SURFICIAL GEOLOGIC MAP OF THE CHENOA 7.5 MINUTE QUADRANGLE, MCLEAN-LIVINGSTON COUNTY, ILLINOIS

Surficial geologic mapping at the 1:24,000 scale has been completed for the Chenoa 7.5 Minute Quadrangle in the north-central part of McLean County, IL. The longitude of the map ranges between 88°45ˈ to 88°37ˈ30”, and the latitude ranges from 40°45ˈ to 40°37ˈ30”. The map was created as a digital PDF file allowing for quick access to the map and introduces compatibility to other mapping programs for local geology to be completed. The map was mostly ground moraine and it was constructed using soils survey data, water well log, and outcrop descriptions. Using well log data, we were able to obtain the thicknesses of some of the formations; The thickness of the Quaternary units can span over 100m in paleovalleys. The youngest Quaternary units present are within the Mason Group consisting of the Peoria Silt and the Cahokia alluvium. The Cahokia formation contains poorly sorted sand, silt, and clay sized grains that are associated with modern stream systems with thicknesses less than 2m thick. The Peoria Formation contains yellow-brown silt and clay, which is interpreted as loess. The Henry Formation is a sand and gravel unit that forms terraces in valley train outwash systems and alluvial fans varying in thickness between 3-20m. The Wedron Group dominates the north east to the central part of the quadrangle and it contains the Lemont Formation, which is a grey diamicton that is deposited on a ground moraine with thicknesses ranging between 5 and 10m. The Cahokia alluvium occurs in the north-central areas of the Quadrangle and the Peoria silt is found in the southwest portion of the quadrangle. In some regions in the north west area of the Quadrangle the Henry Formation is layered within the Lemont Formation. After the map was drafted a field check was conducted to validate contact placement and water well records.