Quaternary, Paleozoic, and Environmental Geology of Chicago
Written by Brandon Curry and Donald Mikulic
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Chicago, the City of Broad Shoulders, the 3rd largest city in the United States, resides on a landscape created by rifting sculpted for more than a million years by glaciers, lakes, and streams. The canvas these agencies worked upon is dominated by dolomite deposited during the Silurian about 430 million years ago at the bottom of a vast inland sea. Found on the edge of the Michigan and Illinois Basins where the bedrock dips gently to the east, resistant ancient reefs are locally exposed at the ground surface where they have been quarried for stone construction materials since the 1820s. As these shallow bedrock resources are becoming depleted, today aggregate production comes in part from underground mines in older Ordovician rocks of the western suburbs. Important bedrock-controlled infrastructure includes TARP, the Tunnel and Reservoir Project, which famously exploits the high rock quality of the Silurian strata as a tunneling medium to direct excess surface runoff to manmade chambers, tunnels, and reservoirs where the water is treated and released to the Des Plaines River.
The resistant Silurian dolomites are also a primary feature controlling the geologic character of the region. Specifically, the bedrock high, in the western part of the metropolitan area, controls the subcontinental divide separating the Lake Michigan and Mississippi River drainage systems. Most of the glacial drift beneath Chicagoland was deposited during the waning stages of the last glaciation from about 22,000 to 16,000 years ago. Glacial deposits are primarily till, outwash sand and gravel, and fine sandy to muddy lake sediment. Below the Loop, glacial drift varies from about 20 to 30 m thick. The nature of the bedrock surface is variable; in places, it was scraped to fresh rock by the glaciers, and in others, the bedrock below the drift is heavily jointed with some joints widened by weathering. Karstic sinkholes, now filled with Quaternary debris, are likely located in some areas.
The drift below the Chicago loop includes, in traditional parlance, the Chicago Hardpan, Blue Clay, sands of Glacial Lake Chicago, and an astounding variety of anthropogenic fill. The hardpan is over consolidated silty and sandy diamicton (till) known to Quaternary geologists as the Haeger Member of the Lemont Formation. Its upper contact with the Blue Clay is one of the easiest subsurface horizons to identify beneath Chicagoland. The Blue Clay is a late-stage glaciogenic succession that includes over consolidated silty and clayey diamicton (the Wadsworth Formation), overlying lacustrine sediment (Equality Formation), and thin littoral sands (Henry Formation). Several generations of fill are below your feet as you navigate the streets of Chicago, including patches of cinder-rich debris from the 1871 Chicago Fire.
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Chicago Geology Map
(Click on Map to Download)
GEOLOGIC MAP OF THE 1° X 2° CHICAGO QUADRANGLE, INDIANA, ILLINOIS, AND MICHIGAN SHOWING BEDROCK AND UNCONSOLIDATED DEPOSITS (1970). Authors: Allan Schneider and Stanley Keller
