The level aspect of Illinois topography has its explanation in the State's glacial history. As late as 25,000 years ago--a tick of the clock in geological time--there was still to be found in Illinois the last of the great ice sheets that had crept down from the North and with a leveling action comparable to that of a road-scraper, effaced hills and valleys carved by centuries of erosion. Ninety per cent of the State's surface was covered by ice; the only unglaciated areas are Jo Daviess County in the extreme northwest, Calhoun County in the west-central section, and the seven southernmost counties. In these areas the rugged terrain, sharply dissected by valleys, indicates the probable appearance of the whole of Illinois before the ice age. Elsewhere, save for sporadic outcrops, the uneven relief lies beneath a mantle of drift averaging 75 feet in depth.
The four ice-sheets that invaded the United States are definitely known to have reached Illinois. The next to last of these covered so great a portion of the State that it has been named the Illinoisan by geologists. Occurring approximately 150,000 years ago, it pushed south to the northern edge of the Ozark Range, and there, halted by increased melting and the barricade of hills, piled up rock debris 20 feet deep on the hillsides. This was the greatest southern penetration of any of the North American glaciers.
The Wisconsin Glacier, which moved into Illinois 50,000 years ago and receded 25,000 years later, covered only the northeast quarter of the State., but because of its geological lateness its effects are more obvious to the layman. The great central portion of the State which was covered by the older Illinoisan sheet, but not overlaid by the Wisconsin, is much more nearly even in relief and mature in drainage. The terminal moraines--ridges of drift piled up where the glacial front stopped--are low and inconspicuous. Those of the last glacier, however, are among the largest known to geologists. Sharply defined and extensive in length, they comprise the chief topographical relief of the northeast portion of the State. The major ones are named for cities that have been built upon them; the Shelbyville, Bloomington, Marseilles, and Valparaiso moraines are four of the most important.
Marked with the characteristics of recent glaciation, the land bordering Lake Michigan near the Wisconsin State line is poorly drained, with many lakes and marshes formed by the melting Wisconsin glacier. Thus was created the lake region of Illinois, major recreational area for the metropolis of Chicago. At the time of recession, the waters of the glacier were impounded between the Valparaiso moraine and the receding edge of ice, forming Lake Chicago, ancestor of Lake Michigan. The site of Chicago lay deep beneath the surface of this ancient lake, and deposition from its waters accounts for the table-top flatness of the city today. In successive stages the water receded north and east.
Glaciation and climate largely explain the agricultural distinction of the Illinois country. The average growing season varies from 160 days in the north to 211 days at Cairo, in the south. The drift laid down by the ice had been gathered from so great a variety of bedrock that an ample percentage of essential minerals was assured. Lying at the southernmost reach of the ice-sheets, Illinois was not strewn with the boulders and heavy débris that pock-mark the land further north. Much of the State is veneered with a layer of loess, the finer particles of drift that were sorted out by the wind and spread across the land. Enriched by prairie grasses during thousands of years, it possesses an even texture which, with the regular terrain, fits Illinois admirably for mechanical cultivation.
Buried beneath the glacial drift, the rock strata of Illinois effect little influence upon the topography, but their minerals yield to the State an income placing it tenth in the country in mineral output. All of the substructure that has been explored by geologists is sedimentary in nature, with the exception of a deep-lying mass of red granite encountered at 3,700 feet near Amboy, in the northern part of the State. At an unknown depth, the entire State is underlain with igneous rock, mother-rock of all formations, but vast processes of sedimentation have buried it beyond reach.
Of the five geological eras, the third, the Paleozoic, was by far the most important both geologically and economically. Beginning some 600 million years ago, it was characterized by repeated submergences and uplifts. What is now Illinois was then covered by a series of shallow seas. In great cycles, the seas advanced, covered the land for millions of years, and then retreated to expose the surface again to weathering and erosion. The strata laid down during each submergence differ sharply from each other, the degree depending upon the depth of the sea and the nature of the land at its shoreline.
The oldest period of the Paleozoic Era was the Cambrian, during which thick layers of sandstone and dolomite were deposited over the entire State. This, like the igneous rock, does not outcrop in Illinois, but slants upward from the south to come to the surface in Wisconsin. Rainfall in the latter area, seeping through surface soil to the sandstone layers, follows these to northern Illinois, where it serves as a reservoir for the wells of many municipalities.
The second period of the Paleozoic Era was the Ordovician, which saw a series of submergences of long duration. Its first deposits, the Prairie du Chien group, included a limestone which was the basis of Utica's natural cement industry, important in the last century but now abandoned. Another of the early Ordovician deposits is a layer of St. Peter sandstone, which outcrops in Ottawa and nearby in a remarkably pure form that has achieved national industrial importance as a source of silica sand used in glass-making and a hundred other processes. St. Peter sandstone also forms the picturesque bluffs that comprise Starved Rock State Park. Platteville limestone, likewise an Ordovician deposit, is used in the manufacture of Portland cement. Late in the period a layer of Galena dolomite was laid down. It bears the lead which gave Galena its name and its early mining boom. Related to this formation is the Kimmswick limestone, source of petroleum in the southwestern field at Dupo.
The third period, the Silurian, laid down several strata of dolomite and limestone. The latter is quarried extensively near Chicago and Joliet for road material, aggregate, and soil replenishment. The following period, the Devonian, is likewise chiefly important for its limestone. Among the Mississippian deposits are the sediments that store the southeastern oil pool, long a steadily producing field and lately the scene of a spectacular boom of revived activity.
Near the end of the Paleozoic Era occurred the Pennsylvanian period, when Illinois' great coal measures were deposited. The coal strata, but a small portion of the Pennsylvanian deposits, far outstrip all other geological periods in the wealth they have yielded. The land at this time was low and marshy, a few feet above sea level. A favorable climate encouraged the growth of giant trees and ferns that subsequent eons compressed into the coal veins that underlie two-thirds of the State. Despite a half-century of extensive mining operations, not more than 2 per cent, it is estimated, of Illinois' coal reserve has been tapped.
Following the close of the Pennsylvanian period, the greater portion of Illinois remained above sea level. Great land movements that raised the Appalachians in the east, folded the land of Southern Illinois into the present Ozark range. The work of the seas was done, and now rain and wind attacked the surface to erode and crease it with great valleys and ridges. But then, following a vast climatic change, snow began to fall in the northern region, year after year, deeper in the winter than the brief summer sun could melt.
So began the glacial period, the deus ex machina in the making of Illinois. Even as the curtain descended upon the State's geological drama, the ice sheets appeared, effaced the ruggedness, and retreated-so recently that Indian legends make awed mention of the Ice God that once came down from the North.