Part Two of the |
Chicago Geology: Paleozoic
Geology - Sedimentary Rocks
General relationsThe geologic history of this part of Illinois is here sketched only in the barest outline.
The rock forming the substructure of this region may be seen in numerous quarries about the city, and is reached by all borings which pass through the drift. The formation exposed at the quarries and first reached in borings is the Niagara limestone.
Information from deep wells
All the deepest wells in this region terminate in the midst of a great sandstone formation, the Potsdam sandstone, This is the lowest known rock formation in the State, and is shown at the bottom of the section. Some of the wells penetrate this sandstone several hundred feet, but none pass through it. In parts of Wisconsin it has a thickness of 1,000 feet.
What underlies this sandstone here is not known. When these formations are traced northward into central Wisconsin it is seen that they gradually rise and, one after another in the order of the succession downward, thin out and disappear as the next formation below rises to the general rock surface, until at length even this lower sandstone reaches the surface, spreads over a great area, thins out, and disappears. From beneath this sandstone there appear crystalline rocks yet remoter age. It is believed that these latter rocks everywhere underlie the formations found by the well borings beneath the Chicago area, and that their erosion originally furnished the material from which the overlying sedimentary formations were made.
Potsdam groupThis lower group of sediments, the Potsdam sandstone, which is very widely distributed in the United States, is of later Cambrian age. Since this is the earliest of the Paleozoic formations in the northern interior, it is evident that a large part of the continent was land area and was exposed to denudation while the earlier Cambrian formations were being deposited in the eastern and western seas.
Beach and shore sands of later Cambrian timeWith the opening of later Cambrian time, however, the ocean advanced upon the land from the south, until of all the northern interior of the United States, only the northern half of the area of Wisconsin and parts of northern Michigan and Minnesota remained above water in this ocean was a characteristic fauna, and in the shallow waters along the advancing shore lines the Potsdam sandstone was deposited, much as great beds of sand are now being deposited along sea shores. That the deposit was made in the ocean and no in an inland lake is shown by the fossil marine fauna, found where the rock outcrops.
Whatever may have been the character of this Wisconsin land and its altitude relative to the advancing Cambrian sea, it is evident the reaction was such that the erosion of its surface and the deposition about its shores gave rise to a very extensive sand bed of considerable thickness. There appears to have been more or less variation in these conditions, so that in some places beds of limestone and beds of shale occur in the upper part of the formation.
The well at Chicago Heights shows 190 feet of limestone and shale in the uppermost part of the Potsdam formation and above this 250 feet more of sandstone, making 1,064 feet of rock to be referred to this group. This bed of limestone and shale indicates an interval of deeper waters, free from beach sands. The limestone probably is to correlated with a similarly located but thinner bed in Wisconsin, known as the Mendota limestone, while the upper 250 feet of sandstone may be correlated with the Madison sandstone of Wisconsin.
These sections show deposition in water somewhat deeper than that which covered the region lying farther east, for instead of so much shore sand, finer silts were deposited.
Lower Magnesian groupOverlying the beds of the Potsdam group is a magnesian limestone formation 160 to 450 feet thick.
Calcareous deposits of early Silurian timeThis formation thickens toward the south and southwest, being 350 feet thick at the Chicago Heights well and 450 feet thick at Joliet.
As a general rule, however, clearer waters prevailed and a new and different fauna was developed. While it is probable that the Lower Magnesian limestone was derived from the calcareous residue of this marine life, well-preserved fossils are very rare in it.
The dolomitic character of the limestone is held by some to have been due to contemporaneous deposits from the magnesian salts of the sea water, and by others to have been due to subsequent alterations.
Sands succeeding the early Silurian dolomite -- St. Peter groupA return to conditions favorable to the deposition of sands and to changes in the fauna of this region marked the formation of the St. Peter sandstone, which overlies the Lower Magnesian group.
This is a very porous, white sandstone, varying considerably in thickness. It ranges from 60 feet thick where it is reached in wells at Goose Island in North Branch of Chicago River, about 2.5 miles northwest of the Chicago Harbor inlet, to 200 feet at Chicago heights and 450 feet at South Evanston.
Magnesian limestone of mid-Silurian time -- Trenton groupLimestones of the Trenton group were deposited in this region during the period which next followed. In the clear waters of this Trenton sea lived a prolific and varied fauna, as we may judge from the fossil remains at places where the rock is not exposed at the surface. This fauna consisted of corals, crinoids, mollusks, crustaceans, and other invertebrates. The limestones are mostly magnesian. Wells in the vicinity of Chicago show strata 270 to 390 feet thick, referable to this group. The upper part of the formation, known as the Galena limestone, is the lead-bearing formation of northwestern Illinois. The lower part is known as the Trenton limestone.
Mud rocks or shale succeeding the Trenton limestone -- Cincinnati or Hudson groupOverlying the Trenton limestones is a formation composed of thin-bedded shales or mud stones. These are, for the most part, composed of the fine, muddy sediments carried into the sea by the drainage of the land. It is possible that these fine sediments so polluted the water as to have a marked effect upon the living organisms therein. At any rate, there was a marked change in the fauna; new species came in, and such as could not adapt themselves to the new conditions were extinguished or forced to emigrate.
Under the Chicago area this shale formation is shown by wells to vary in thickness from 105 to 250 feet. It was found to have a thickness of 250 feet at the first Union Stock Yards well. A 20-foot bed of limestone occurring in the formation at this place shows an interval of clearer water.
At the close of the Hudson epoch two great islands developed in the interior continental sea in the region of Indiana, Ohio, Kentucky, and Tennessee, which very materially affected the rock-making conditions. No marked disturbances of the strata are found in the vicinity of Chicago, but the sea was withdrawn for a considerable period, as is shown by the fact that formations which were elsewhere deposited between the Cincinnati group and the next overlying formation show that an interval occurred in which the marine fauna was almost entirely changed. When the sea again submerged the area a fauna composed almost entirely of new species made this their habitat.
Niagara groupUpon the Hudson shale lies a great limestone formation, the Niagara limestone, which immediately underlies the drift throughout this region (figs. 2 and 3.) This formation here varies in thickness from 254 to 409 feet, and it is probable that even this is not its original thickness, since there was a great opportunity for erosion before it was covered by the protecting mantle of drift.
During the deposition of this limestone clear water conditions prevailed over wide areas and a new and wonderful fauna developed [The Silurian fauna interpreted on the epicontinental basis by Stuart Weller: Jour. Geol., Vol. VI, 1898, pp.692-703]. Just where this new fauna came from is not known, but a careful study of the distribution of the rock of this age and of the forms of life represented has led to the suggestion that the interior sea extended far northward and connected with Europe across the polar regions, and along the shores of this sea the species may have come to the region of northeastern Illinois.
Exposures of Niagara limestone in and about ChicagoSince the Niagara limestone immediately underlies the drift throughout the Chicago district, there is abundant opportunity for its examination and study at the numerous quarries and outcrops.
As shown by the sections (figs. 2 and 3), the thickness of the Niagara limestone varies considerably within this area. The character of the rock at the various outcrops and quarries is indicated in the following description:
The term "marble," as used here evidently means even-grained, close textured limestone.
Quarries near Douglas Park, at Bridgeport, and at Hawthorne
Quarry near Blue IslandAbout a mile southwest of the village of Blue Island the limestone has been quarried. Over a considerable area southwest of this it is but thinly covered. The beds are nearly horizontal, having a dip of 2 degrees to 4 degrees SE, and form no elevation above the plain. The rock at the surface is a thin-bedded shaly limestone showing obscure glacial markings. The quarries being filled with water at the time of examination, the lower beds could not be seen. Dr. Bannister says that a bed of bluish impure limestone was formerly worked here as hydraulic rock. [Geology of Cook County by H. M. Bannister: Geol. Surv. Illinois, Vol. III, 1868, p. 244.]
The Niagara again appears about a mile to the southwest, on the place of Mr. Henry Schwartz, where it has been stripped of 3 to 6 feet of bowlder clay.
Exposures and quarries near ThorntonAt Thornton, on the Chicago and Western Indiana Railroad, there is a low elevation of limestone covering an area of 1.5 to 2 square miles. The village is on the southeastern part of the elevation. The rock has a relief of 10 to 18 feet above the plain to the north, and is but thinly covered by drift. About one-half mile north of the village, on the road to Blue Island, the rock is exposed with a dip of 20 degrees NE. In the southwestern part of the village is the quarry of the Brownwell Improvement Company. The strata have here a dip of 12 degrees SE, and the upturned edges have been smoothed and striated by glacial action. The quarry shows at present a 95-foot section. The beds increase in thickness downward from about 4 inches to 4 feet and are much fractured, so that no dimension stone is obtained. Near the surface the rock is buff and somewhat shaly, passing downward into a dense, blue gray, siliceous limestone. Mr. Brownwell states that the rock contains much bitumen or asphalt in pores and small cavities, and it also highly fossiliferous. In the bed of Thorn Creek, just south of the village bridge, the rock has a dip of about 24 degrees NNE. About 100 yards to the south the dip is in the opposite direction, toward the southwest, at a like angle, indicating a gentle local anticline. At this place the glacial striations are well shown.
Near GlenwoodOne mile south of Thornton, near the road to Glenwood, the rock is again exposed in the bed of Thorn Creek. About 2.5 miles south of Glenwood the Niagara is seen at Miller's limekiln and also in a road cut on the hill slope. The rock is an impure buff fossiliferous limestone, containing bitumen.
Outcrop in the vicinity of ElmhurstWest of the city, the northernmost exposure is seen a mile west of Elmhurst, on the Chicago and Northwestern Railway. The rock is here well bedded (see Illustration sheet 1, fig. 16) and shows abundant traces of organisms. The strata form a gentle anticline with northeast-southwest strike, and the surface shows glaciation. Some dimension stone is quarried.
About 3 miles southeast of Elmhurst, near the Illinois Central Railroad, at the quarry of Kogle and Smith, is an outcrop of a grayish limestone, weathering to a dark-buff or brown color. The upper portion is somewhat decomposed and crumbling. Crushed stone and some building stone are obtained. The rock surface shows abundant glacial striations. As the rock is also seen in the railroad cut one-quarter of a mile southwest from the quarry, it probably forms the body of the small elevation at this place.
About 3.5 miles south of Elmhurst, horizontal thin-bedded limestone is seen in the western bank of Salt Creek. The upper 2 or 3 feet are porous and yellow, passing below into an even-textured, light-gray limestone containing nodules of chert. The beds are fossiliferous.
Near LagrangeAbout a mile northwest of Lagrange, on the banks of Salt Creek, is a small quarry in light gray fossiliferous limestone. The rock is overlain by a few feet of fine stratified gravels and bowlder clay.
Limestone exposed in Desplaines Valley
Details of rock surface in Desplaines ValleyThe surface of the rock underlying the valley bottom, where exposed in the excavations for the Sanitary and Ship Canal, has given much evidence of strong river erosion. Large cavities, 5 to 10 feet wide and of a corresponding depth, have been cut through by the canal section. These have been taken to be potholes, ground out by stones in the eddying currents. Dr. Bannister reports it as stated that the potholes in the surface layers at the Athens (Lemont) quarries, when of sufficient depth to penetrate one layer and enter another, are occasionally found to be dislocated; that is, one layer has slipped upon the other, so that the upper and lower portions of the pothole are, in some cases, entirely separated from each other. The phenomenon was not observed by Dr. Bannister, but seemed to be well attested. As he stated, this would see to indicate a horizontal disturbance of limited amount at a comparatively recent period.
Mr. Frank Leverett pictures and notes heavy surface grooves in the rock exposed in the excavation for the diversion channel of Desplaines River at Lemont, near the Santa Fe Railway bridge. [The Pleistocene features and deposits of the Chicago area by Frank Leverett: Chicago Acad Sci., Bull. II Geol. and Nat Hist. Survey, 1897, p. 53, fig. 7. Also, The Illinois glacial lobe by Frank Leverett: Mon. U.S. Geol. Survey, Vol. 38, 1899, p. 417]
Mr. Ossian Guthrie reported them to have a bearing of about S. 60 degrees W. These have been regarded by some as glacial groovings, but Mr. Leverett thinks they are probably the product of river erosion.
Exposures south of Desplaines Valley
The exact stratigraphic relations of the beds seen in these various outcrops can not be determined. The character of the rock varies considerably within the area. In places many fossils have been collected, while in others the beds are almost entirely barren.
Rock shoals in the lakeBesides the land exposures of the underlying rock, there are numerous rock shoals off the shore of Lake Michigan, on the city front, as shown by the United States coast charts of the survey of the northern and northwestern lakes. These are
Evidence of Devonian sedimentsUntil recently it has been thought that the Niagara limestone was the last of the indurated rock formations of which any trace remained in this vicinity. Such, however, is not the case.
In northwestern Indiana deposits of lower Helderberg and Corniferous age and later Devonian shales are known to occur beneath the heavy mantle of drift, and in Milwaukee and Ozankee counties, Wis., deposits referred to the Onondaga and Hamilton epochs are well developed. It is also probable that the basin of Lake Michigan is very largely excavated in deposits of Devonian age. So it is not at all surprising that Devonian deposits should have extended over the region about Chicago.
Fossils in the quarry near ElmhurstAt the quarry near the Chicago and Northwestern Railway, 1 mile west of Elmhurst, in eastern Dupage County, evidence has been found showing unmistakably that the Niagara limestone was once overlain by beds of later age. Of a rock specimen purporting to come from this quarry, one-half was Niagara dolomite and the other half was a black shaly rock containing abundant fragments of small fish teeth. A careful examination of the stripped rock surface at the quarry failed to show any black shale, nor was any noted in the overlying drift. Further examination, however, revealed the origin of the specimen. The following description of the deposit is taken, with slight modifications, from Stuart Weller [A peculiar Devonian deposit in northeastern Illinois by Stuart Weller: Jour. Geol, Vol. VII, 1899, p. 483 et seq.]
At this locality the limestone is much fractured by two sets of gentle folds, and joint cracks are well developed. Some of these cracks are several inches in width, and they are in general filled with black or blue clay. At one point in the east quarry face, about 18 feet below the glaciated surface of the rock, one of these joints is somewhat enlarged to form a narrow, triangular opening about 6 inches in width at the base and about 16 inches in height. This opening, instead of being filled with clay, as are all the other large joints in the quarry, is filled with a breccia composed of angular fragments of the adjacent limestone embedded in a dark-brown, arenaceous matrix. This matrix is abundantly fossiliferous, containing an immense numbers [sic] of fish teeth and a smaller number of Lingula shells and other brachiopods which indicate its Devonian age.
The situation of this most peculiar occurrence of Devonian fossils, deeply buried in the Niagara limestone, is shown in fig. 16, Illustration sheet 1. The triangular opening filled with Devonian material is just to the left of the hammer and about 18 feet below the upper surface of the rock.
The species of fossils recognized are as follows:
The most abundant species is Ptyctodus calceolus, whose tritors are present literally by the hundreds. This species is characteristic of the middle and upper Devonian faunas of the interior of North America. The remaining fish teeth are two new species of Diplodus, which have been described by Dr. C.R. Eastman [Description of new species of Diplodus teeth from the Devonian of northeastern Illinois by C.R. Eastman, Jour. Geol. Volume VII, 1899, pp. 489-493.]
Correlation and geographic conditions during Devonian timeThe result of the study of the fossils is stated by Mr. Weller thus; "The presence of Ptyclodus is certainly indicative of the Devonian age of the fauna, but Diplodus has previously been recognized in Carboniferous strata, and the presence of two species of this genus with the Waverly species of Orbiculoidea would seem to indicate a very late Devonian age."
The presence of the remains of this fauna of late Devonian age, deeply buried in the Niagara limestone, is very interesting and most significant. The absence of any species of earlier Devonian age would seem to indicate that during the earlier part of this age the strata were so far elevated as to become land. This land is thought to have been part of what was probably a large land surface, stretching from the Wisconsin land on the north to the Ozark land of Missouri on the south. Doubtless the shore line was no very far east of the Chicago district, since part of eastern Wisconsin and part of northwestern Indiana were submerged. During this elevation slight flexing of the rock strata produced the joint cracks in the Niagara limestone, and these were doubtless somewhat enlarged by solution by percolating waters. Doubtless, also, more or less of the upper part of the Niagara formation was cut away by erosion during the interval of exposure.
Near the close of the Devonian age the land was submerged beneath the sea waters and the cracks in the sea floor were filled by sand and fine silts, in which were small shells and fish teeth. Coatings of this same material on the sides of cracks leading to the surface of the Niagara limestone show clearly that this was the mode of deposition of the material found.
Besides this filling of the cracks in the sea bottom, doubtless beds of some thickness covered much of the surface of the Niagara limestone in this area. Were these beds, however, so friable as this remnant found at Elmhurst, they would have been readily removed by erosion during their exposure on the emergence of the area which followed.
Heretofore, so far as known, the marine life of this region was confined to invertebrate animals. In some other places fossil remains seem to show that vertebrates appeared at an earlier time, but not until the Devonian submergence to these forms appear to have reached the northern interior sea. The evidence of so great an advance in the life forms thus give to those Elmhurst deposits an especial interest. Since the Devonian sea reached northern Indiana, Michigan, and eastern Wisconsin somewhat earlier than the time of its encroachment on the Chicago area, it is probable that vertebrate life was already flourishing in its waters when the submergence of the Chicago area took place.
The region long a land areaSo far as known, the close of Devonian time was marked by the final withdrawal of the sea from the Chicago area. Certain evidence developed has suggested a possible resubmergence in marine waters at a much later date. This evidence is noted on page 10 of this folio. No indurated rock formations later than the Niagara limestone and the slight remnant of Devonian rock are known within the district. Upon these formations lies the heavy mantle of unconsolidated material referred to as drift (figs. 2 and 3). This drift not only overlies the rock formations here represented, but, extending out over the greater part of Illinois, and in fact over a large part of northern United States and Canada, it overlies rock formations of all ages, even the most recent; whence it is evident that the drift is the latest of all the great geologic formations of North America. It was deposited in the Pleistocene, the era immediately preceding our own.
A large part of the rock formations of the continent was formed subsequently to the Devonian period, so it is evident that the interval between the deposition of the rock exposed in this district and of the immediately overlying drift was of enormous duration. During the whole of this interval, so far as known, the northern part of Illinois stood above the level of the sea. It is true there may have been, and indeed probably were, many changes in the conditions, due to variations in the elevation of the land and to other causes. There may have been intervals during which the sea waters again encroached upon this land, but no evidence other than that noted on page 10 of this folio has been found. The work of geological agents did not, however, cease in this region during this great interval. Up to this time the area of northeastern Illinois had been receiving contributions principally from the Wisconsin land to the north; during this interval the rock beds of this area were subjected to erosion and the material thus obtained was carried off southward by the streams, to help in the formation of other rock beds in the sea. So great was this interval of rock disintegration and erosion that the Devonian beds and whatever subsequent rock formations may have been present in this area were entirely removed. More or less of the Niagara limestone was also cut away at this time.
As the rock disintegrated into soils vegetation developed and faunas of the air and land appeared, yet no traces of these faunas and floras here remain. The only evidence here is of destruction; one must go elsewhere to find the results of constructive work of post-Devonian and pre-Pleistocene time. With the deposition of the drift the process of rock destruction in this area practically ceased.
|This ends part 2 of The Chicago Folio by William Alden, 1902.|
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