Glaciers Smooth the Surface
Late in the Cenozoic Era, the world cooled down. From about 2 million years to about 10,000 years ago, glaciers of the Pleistocene Epoch—or "Ice Age"—profoundly affected the topography of Illinois (figures 4 and 5). The modern Illinois landscape is a result of bedrock carving and associated sediments deposited during the episodes of glaciation that occurred during this time. The network of deeply eroded valleys in the bedrock surface that formed in preglacial times was filled with glacial deposits that covered most of the state. Much of the present land surface of the glaciated areas of Illinois does not reflect the underlying bedrock surface.
FIGURE 4: Maximum extent of (a) early Pre-Illinois glacial episode (1,000.000±years age); Driftless Area shown by stippled pattern; arrow indicates direction of ice movement; (b) late Pre-Illinois glacial episode (600,000±years ago); (c) Illinois Glacial Episode (250,00±years agae); (d) late Wisconsin Glacial Episode (22,000 years age).
FIGURE 5: Timetable illustrating the glacial and interglacial events, sediment record, and dominant climate conditions of the Ice Age in Illinois (from Illinois' Ice Age Legacy).
How did this happen? What forces filled valleys, diverted rivers, and smoothed the landscape? During the Pleistocene Epoch, much of North America was covered repeatedly by huge glaciers. These continent-size ice masses formed in eastern and central Canada and advanced southward. As the glaciers flowed, they eroded rock along their path and carried this debris into Illinois; these materials were dropped out from the glaciers as the ice melted. Because the identification of the early glacial episodes in Illinois is obscured by reworking and burial during later glacial advances, they are collectively known simply as the pre-Illinois glacial episodes. These episodes ended about 425,000 years ago and were followed by a long warm interval between glaciations (the Yarmouth) that lasted about 125,000 years. A deep weathered zone, the Yarmouth Geosol, formed during that time.
Approximately 300,000 years ago, the Illinois Episode of glaciation began. During the 175,000 years of this episode, the ice advanced three times out of the northeastern center of accumulation. During the Illinois Episode, North American continental glaciers reached their southernmost position, located just north of the Shawnee Hills in southern Illinois (see figure 4c ). During the first of these advances, ice of this episode reached westward across Illinois and into Iowa. After the Illinois Episode, during the Sangamon interglacial interval, another major soil, the Sangamon Geosol, developed.
Next came the Wisconsin Episode of glaciation, which created the landscape we see today. The Wisconsin glaciers advanced into Illinois about 25,000 years ago. Although late Wisconsin glaciers advanced across northeastern Illinois, they did not reach southern or western Illinois (see figure 4d). As the glaciers advanced, earth materials from previous times were buried, transported, or otherwise rearranged. As the Wisconsin ice retreated, large ridges of deposited earth materials—called moraines—were formed. The Illinois Episode left few moraines, and all are markedly smaller in size than the later Wisconsin moraines. Illinois moraines are only preserved south of the limits of the younger Wisconsin age glacier.
The glacial deposits of Illinois consist primarily of (1) till—pebbly clay, silt, and sand, deposited directly from melting glaciers; (2) outwash—mostly sand and gravel, deposited by the rapidly flowing meltwater rivers; (3) lacustrine deposits—silt and clay that settled out in the clear, quiet water of glacial lakes and sand and gravel that waves concentrated at or near shorelines; and (4) sand dunes and loess (windblown silt). During the major glacial advances, outwash deposits of silt, sand, and gravel were dumped along the Mississippi and Illinois River valleys. When these deposits dried out during the winters, strong prevailing winds from the west (the westerlies) winnowed out the finer materials, such as fine sand and silt, and carried them eastward across most of the state.
Scattered along the Mississippi and Illinois River floodplains are several areas of sand dunes. These dune areas generally contain a random arrangement of small hills, mounds, or elongated ridges that are almost entirely made of sand. Dunes are formed by the piling up of sand by the wind and can develop in any region with a readily available source of sand and occasional strong winds.
The loess that mantles most of the state of Illinois was laid down by the wind during the Illinois and Wisconsin Glacial Episodes. In general, the thickness of the loess deposits is greater along the eastern boundaries of the Mississippi and Illinois Rivers, and the thickness decreases to the east. The loess deposits are up to 25 feet thick along the Illinois River valley and more than 50 feet thick, in some localities, along the east edge of the Mississippi River valley. Illinois is the only state with an almost complete blanket of loess, because of the westerly winds crossing the combined Mississippi and Illinois River valley outwash plains, which extend the full length of the western edge of the state. These loess deposits form the parent materials for a large majority our modern soils.
The glaciers also brought in large boulders of igneous and metamorphic rock that geologists call erratics. These large erratic boulders were plucked from bedrock in Michigan, Wisconsin, and Canada and now are scattered throughout Illinois. Often, these erratics can be seen along country fence rows where farmers have cleared them from their fields.
Although the glaciers left Illinois about 13,500 years ago, they left behind many kinds of erratics, including quartzite, schist, granite, and gabbro.