s 
 
 14.GS: 
 CIR230 
 c. 1 
 
 STATE OF ILLINOIS 
 
 WILLIAM G. STRATTON, Governor 
 
 DEPARTMENT OF REGISTRATION AND EDUCATION 
 
 VERA M. BINKS, Director 
 
 SUBSURFACE DOLOMITE AND 
 LIMESTONE RESOURCES OF 
 GRUNDY AND KENDALL 
 COUNTIES 
 
 Meredith E. Ostrom 
 
 DIVISION OF THE 
 
 ILLINOIS STATE GEOLOGICAL SURVEY 
 
 JOHN C. FRYE, Chief URBANA 
 
 CIRCULAR 230 
 
 1957 
 
Digitized by the Internet Archive 
 
 in 2012 with funding from 
 
 University of Illinois Urbana-Champaign 
 
 http://archive.org/details/subsurfacedolomi230ostr 
 
SUBSURFACE DOLOMITE AND LIMESTONE RESOURCES OF 
 GRUNDY AND KENDALL COUNTIES 
 
 Meredith E. Ostrom 
 
 ABSTRACT 
 
 This report, covering Grundy and Kendall counties in north- 
 eastern Illinois, was prepared in response to increasing interest 
 in shallow dolomite and limestone resources in counties neigh- 
 boring the Chicago area. The resources, especially those not ex- 
 posed at the surface, are indicated on maps (based on well data 
 and outcrops) that show the distribution of the bedrock forma- 
 tions, depth to bedrock, depth to dolomite or dolomitic limestone, 
 and thickness. Dolomite and dolomitic limestones are the domi- 
 nant carbonate rocks. Seven dolomite or limestone formations, 
 ranging up to 379 feet thick, contain significant thicknesses of 
 carbonate rocks. Several areas are underlain by such rocks at 
 shallow depths. 
 
 The available well data regarding the rocks do not permit 
 accurate determinations of the physical and chemical character- 
 istics which might prescribe their potential uses. However, de- 
 posits of stone which might be suitable for road material and ag- 
 ricultural limestone are available. Less common, but within the 
 limits of possibility, are deposits of stone suitable for concrete 
 aggregate and railroad ballast. High-purity dolomite may be 
 present locally. 
 
 The weathered character of many deposits and the possibil- 
 ity of groundwater in all of the dolomite and dolomitic limestone 
 formations are problems that affect their potential uses. Test 
 drilling is requisite for a thorough evaluation of the resources. 
 
 INTRODUCTION 
 
 The dolomite and limestone resources of Grundy and Kendall counties 
 have become of increasing interest because of their proximity to Chicago. 
 In general the counties do not have pronounced topographic relief, and as a 
 result bedrock outcrops are relatively few and not of great thickness. Most 
 of the outcrops are known, having been described in other, reports (Worthen, 
 1870; Krey and Lamar, 1925; Culver, 1922; Lamar and Willman, 1931, 1933; 
 and Lamar, et al., 1934). 
 
 Some of the outcrops occurring along the Fox River in northeastern 
 Kendall County are in residential or industrial areas or in areas that are 
 potentially valuable for town sites or industries. This is also true regarding 
 outcrops in some other places. There are large nonresidential and nonindus- 
 trial areas, however, where dolomite or limestone is relatively shallow, al- 
 though unexposed or but poorly visible. 
 
 [3] 
 
4 ILLINOIS STATE GEOLOGICAL SURVEY 
 
 It is toward such shallow occurrences that primary attention is di- 
 rected here. To this end information is given regarding depth to limestone 
 or dolomite in both counties, even though the rocks may be considerably 
 deeper than is now regarded as within the range of depth of economical 
 quarrying or underground mining. Inasmuch as the growth of northeastern 
 Illinois or technological developments may change the depth to which quar- 
 rying is feasible, the information will serve to indicate those areas of pos- 
 sible present interest as well as those which may have future possibilities. 
 
 The data available from the well records involved in the study do not 
 provide detailed chemical or physical analytical data on which to base 
 specific recommendations regarding uses for the dolomites or limestones 
 encountered. However, certain inferences are made on the basis of the gen- 
 eral geological character of the various formations and the uses for some 
 of them in other areas where they have been quarried. 
 
 It has not been feasible to include the detailed records of the numer- 
 ous wells involved in this study, however, they are available for examina- 
 tion at the offices of the Survey in Urbana. 
 
 The assistance of J. E. Lamar, Robert E. Bergstrom, James E. 
 Hackett, H. B. Willman, T. C. Buschbach and George E. Ekblaw, of the 
 Geological Survey, and Robert Sasman of the State Water Survey, in connec- 
 tion with certain phases of this investigation is gratefully acknowledged. 
 
 PRESENT QUARRYING INDUSTRY 
 
 Most of the dolomite and limestone formations which crop out in 
 Grundy and Kendall counties have been worked in a comparatively small 
 way in the past. At present the only active quarry is that of the Central 
 Limestone Company located at Central in the south-central part of Kendall 
 County where about 45 feet of Galena-Platteville formation is being quar- 
 ried. The rock is largely medium-grained gray limestone, parts of which 
 are mottled with irregularly shaped bodies of light brownish dolomite up to 
 8 inches in size. The stone is sold for road surfacing, agricultural lime- 
 stone, and metallurgical flux. 
 
 SOURCE AND CHARACTER OF DATA 
 
 The depth to bedrock, depth to dolomite or limestone, and the thick- 
 ness of dolomite or limestone penetrated are shown on maps (figs. 1-6). 
 The maps are based principally on well records supplied by well drillers 
 and to a lesser extent on samples saved during drilling and subsequently 
 studied by Survey geologists. Because the records have been supplied by 
 a number of drillers, there is probably a variation in the terms used to 
 describe the formations penetrated and in the accuracy with which the well 
 records were kept. As a result, the accuracy and uniformity of the maps 
 vary. They do, however, present the currently available data regarding 
 dolomite and limestone resources and are believed to give a reasonably 
 good idea of the distribution, depth, and general character of the dolomites 
 and limestones. 
 
DOLOMITE AND LIMESTONE RESOURCES 5 
 
 ROCK FORMATIONS 
 
 The bedrock in Kendall and Grundy counties consists of rocks of Ordo- 
 vician, Silurian, and Pennsylvanian age which are overlain by unconsolidated 
 glacial deposits of clay, silt, sand, and gravel in varying proportions, from 
 to more than 250 feet thick. 
 
 The bedrock units which crop out or are penetrated in drilling are com- 
 posed of the rock units listed in table 1. 
 
 GENERAL DISTRIBUTION OF ROCK FORMATIONS 
 
 The following series of maps show the distribution and depth of the 
 various bedrock formations in Grundy and Kendall counties. 
 
 Figure 1 is a generalized geological map showing the probable distri- 
 bution of the various bedrock formations as it would appear if all the uncon- 
 solidated materials were removed from above the bedrock. The map shows 
 that the southern three -fourths of Grundy County is underlain by Pennsylvan- 
 ian formations. These formations offer little possibility for commercial 
 limestone deposits because they contain only a few limestones, and such 
 strata as are present are thin (table 1). The lower and upper Maquoketa 
 shales underlie a considerable part of north-central Kendall County, and the 
 St. Peter sandstone is the bedrock in the southwestern corner of Kendall 
 County. Elsewhere the thickest dolomite and limestone formation of wide- 
 spread distribution is the Galena -Platteville formation. The Divine dolomite 
 and limestone formation also underlies a considerable area. 
 
 DEPTH TO DOLOMITE AND LIMESTONE 
 
 The approximate depth to the first dolomite or limestone encountered 
 in wells in Grundy and Kendall counties is shown in an over -all view in fig- 
 ure 2. The shallowest dolomite or limestone occurs in southern Kendall and 
 northeast and north-central Grundy County with other shallow areas in north- 
 east and northwest Kendall County. By comparing figures 1 and 2, the iden- 
 tity of shallowbedrock dolomite and limestone formations may be determined. 
 
 DEPTH TO DOLOMITE OR LIMESTONE, 
 THICKNESS OF DOLOMITE OR LIMESTONE, 
 DEPTH TO BEDROCK 
 
 The depth and thickness of the first dolomites and limestones encoun- 
 tered in Grundy County wells is shown in figure 3. Outcrops also are indi- 
 cated. At comparatively shallow depths deposits of dolomite and limestone 
 formations of moderate thickness occur in the northeastern part of the 
 county. In the north -central part, moderately thick to thick dolomite or lime- 
 stone formations occur at shallow depths. Figure 4 indicates the depth to 
 bedrock in wells encountering dolomite or limestone in Grundy County. It 
 indicates that certain of the shallow deposits shown in figure 3 have no bed- 
 rock overburden or only very thin overburden. Their overburden is likely 
 to be clay or silt and possibly sand and gravel at some places. 
 
ILLINOIS STATE GEOLOGICAL SURVEY 
 
 l*ML.*3 Lower Moquoketo 
 
 sG^pn! Goleno - Plotleville 
 
 HaFI st Pe,er 
 
 K;s"kv| Shohopee 
 
 tj»jT3 New Richmond 
 
 ?0N?1 Oneolo 
 
 Fig 
 
 ig. 1. - Distribution of bedrock formations in Kendall and Grundy counties 
 
DOLOMITE AND LIMESTONE RESOURCES 
 
 R 6 E 7 R8E 
 
 '"""///'■* Dole 
 
 W/M /''''•'•'•\ Lil 
 
 ////////'"SI 
 
 'SA'/AV/S/A 
 
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 t/s/sssssfSA Depth to 
 
 Dolomite or 
 .tmestone 
 
 ( feet ) 
 
 50-100 
 
 □ 
 
 Scole of miles 
 Q 2 
 
 00*0 m Grundy County south of this 
 [ line are too sparse for contouring \ 
 
 Fig. 2. - Approximate depth to dolomite or limestone bedrock 
 
ILLINOIS STATE GEOLOGICAL SURVEY 
 
DOLOMITE AND LIMESTONE RESOURCES 
 
 
 
 
 
 
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 ILLINOIS STATE GEOLOGICAL SURVEY 
 
DOLOMITE AND LIMESTONE RESOURCES 
 
 11 
 
12 
 
 ILLINOIS STATE GEOLOGICAL SURVEY 
 
 Kind of 
 
 Ma: 
 
 ximum 
 
 rock 
 
 thic 
 
 :kness, ft, 
 
 Clay, silt, sand, 
 
 
 
 gravel 
 
 
 250 
 
 Shale, sandstone, 
 
 
 
 clay, coal and 
 
 
 230 
 
 thin limestone 
 
 
 
 Dolomite 
 
 
 28 
 
 Dolomite or 
 
 
 
 Table 1. - Formations Cropping Out Or 
 Encountered in Wells 
 
 Geologic name 
 of rock unit 
 
 Pleistocene* 
 
 Pennsylvanian' 
 
 Niagaran 
 
 Alexandrian* 
 
 Geologic 
 system 
 
 Pleistocene 
 
 Pennsylvanian 
 
 Silurian 
 
 
 
 Maquoketa 
 
 Ordovician 
 
 Shale 
 
 50 
 
 Upper* 
 
 
 Dolomite & Limestone 
 
 70 
 
 Divine* 
 
 
 Shale 
 
 115 
 
 Lower* 
 
 
 Dolomite or 
 
 
 
 
 dolomitic limestone 
 
 379 
 
 Galena-Platteville* 
 
 
 Sandstone, shale and 
 
 thin dolomite beds 
 
 89 
 
 Glenwood 
 
 Sandstone 
 
 335 
 
 St. Peter* 
 
 Dolomite, mostly cherty 
 
 80 
 
 Shakopee* 
 
 Sandstone 
 
 77 
 
 New Richmond 
 
 Dolomite 
 
 215 
 
 One ota* 
 
 Dolomite, sandstone, 
 
 
 Gunter (Ordovi 
 
 and shale 
 
 20+ 
 
 and Jorda 
 
 Dolomite 
 
 240 
 
 Trempealeau 
 
 Cambrian 
 
 * Indicates outcrops of this formation occur in Grundy or Kendall counties. 
 
DOLOMITE AND LIMESTONE RESOURCES 13 
 
 Figure 5 gives details regarding the depth and thickness of the first 
 dolomites and limestones penetrated by wells in Kendall County. The great- 
 est concentration of wells encountering shallow dolomite or limestone is in 
 the southern part of the county. In the central part shallow stone is uncom- 
 mon, but such rock is again present at scattered points in the northern part 
 of the county. Outcrops are most abundant in the northern part of the county. 
 
 The depth to bedrock, regardless of its kind, in wells reporting more 
 than 5 feet of dolomite or limestone is indicated in figure 6. It is apparent 
 that in the areas of shallow dolomite and limestone shown in figure 5, the 
 overburden on the bedrock is largely unconsolidated materials. There is 
 likely to be clay and silt, and possibly sand and gravel, in some places. 
 
 RELATION BETWEEN MAPS SHOWING 
 
 "DEPTH TO BEDROCK" AND 
 
 "DEPTH TO DOLOMITE AND LIMESTONE" 
 
 It is possible that, if underground mining of dolomite or limestone in 
 Grundy or Kendall counties should be feasible at some future time, it would 
 be desirable to work a dolomite or limestone formation which has another 
 type of bedrock above it, either because the particular dolomite or limestone 
 formation is the most desirable for the intended purposes, in order to exclude 
 an influx of water from overlying unconsolidated earth materials, or for some 
 other reason. 
 
 In this connection, by comparing the data in figure 3 with those of fig- 
 ure 4 (or the data in figure 5 with those of figure 6) it is possible to determine 
 whether a given dolomite or limestone formation at any particular place is 
 overlain by another bedrock formation, or by only unconsolidated glacial 
 materials. If the depth to bedrock and the depth to dolomite and limestone 
 are the same, the dolomite or limestone has no bedrock overburden. How- 
 ever, if the depth to dolomite or limestone is greater than the depth to bed- 
 rock, the difference between the two indicates the thickness of bedrock over- 
 burden on the dolomite or limestone. The nature of this overburden may be 
 determined by consulting the geological map (fig. 1) and the geologic column 
 in table 1, which gives some idea of the character of the various bedrock 
 formations. 
 
 CHARACTER AND ECONOMIC POSSIBILITIES 
 OF DOLOMITE AND LIMESTONE FORMATIONS 
 
 The dolomite or limestone formations which underlie unconsolidated 
 earth materials, those which are the first dolomite or limestone formations 
 encountered in the bedrock, or those which are dolomites or limestones that 
 are encountered within 200 feet of the ground surface in Kendall and Grundy 
 counties, are described below beginning with the geologically oldest forma- 
 tion. The descriptions are based on information obtained from well records 
 and to a lesser extent from outcrops. Data on chemical composition of the 
 rock and suggested uses are taken or inferred from the same sources. The 
 data should be confirmed by test drilling or other means before attempting 
 commercial exploitation of deposits. 
 
14 
 
 ILLINOIS STATE GEOLOGICAL SURVEY 
 
 
 
 
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DOLOMITE AND LIMESTONE RESOURCES 
 
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 KEY 
 
 Depth to Dolomite or Limestone 
 • 0- 50 feet 
 
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 A Outcrop 
 
 *X Quarry, active or abandoned 
 
 Depth to dolomite or limestone 
 
 36 
 39 
 
 ndicates 
 
 Thickness of dolomite or limestone 
 Uncertain doto 
 
 indicates bottom of dolomite or limestone 
 was reached ond passed 
 
 stone in wells and outcrops in Kendall County 
 
16 ILLINOIS STATE GEOLOGICAL SURVEY 
 
 Trempealeau Formation 
 
 The Trempealeau dolomite was penetrated at a depth of 192 feet in west- 
 central Kendall County. It is light buff to brown, very fine- to coarse-grained, 
 and slightly clayey. Parts of the formation are cherty and sandstone strata 
 are present in some places. Some of the dolomite contains small quartz-lined 
 cavities. The formation is about 125 feet thick. It is reported to contain mas- 
 sive deposits of reef type of dolomite up to 10 feet thick in other parts of north- 
 ern Illinois. Its most shallow reported occurrence in Kendall County is at a 
 depth of 192 feet in the NW 1/4 NW 1/4 NW 1/4 sec. 5, T. 36 N., R. 6 E. It is 
 not reported directly below glacial material in any well records and it does 
 not crop out. 
 
 Some beds of the Trempealeau dolomite might be suitable for use as 
 agricultural limestone or road material. The presence of chert, beds of sand- 
 stone, and cavities lined with quartz, limit the use possibilities of the stone. 
 
 Gunter and Jordan Formations 
 
 The Gunter and Jordan formations are not distinguishable in Kendall 
 County (Workman and Bell, 1949). They consist of thin-bedded silty and sandy 
 dolomite, and clayey fine- and coarse-grained sandstone, both of which contain 
 chert. A maximum reported thickness of 20 feet occurs at a depth of 300 feet 
 in the SW 1/4 NW 1/4 NW 1/4 sec. 5, T. 36 N., R. 6 E. in Kendall County. 
 They are not exposed nor are they encountered directly below unconsolidated 
 materials in any wells. The Gunter and Jordan formations are not believed 
 to have economic possibilities. 
 
 One ota Formation 
 
 The Oneota formation is predominantly a light gray to bluish gray, 
 coarsely crystalline, dense, mostly cherty dolomite. However, studies of 
 samples from two wells located in west-central Kendall County, one of which 
 penetrated the lower 50 feet of the formation and another which goes through 
 123 feet and on into the underlying formation, do not mention the occurrence 
 of chert in the Oneota. It is about 215 feet thick and occurs in massive beds 
 up to 15 feet thick. The lower one -eighth to one -fourth of the Oneota is ordi- 
 narily clayey and somewhat sandy (Willman and Templeton, 1952). 
 
 The Oneota dolomite crops out at three places about one mile northwest 
 of Millbrook in west-central Kendall County (Willman and Payne, 1943; Will- 
 man and Templeton, 1952), and has been encountered directly beneath the 
 glacial drift in one well which is located in sec. 5, T. 36 N., R. 6 E. The 
 greatest thickness of Oneota exposed is located in the SE 1/4 NW 1/4 SE 1/4 
 sec. 8, T. 36 N., R. 6 E., where 12 feet crops out in a stream channel at a 
 breached earth dam. 
 
 The widespread occurrence of considerable amounts of chert through- 
 out the Oneota formation restricts its use. However, there seems to be a 
 reasonable possibility that thicknesses up to 123 feet, particularly near the 
 base of the Oneota, might be essentially chert-free and, therefore, might 
 supply a suitable stone for a variety of purposes. If chert-free beds are 
 
DOLOMITE AND LIMESTONE RESOURCES 17 
 
 present they might be suitable for agricultural limestone, or road material 
 and other purposes; conceivably some strata may be high-purity dolomite. 
 
 Shakopee Formation 
 
 The Shakopee formation consists of buff to light brown and light gray, 
 fine- to medium-grained dolomite, most of which is clayey, silty, and shaly. 
 It is reported to be about 165 feet thick and is restricted to a narrow area 
 adjacent to the western margin of Kendall County. It is not encountered in 
 wells, but is known from several abandoned quarries north of Millington 
 along the Fox River (Willman and Templeton, 1952). 
 
 The maximum thickness exposed is in an abandoned quarry about 11/2 
 miles northeast of Millington, on the west side of the Fox River, near the 
 center of the NE 1/4 sec. 19, T. 36 N., R. 6 E. where 25 feet of sandy and 
 clayey dolomite can be seen. 
 
 Commonly the Shakopee formation consists of irregular, alternating, 
 varicolored units of dolomite, shale, siltstone, and sandstone. In addition 
 the dolomite varies in purity. The major uses for the purer dolomite beds 
 appear likely to be road stone and possibly agricultural limestone. 
 
 Galena -Platteville Formation 
 
 The Galena -Platteville formation consists of dolomite and dolomitic 
 limestone. Its lower part, having a maximum thickness of about 150 feet, is 
 brownish gray, finely crystalline, compact dolomite and .fossiliferous, brown 
 and gray, fine- to very fine-grained dolomitic limestone. At various horizons 
 in this lower portion, but particularly in its upper part, there are lenses, 
 nodules, and beds of chert. The lower part of the formation can be distin- 
 quished from the upper part of the formation by the former's usually mottled 
 brown and gray color and the general absence of shaly partings. 
 
 The upper part of the Galena-Platteville formation has a maximum 
 thickness of about 220 feet and consists of brown to buff, medium to coarsely 
 crystalline, partly cherty dolomite or dolomitic limestone. The dolomite 
 usually has small cavities, many of which are lined with calcite crystals. 
 The coarser grain size and greater porosity of the upper part of the Galena- 
 Platteville formation distinguishes it from the lower part. Chert is most 
 common in the lower one -half or one -third of the upper part of the formation 
 although it often occurs in variable amounts throughout this upper section 
 (Willman, 1943). 
 
 The Galena-Platteville formation occurs as a wide belt that extends in 
 a southeasterly direction from the northwest corner of Kendall County to the 
 vicinity of Morris, in Grundy County, south of which it is covered by Pennsyl- 
 vanian rocks. The formation crops out at several localities in northwestern 
 and south-central Kendall County and in northern Grundy County (Culver, 
 1922); it is the source of stone for the only active quarry in the two counties. 
 In outcrop the rock is commonly brownish gray to gray and occurs in beds 
 from one to ten inches thick. 
 
18 
 
 ILLINOIS STATE GEOLOGICAL SURVEY 
 
 Fig. 6. - Depth to bedrock in wells encounter- 
 
DOLOMITE AND LIMESTONE RESOURCES 
 
 19 
 
 KEY 
 
 Depth lo Dolomite or Limestone 
 • 0- 50 feet 
 
 a More than 50 feet, up to 100 feet 
 o More than 100 feet 
 
 Ogo Indicates depth to bedrock 
 
 * Outcrop 
 
 tf Quarry, active or abondoned 
 
 ' Uncertain data 
 
 ing dolomite or limestone in Kendall County 
 
20 ILLINOIS STATE GEOLOGICAL SURVEY 
 
 As indicated in table 1, the maximum thickness of the Galena -Platteville 
 formation encountered in wells is 379 feet. The thickness of the formation 
 varies from this maximum down to zero as a result of erosion of the forma- 
 tion by glaciers and streams. 
 
 The chemical composition of the Galena -Platteville dolomite varies from 
 relatively high purity to impure. The same statement applies to the dolomitic 
 limestone which occurs in the formation. Judging from the well log descrip- 
 tions of some of the dolomite beds in the upper part of the formation and the 
 character of the formation where it crops out in other parts of the State, strata 
 of high-purity dolomite may be present. In the lower part of the formation con- 
 siderable thicknesses of dolomitic limestone of high purity may be present 
 locally. Usually the amount of MgO present is around 5 percent or more, but 
 in places it is possible that limited thicknesses of stone containing less than 
 5 percent MgO may be present. Such stone might furnish raw material for 
 the making of Portland cement. 
 
 Certain parts of the Galena-Platteville, particularly the lower one -third 
 of the lower part, might pass specifications for concrete aggregate and rail- 
 road ballast providing that the rock were fresh and unweathered. The possi- 
 bilities of the upper part of the formation appear less promising because the 
 rock is coarse and somewhat sandy. Agricultural limestone and road stone 
 could doubtless be obtained from the formation. 
 
 Maquoketa Formation, Divine Member 
 
 The Divine "limestone" member of the Maquoketa formation is 70 feet 
 thick in some places. The upper part of the Divine member is usually white, 
 light gray or buff, locally pink or brown limestone which grades downward to 
 gray to dark gray, fine- to medium- to coarse-grained, crystalline and med- 
 ium-grained, locally porous dolomite. The magnesium carbonate increases 
 from about 1 percent near the top to 39 percent near the base as revealed in 
 cores from borings in Grundy County. 
 
 South of the Sandwich fault zone the Divine member covers an area 
 approximately four miles wide, from east to west, in northeastern Grundy 
 County and southeastern Kendall County. North of the fault zone it occurs as 
 an irregular narrow north-south trending belt which passes east of the towns 
 of Yorkville and Bristol. Outcrops reveal only a limited portion of the total 
 thickness of the Divine member due to the low topographic relief of the area 
 and the nearly horizontal attitude of the beds. Exposures of the Divine occur 
 along the banks and in the bed of Aux Sable Creek in southeastern Kendall 
 County and northeastern Grundy County (Worthen, 1870; Krey and Lamar, 1925) 
 as well as in the area in Grundy County just west of the junction of the Des 
 Plaines and Kankakee rivers, on both sides of the Illinois River (Lamar and 
 Willman, 1931, 1933). 
 
 A large part of the area in sees. 25, 26, 35 and 36, T. 34 N., R. 8 E., 
 consists of land reported to be preempted as a future site for the construction 
 of the proposed Dresden Nuclear Power Plant and, therefore, may not be avail- 
 able for quarrying. 
 
DOLOMITE AND LIMESTONE RESOURCES 21 
 
 Locally the upper part of the Divine is a high-calcium limestone, but in 
 observed outcrops the thickness of the high-calcium part is less than about 
 5 feet. So far as can be told from well records the possibilities for high- 
 calcium limestone in the upper Divine are limited, even where the formation 
 is at its maximum thickness. 
 
 Parts of the Divine member might serve as a source of agricultural 
 limestone, road materials, and possibly railroad ballast and concrete aggre- 
 gate. Possibilities for high-purity dolomite exist but cannot be accurately 
 determined from the data at hand. 
 
 Alexandrian and Niagaran "Formations" 
 
 The lower part of the Alexandrian "formation" is a greenish gray, 
 sandy limestone and dolomite with thin clay partings and interbedded clayey 
 dolomite and silty shale. These beds grade upward into buff to gray, finely 
 crystalline, thin-bedded dolomite or dolomitic limestone with some chert 
 which comprise the upper portion of the formation. 
 
 The overlying rocks of the Niagaran "formation" are commonly brown- 
 ish gray, dense to vesicular, finely crystalline dolomites and dolomitic lime- 
 stones. Chert occurs in the upper portion of the Niagaran and there is some 
 scattered fine sand at some places in the middle portion. The lower Niagaran 
 beds are more massive and are usually chert free. The nature of the well 
 data available did not permit distinction of the line of separation between 
 Alexandrian and Niagaran rocks. 
 
 Rocks of Silurian age, probably Alexandrian, are exposed along the Fox 
 River north of Oswego. They consist primarily of light buff to brownish-gray, 
 finely crystalline, vesicular dolomite with beds from two inches to one foot 
 thick. A maximum of 70 feet of Alexandrian and 26 feet of Niagaran rocks are 
 reported in Kendall County. 
 
 The Alexandrian rocks appear in general to have limited commercial 
 possibility, though in some places parts of the formation may provide stone 
 for agricultural limestone, roads, and possibly other uses. The uses for the 
 Niagaran rocks are uncertain but may include agricultural limestone, road 
 stone, and possibly concrete aggregate and railroad ballast. High-purity dol- 
 omite also is a possibility. 
 
 STRUCTURAL FEATURES 
 
 A zone, possibly more than one -fourth to one -half a mile wide, in which 
 there is a pronounced dislocation of the bedrock strata, the Sandwich fault 
 zone, traverses Kendall County from northwest to southeast (fig. 1). The 
 rocks were down-dropped along the northeast side of the fault zone and were 
 uplifted on the southwest side (fig. 7). In the western part of Kendall County 
 in the vicinity of Millington, up-folding of the rocks accompanied the faulting. 
 Erosion of the arch thus produced has exposed the Oneota dolomite and geo- 
 logically younger formations. 
 
 Elsewhere in both Kendall and Grundy counties the rock strata slope 
 gently in a roughly easterly direction (fig. 6), though deviations from this dip 
 occur in some places. 
 
22 
 
 ILLINOIS STATE GEOLOGICAL SURVEY 
 
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DOLOMITE AND LIMESTONE RESOURCES 23 
 
 GROUNDWATER CONDITIONS 
 
 Groundwater conditions are significant in determining the suitability of 
 an area or rock unit for mining by open pit or underground methods. Where 
 dolomite or limestone occurs above the water table it is unlikely that a ser- 
 ious water problem will be encountered. However, where such rock is below 
 the water table the possibility of water must be considered. 
 
 Groundwater is most likely to occur in limestone or dolomite in open 
 crevices and channels dissolved out of the rock. This solution frequently 
 happens when the rock is at or near the surface of the ground. Thus, dolomite 
 or limestone formations now at or near the ground surface, as well as those 
 which occupied this position at some earlier geologic time, may have water- 
 bearing crevices and channels in them. More specifically, anywhere dolomite 
 or limestone is the first bedrock encountered in wells in Kendall and Grundy 
 counties the upper parts of the formations may be water bearing except as 
 subsequently noted. 
 
 Further, as the top of the Galena -Platteville formation was once at the 
 surface of the ground, its upper part is likely to be a water-bearing zone even 
 though the formation is now covered by the Maquoketa shale. This is not to 
 say that water will be present throughout the entire extent of the upper parts 
 of the dolomites or limestones mentioned, as there are no data to prove this, 
 but the possibility should be recognized and considered in connection with 
 prospecting or development, should they be undertaken. 
 
 Exceptions to the foregoing are topographically high areas underlain by 
 bedrock at a shallow depth and bedrock hills with thin overburden. These are 
 believed unlikely to contain enough groundwater to prohibit open-pit quarry- 
 ing or mining at relatively shallow depths. Areas of this sort are shown in 
 figure 2 as areas in which dolomite or limestone is from "0 to 25 feet deep." 
 
 Areas in which the Oneota, Galena-Platteville, Divine, and Silurian 
 dolomite or limestone "formations" occur directly beneath the unconsolidated 
 material are shown in figure 1. These formations are widely used as sources 
 of relatively small quantities of groundwater. Records in the files of the 
 Illinois State Water Survey indicate that production from farm and domestic 
 wells in these counties commonly ranges from 5 to 15 gallons per minute. 
 However, many of these wells probably are not pumped to capacity because 
 of the small amounts of water required. Only one well, located in east- 
 central Grundy County, is known to provide enough water from these rocks 
 for a municipal water supply, and it is in the Galena-Platteville formation. 
 In general, therefore, the amount of water that the majority of these wells 
 could supply is unknown and predictions regarding the amount of water that 
 might be encountered in underground mining operations cannot be made pn 
 the basis of the available information. 
 
 The possibility that the volume of water supplied by wells in the vicin- 
 ity of a quarry or mine might be decreased, perhaps markedly, by removal 
 of water from the quarry or mine should be borne in mind as well as the pos- 
 sibility that the mine or quarry water may have economic use. 
 
24 ILLINOIS STATE GEOLOGICAL SURVEY 
 
 DEPTH OF WEATHERING 
 
 The suitability of rock for many of the commercial purposes mentioned 
 depends upon its being unweathered, or unaltered. Weathering of a dolomite 
 commonly changes the color from brownish gray to light grayish brown to a 
 light brown or light reddish brown, and tends to give the stone a sandy or a 
 "loose -grained" character. Most of the bedrock dolomites or limestones of 
 Grundy and Kendall counties are at least partially weathered. This weather- 
 ing usually is most prominent at the contact of the rocks with the overlying 
 glacial deposits. • The least weathered dolomites and limestones occur where 
 the stone is covered by other rock units, especially shale. 
 
 Comparatively unweathered stone is available where there are bedrock 
 hills lying close to the surface, as can be seen at many of the quarries through- 
 out the area but most strikingly in the quarry at Central where very noticeable 
 weathering of the stone has taken place to a depth of only about 5 feet. Dolo- 
 mite, judged by its brown color to be weathered, is recorded in wells penetrat- 
 ing the Galena -Platteville formation at a depth of 65 feet below its top. Some 
 wells penetrate apparently unweathered dolomite and then enter weathered 
 stone, as though the weathering were controlled by the action of groundwater 
 along and adjacent to underground water channels. 
 
 A careful exploratory drilling program is essential, prior to opening a 
 deposit, if the proposed uses for the stone depend upon its state of weathering. 
 
 CLAY -FILLED DEPRESSIONS AND JOINTS 
 
 Some of the dolomite and limestone rock units in Kendall and Grundy 
 counties are known to contain locally clay-filled depressions and/or clay- 
 filled joints. The pockets and joints are of various sizes and shapes. Pock- 
 ets have been reported as large as 5 feet high and slightly greater in width 
 (Willman and Payne, 1942). Joints up to 3 feet wide, extending to depths 
 greater than 45 feet below the top of a bedrock dolomite or limestone, are 
 known to exist. 
 
 Clay-filled depressions and joints may occur anywhere that the bedrock 
 is shallow enough to permit the weathering and filling of pockets and the fill- 
 ing of joints. These features hinder quarry development and production be- 
 cause they create problems in the quarrying process. 
 
DOLOMITE AND LIMESTONE RESOURCES 25 
 
 REFERENCES 
 
 Culver, Harold E., 1922, Geology and mineral resources of the Morris quad- 
 rangle: Illinois Geol. Survey Bull. 43B. 
 
 Krey, Frank, and Lamar, J. E., 1925, Limestone resources of Illinois: Illinois 
 Geol. Survey Bull. 46. 
 
 Lamar, J. E., and Willman, H. B., 1931, High-calcium limestone near Morris, 
 Illinois: Illinois Geol. Survey Rept. Inv. 23. 
 
 Lamar, J. E., and Willman, H. B., 1933, Results of test-drilling of limestone 
 near Morris, Illinois: Illinois Geol. Survey Inf. Circ. 40. 
 
 Lamar, J. E., and Willman, H. B., Fryling, C. F., and Voskuil, W. H., 1934, 
 Rock wool from Illinois mineral resources: Illinois Geol. Survey Bull. 
 61. 
 
 Willman, H. B., 1943, High-purity dolomite in Illinois: Illinois Geol. Survey 
 Rept. Inv. 90. 
 
 Willman, H. B., and Payne, J. N., 1942, Geology and mineral resources of the 
 Marseilles, Ottawa, and Streator quadrangles: Illinois Geol. Survey 
 Bull. 66. 
 
 Willman, H. B., and Payne, J. N., 1943, Early Ordovician strata along Fox 
 River in northern Illinois: Illinois Geol. Survey Circ. 100. 
 
 Willman, H. B., and Templeton, J. S., 1952, Cambrian and lower Ordovician 
 exposures in northern Illinois: Illinois Geol. Survey Circ. 179. 
 
 Workman, L. E., and Bell, A. H., 1949, Deep drilling and deeper oil possibil- 
 ities in Illinois: Illinois Geol. Survey Rept. Inv. 139. 
 
 Worthen, A. H., 1870, Geology and paleontology; Geological Survey of Illinois, 
 vol. IV, Grundy County, p. 190-206. 
 
 Worthen, A. H., 1870, Geology and paleontology; Geological Survey of Illinois, 
 vol. IV, Kendall County, p. 136-148. 
 
 Illinois State Geological Survey Circular 230 
 25 p., 7 figs., 1 table, 1957 
 
■IHI.'MH | 
 
 CIRCULAR 230 
 
 ILLINOIS STATE GEOLOGICAL SURVEY 
 
 URBANA