Geology and Topography

The current biodiversity of the Chicago Region and Oakton's Natural Areas has been strongly affected by gradual, geological processes that span timescales of millions of years. Processes like glaciation literally change the landscape, altering the stage on which ecological interactions among species play out. Land forms, soils, and networks of rivers and lakes are all shaped by geology, slowly but surely. Thus, it is important to account for some of the significant, geological changes in the region over the last 2.6 million years - what geologists have termed the Quaternary period within the Cenozoic era (65 million years ago to present).

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On geologic timescales, the Quaternary is a very short amount of time, given the 4.6 billion-year history of the Earth. Even so, it is fair to say that a lot has happened in the Chicago Region during the geologic blink of an eye. Geologists have subdivided the Quaternary into two epochs: the Pleistocene (2.6 million to 11,700 years ago) and the Holocene (11,700 years ago to present). The Pleistocene is generally thought of as an ice age, with four major ice advances into the Chicago Region. The Holocene is a somewhat artificial division and merely corresponds to the most recent interglacial period of warming since the Last Glacial Maximum about 13,000 years ago.

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There are not any active volcanoes near the Chicago Region, so our recent geologic history is not a story of fire, but it is certainly a story of ice. Glaciers essentially constructed the current landscape that we see today. A glacier is a dense, slowly moving river of ice powered by its own prodigious mass and the accumulation of snow over thousands of years. The most recent major overhaul to the landscape began about 26,000 years ago, at the start of the Wisconsinan stage, as glaciers advanced from the north into the region. The Wisconsinan stage glacial advance reached about 200 miles south of present-day Des Plaines, Illinois before the glacier began its retreat about 13,000 years ago. The glaciers that scoured the region were 1,300 feet thick and had a profound effect on the land and waters of today.

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Despite the massive power of the glaciers, the existing land underneath largely channeled where they went. In the Midwest, the ice followed river valleys and gouged those valleys into the deep basins that now comprise the Great Lakes. The gravel, sand, silt, and clay that the glaciers deposited in the region upon their final retreat covered a bedrock of sedimentary rocks that once formed the bottom of an ancient sea. The most common type of bedrock in the Chicago Region is a limestone called dolomite, which is rich in magnesium. This was laid down in the Silurian period about 400 million years ago. The bedrock does vary in topography, but it has little connection to the surface topography in the area today. These modern landscapes are shaped by all the material that the glaciers left behind as they melted. In some places the accumulated material is nearly 400 feet thick. Only along the Des Plaines River in southwestern Cook County is there significant exposed bedrock with just a few inches of soil above it that actually interacts with the organisms of today.

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Glacial drift refers to the material deposited by glaciers: till is material directly deposited; and outwash is material deposited by meltwater flowing from the glacier. As glaciers slowly scrape across the ground, a hodgepodge mixture of boulders, gravel, sand, silt, and clay gets carried hundreds of miles across the landscape and drops out as the glaciers melt and retreat. Mounds of drift can form hills hundreds of feet high. Giant blocks of ice may break off from the melting glacier and get buried in the till, forming ponds and lakes. The ridges or mounds of unsorted till left behind at the edge of a glacier's advance are called moraines. The Chicago Region's topography is essentially a network of moraines left behind from the glacier's most recent retreat.

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In between moraines are areas created by glacial outwash. When a block of ice as immense as a glacier melts, it floods the land with massive sheets of water. Powerful rivers a mile wide sculpted out valleys, and the flowing water carried and sorted particles of all sizes. For example, the sandstone canyons along the Illinois River at Starved Rock State Park near Ottawa, Illinois were gouged out by a massively powerful flood of glacial meltwater called The Kankakee Torrent that occurred about 19,000 years ago. It resulted from a breach of moraines forming a large glacial lake fed by the melting of the Late Wisconsinan Laurentide Ice Sheet. The torrent must have been almost unimaginably powerful indeed - there is evidence of the flood's great scouring power in the uplands of Starved Rock State Park, up to 640 feet above sea level which is about 180 feet above the current level of the Illinois River. Mighty torrents scoured deep canyons in sedimentary rock and moved large boulders, but as the current slowed over time, particles began to settle to the bottom of rivers and lakes. In fact, many of the Chicago Region's sand prairies are places where the massive flood waters ebbed and slowed down and deposited large quantities of sand. Much of the soil in the region's lower-lying areas - such as Oakton's Des Plaines campus - is comprised of glacial outwash.

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The glacial till deposited about 10,000 - 13,000 years ago forms the raw material of today's soils, and those soils support plant life that in turn supports animal and fungal life to collectively comprise the biodiversity of the Chicago Region. Mature soils develop over thousands of years through varied physical and biological processes such as the growth and dying of plant roots, which add more organic material to the soil. Even so, the biodiversity of organisms in the Chicago Region and on Oakton's Des Plaines campus  - and their particular distributions and abundances - are intimately connected to the detailed history of the most recent glaciation in the region. Those geological processes formed the very ground on which we walk and have set the stage for the ecological interactions among diverse organisms that we can observe today in the natural area remnants and restorations on Oakton's Des Plaines campus.

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The Chicago Region: