Evidence for Glaciation

One legacy of Quaternary glaciation in Illinois is the landforms that were created. Most obvious, perhaps, are the end moraines that formed along the glacier's margin, particularly in northeastern Illinois during the Wisconsin Episode glaciation. These broad arc-shaped ridges are evidence that even as the glacier melted back from its maximum position, it continued to flow and deliver sediment to its leading edge. Also obvious are the large valleys with seemingly too-small streams that lead away from the glaciated areas. These streams are known as underfit streams, because the valleys were created by much larger streams than flow through them today. A good example is the Illinois Valley, which was cut by the much larger meltwater streams draining away from melting glacier. The valley floors are relatively flat and are underlain by outwash sand and gravel, called valley train, below the finer waterlain sediment of the postglacial Illinois River floodplain.

The most apparent evidence is of course the glacial drift itself. Glacial drift refers to the rock material ground up and transported by a glacier and deposited by or from the ice (till) or in water derived from the melting of ice (outwash or lake sediment). It is not generally taken to include loess or other windblown material. However, in glaciated areas, loess and interglacial sediments (soils, slopewash, and river and lake deposits) are also included on drift thickness maps.

Illinois' glacial heritage can also be seen in the types of rocks that adorn fields, parks, and lawns. Many of these rocks are unlike the local sedimentary bedrock (limestone, dolomite, sandstone, and shale) we find in quarries, stream cuts, and road cuts. Instead these exotic rocks, called erratics because they are out-of-place among the local rocks of the State, are natives of places farther north and were transported here by the glaciers.

Some erratics are of distinctive rock types that are not widespread, but instead are known only to occur in unique areas. Such erratics provide evidence of glacial flow lines. Examples include omars, jasper conglomerates, and tillites.

Other evidence for glaciation is recorded on some bedrock surfaces beneath the glacial drift. Scratches made by rocks frozen into basal ice scraping over the bedrock are called striae. They show the orientation of iceflow and in Illinois are best preserved on limestone and dolomite rocks.