Illinois State Museum

• Glaciers

• Glaciers and Landforms

• Glaciers and Agriculture

• Narrative

• Terrain

Lake Deposits

Sometimes we find fine-grained silt and clay deposits in the middle of fields. These are the beds of ancient ponds and lakes that were filled with muddy glacial runoff water. The silts settled to the bottom. The ponds or lakes eventually dried up and disappeared except for the deposits.

Erratics

Erratics is a name given to boulders found in Illinois that do not match the Illinois bedrock. Those rocks match those found in the bedrock of Canada and northern states such as Minnesota. We know they were not formed in Illinois because they are igneous (formed from hot, liquid rock such as lava) or metamorphic (rock that is changed by pressure, heat, or water). Illinois bedrock is sedimentary (settled materials brought by wind, water, or glaciers). Those glacier-moved rocks are still visible today in the far northwestern and the far southern tips of the state. These areas are called driftless because they have no layers of loess or glacial deposits.

End Moraines

An end moraine is a large, curved ridge of till that was once the end or margin of a glacier. They formed when the rate of the glacier's advance was equaled by the rate of its melting (meaning the edge of the glacier was staying in one place on the land). The ice kept moving forward with its debris, dropping it as the edge melted. These end moraines can be seen near Marseilles, Bloomington, and Shelbyville. (see the map for details)

Model of end moraine made by Mona Colburn, ISM research assistant

Watch this animation of an end moraine.
Visit this Web site on moraines by the Illinois State Geological Survey.