SURFICIAL GEOLOGIC MAP OF THE PRAIRIE CENTER 7.5’ QUADRANGLE, LASALLE COUNTY, ILLINOIS

The Prairie Center 7.5’ Quadrangle is located in LaSalle County, Illinois. It covers the area between 88°52’30” and 89°00’00" west longitude and 41°22’30” and 41°30’00" north latitude. A surficial geologic map of the Prairie Center Quadrangle was constructed using soil maps from the Soil Survey of LaSalle County, published by the University of Illinois Agricultural Experiment Station. The soils and their descriptions, designated by numbers on the soil maps, were interpreted and sorted into formations present in Illinois. The soils were sorted into eight formations that were then named based on their relationship to Illinois formations: the Cahokia formation (QHc), Equality formation (Qe), Henry formation (Qh), Peoria silt (Qps), Lacon formation (Qla), Yorkville member of the Lemont formation (Qly), Batestown member of the Lemont formation (Qlb), and Bedrock. Of these eight types, four are present in the Prairie Center Quadrangle. The Peoria silt, which covers a large portion of the quadrangle, is composed of Wisconsin episode loess reported to be greater than 5 feet in depth. The formation is generally yellow-tan to gray clayey silt and does not contain many significantly weathered areas. The Yorkville member of the Lemont formation also covers a large section of this map. It is composed of 35 different soil types that fall into the category of Wisconsin Episode till. The member consists of dark gray silty clay or silty clay loam diamicton and oxidizes to an olive brown color. The third formation found in Prairie Center are small areas of the Equality formation, most often associated with the areas of the Peoria silt but in some cases are isolated within the Yorkville member of the Lemont formation. The Equality was distinguished from the other soils as lake sediments and bedded silts and clays. A small area of the Henry formation is present in the northeast section of the Prairie Center quadrangle. The Henry Formation intertongues with the Equality Formation throughout Illinois. It is composed mainly of stratified sand and gravel, and the soils within the Henry formation group are typical of Wisconsin Episode channel deposits. This project is part of the long-term goal of the Geology Department at Illinois State University to map all of LaSalle County using the Soil Survey method. It is being funded by the USGS, ISGS, and NICOR Inc.