor is more important than the quality of the machinery. The lowest bidder is often the most ex¬ pensive in the long run. Future of Underground Waters Many wells have been abandoned in Illinois and elsewhere because it was assumed that the supply of water had been exhausted. Study of old wells shows definitely that for the most part they were of such small diameter that they soon bridged or filled with cavings. In some com¬ munities, when more water was required new wells were drilled so close to the older ones that the available supply was simply divided. It is absolutely essential that wells be spaced over considerable areas. In Chicago the water levels have been greatly lowered as has been shown by Anderson. 38 The condition appears to be local, although it was noted that at West Chicago, 30 miles away, the water in the Dresbach is now lower than the level of Lake Michigan. The reduction in water pressure has probably resulted in settling and compacting of the sand¬ stones so that the porosity of the rocks has been permanently lessened. The situation in the Stock Yards district is serious and from the stand¬ point of conservation it seems a pity that such vast quantities of pure water are used for washing, cooling, boilers, and other industrial pur¬ poses for which lake water would serve as well. It is probable that in smaller cities sufficient water for a public supply will always be avail- abe. To this end the use of underground waters for industries that do not require pure water should be discouraged in the future, at least where surface waters are obtainable at reasonable cost. Before concluding that public supplies from wells must be aban¬ doned, engineers should look to see if the shortage cannot be relieved by repair of the wells. Methods of increasing the supply from old wells comprise recasing, cleaning, reaming, shooting, and deepening. Many drillers state that reaming is in most cases more expensive than a new well and that shooting, although an undoubted benefit if crevices are opened in the more brittle rocks, often causes permanent caving condi- 38Anderson, C. B., The artesian waters of northeastern Illinois: Illinois State Geol. Survey Bull. 34, pp. 93-95, 1919. 48 STRATIGRAPHY AND GEOLOGIC STRUCTURE OF NORTHERN ILLINOIS tions which soon offset the increase of yield. The writer therfore hesi¬ tates to recommend shooting- as a certain means of increasing well yields. 39 Well Logs The writer wishes to emphasize the desirability of obtaining accu¬ rate well logs based both on drillers’ records and on the study of cut¬ tings from every 5-foot screw. Bags for the collection of samples are furnished free by the State Geological Survey at Urbana. Records based on examination of samples by an experienced geologist furnish much more detailed data than can be obtained from the drillers’ log alone, but the latter should not be neglected, for in many cases the action of the tools may tell as much concerning the underground conditions as do the samples. It is a common fallacy among both geologists and en¬ gineers to think that samples from churn drill holes are worthless on account of caving. Such is not the case, for if caving equaled the amount of new hole little progress would be possible. Although it is true that caved material is sometimes troublesome in the study of cut¬ tings, it can generally be distinguished by the large size and unworn character of the fragments and its presence can always be noted by an experienced driller. The principal source of error is the tendency of drillers to take samples from the slush pit when going off shift instead of direct from the bailer. Records should include a casing log which shows the length and position of each string of pipe as well as the sizes of pipe and of hole. Whenever repair work or new wells are needed such detailed records are of immense value. Attention should also be given to the fact that study of underground temperatures is a guide to the relative amounts of water from different producing formations. Many temperatures of waters were collected by Anderson and Habermeyer 40 which, when compared with those observed by R. C. Lentz 41 at West Chicago, seem to indicate that much water is derived by uncased wells from the formations above the Dresbach sandstone. The State Water Survey Division has many records of well yields and analyses of water samples which are available to persons who con¬ template installing wells. For municipalities, mineral analysis of waters will be made free of charge and assistance will be given during tests of yields whenever possible. SfNorton, W. LI., Underground ■water resources of Iowa: U. S. Geol. Survey Water- Supply Paper, 293, p. 131, 1912. Kirchoffer, W. G., Increasing the capacity of ground water supplies: Amer, Water Works Assoc. Jour., vol. 15, pp. 144-151, 1926. 40Anderson, C. EL, The artesian waters of northeastern Illinois: Illinois State Geol. Survey Bull. 34, pp. 42-50, 1919. Habermeyer, G. C., Public ground-water supplies in Illinois: Illinois State Water Survey Bull. 21, 1925. 43Temperature at 1650 62° F., at 20S2 75° F. Temperature in well at Madison, Wis., at 840 55° F. STRATIGRAPHY AND GEOLOGIC STRUCTURE OF NORTHERN ILLINOIS 49 In conclusion the writer wishes to urge that engineers and well owners cooperate with the State Geological Survey and the State Water Survey in collecting accurate data regarding underground waters. There can be little serious question but that a properly constructed well is a nearly fool-proof source of pure water with which no man-made filter can compare in efficiency and safety. f r ■ I ■ PLATE ONE Illinois State Geological Survey Report of Investigations No. 13, Plate I Galena Waukoqan .ibortyvillo Rondout SO • < Rocklord 10 1 Hanover Mundolom; (Aron) Bolvidero LEGEND r 2 i Savanna Hampshire Ii Ml. Carroll .Wlnnolka ML Moms Contour on lop of Drosbaoh -jondslono Oregon 1 *^ Inlorval bolwoon lop of Si. Poler sondslono and lop of Drosbach sand9lono Bonsunvlllnl/ I ' * ' " SCALE •l Rochelle ihuflosi iHICAGO Lombard' West Cliicago Bulnvlu '4 Morrison Sterling [ 20 j)ltfporvillulO •Amboy Pawpaw r -i 4101 •Plano Lookport t|Oi rJi/Chioiwo Height* Geneseo ( 2 i Otlawa Kewaneo Structure map of northern Illinois. See table 14, pp. 36-39, for data used in the construction of the map. & IF ' \ ■ \ ■ \ - ; : V \ \ ^ «* u I* ^3 fo • l \ ^ \ ' gj 03 -—;-ix l- j—- i- 1 - - -.— — •»* 1 3 - -■ ■ . -t -J tL —— m *• . \ \J \ KP lh [mi <* I is is t J \ . V I \ ! \ K* #0 http? 3 Illinois State Geological Survey Report of Investigations No. 13, Plate II UNIVERSfTY OF illinois-urbana 30112 121958992