Quaternary Deposits in Cook County

The Quaternary age deposits in Cook County are mostly the result of glacial processes, but they also include post-glacial sediments. Sand and gravel used for road and building construction are recovered from some of these deposits.

Locally, layers of sand and gravel in the Quaternary deposits supply residential users with good quality groundwater. The greatest threat to the groundwater resources in these areas is from surface contamination of relatively shallow aquifers.

More than half of Cook County is underlain by glacial till of the Wadsworth Formation, a clay-rich diamictite. The low, flat plain in the east-central part of the county is mostly silt and clay of the Equality Formation. These sediments were deposited in the former glacial lake, are composed of silt and clay and are not considered aquifers.