SURFICIAL GEOLOGIC MAP OF THE WASHINGTON 7.5 MINUTE QUADRANGLE, TAZEWELL COUNTY, IL

Abstract

This study used soil surveys as well as LIDAR information to convey different Quaternary units located in Tazewell County, Illinois. A 7.5 minute map of the Washington Quadrangle (at a scale of 1:24,000) to display several different units located in that area. There are four major units located in this study: The Peoria Silt, the Tiskilwa Formation, the Henry Formation, and the Cahokia Formation. The majority of this map is covered in the Peoria Silt, a wind blown sediment that is silt-clay in size and was derived from the recent (Wisconsin) glaciation; this silt increases in thickness from west to east. The Tiskilwa formation is another major unit in this area and is a glacial till (diamicton) that has a clay-silt matrix, large boulder clasts, and is often red in color. The Henry Formation is composed of gravel and sand and is valley outwash plains, and is mostly found next to stream cuts along side the Cahokia formation. The Cahokia formation is a clay and silt sediment interbedded with sandy layers and is interpreted to be post-glacial derivation of sediments deposited in stream terraces from the recent glaciation events.