SURFICIAL GEOLOGIC MAP OF THE GERMANTOWN HILLS 7.5 MINUTE QUADRANGLE, WOODFORD COUNTY, ILLINOIS

The Germantown Hills Quadrangle is located in Woodford County and spans 40⁰45’0” to 40⁰52’30” north latitude and 89⁰30’0” to 89⁰22’30” west longitude. The surficial geology of the area primarily consists of tills and floodplain geology with the exception of minor deposits of organic material near the Illinois River channel in the northwestern corner of the quadrangle. Quaternary units exceed 100 meters in thickness where they overlie buried Pennsylvanian bedrock. Major Quaternary units within the Germantown Hills quadrangle consists of the Wedron Group till units, Mason Group outwash units, and Cahokia alluvium deposits. The Cahokia alluvium exists with a thickness of about 5 meters and is associated with Holocene-aged drainage systems and along the Illinois River. Intertonguing the Cahokia alluvium, Grayslake Peat are local deposits of 1 to 10 feet thick found near the river. The Henry Formation, a unit of the Mason Group, is also located along the river channels. The Henry Formation is associated with yellow to grayish brown calcareous sand and gravel deposits ranging from 5 to 15 feet thick. The Lemont Formation of the Wedron Group is a till unit positioned surrounding the alluvium deposits near the river channels. Till units are predominately diamict clays and gravel which range in color from a red to grayish-blue with a thickness of 70 meters. Covering the rest of the map is an outwash plain comprised of sand and gravel known as the Henry Formation from the Mason Group, which can be 7 to 20 meters thick. The surficial geology map was prepared using soil maps from the Woodford County Soil Survey. Parent materials of each formation were identified and grouped consequently creating formation boundaries. Units were required to be at least 5 meters thick in order to be mapped. In order to qualify the viability as mappable units, evaluations of water well data provided from the Illinois State Geologic Survey were administered to specify the thickness of surficial deposits. Once boundaries were defined, a field check was conducted to modify contact locations.