SURFICIAL GEOLOGIC MAP OF THE ARROWSMITH 7.5 MINUTE QUADRANGLE, MCLEAN COUNTY, ILLINOIS

A surficial geologic map of the Arrowsmith 7.5’ Quadrangle was constructed, spanning from 40° 22’ 30’’ to 40° 30’ north latitude and 88° 45’ to 88° 37’ 30’’ west longitude. Sediment unit parent materials were identified using McLean County Soil Survey data, they were then grouped together to create formation boundaries. Thickness of major units was determined using water well data, and only considered present at the surface if greater than 2 meters in thickness. Quaternary sediment units deposited during the Woodfordian glacial advance and modern stream alluvium dominate the surficial geology. Quaternary units reach over 100 meters in thickness where they overlie buried bedrock valleys. Major Quaternary mapping units include the Wedron Group, the Henry Formation, the Peoria Formation, and the Cahokia Alluvium. The Wedron Group includes the Tiskilwa till and Lemont Formation, which consist of diamicton units that are interbedded with proglacial river and lake sediments. The Lemont Formation is grey in color and exists to the north of the Bloomington Moraine, while the Tiskilwa till is pink in color and lies to the south of the Bloomington Moraine. The Henry Formation is prominent in the northern half of the quadrangle (up to 10 meters thick) and consists of sand and gravel that was deposited in glacial rivers and outwash fans. The Peoria Formation (up to 3 meters thick) forms the top layer in much of the southern half of the mapping area and consists of fine-grained yellow silt/clay and is interpreted as loess deposits. The Cahokia Alluvium (less than 5 meters thick) is made up of modern river deposits of sand, gravel and silt. A traditional field check was later conducted to ensure that contacts were properly placed. Cross sections were also constructed by subsurface correlation of water well records.