SURFICIAL GEOLOGIC MAP OF THE TONICA 7.5-MINUTE QUADRANGLE, LASALLE COUNTY, ILLINOIS

The Tonica 7.5 minute Quadrangle is located in 89°07'30" and 89°00'00" west longitude and 41°15'00" and 41°07'30" north latitude. The goal of this research is to geologically map all of LaSalle County, Illinois. Funding for the project was provided by the Illinois State Geologic Survey, United States Geologic Survey and Nicor Inc. The geologic map was constructed using soils maps, borehole data and traditional field methods. The University of Illinois Agricultural Experiment Station published the soils maps used during the construction of this map. Well data provided by the Illinois State Geologic Survey was also used to determine bedrock depth and stratigraphy. Soil descriptions from Illinois were separated into groups with similar qualities. Soil groups were then assigned to stratigraphic units. The Cahokia, Peoria, Henry, Equality, and Lemont Formations are all present in the Tonica Quadrangle. Loess which is greater than 5 feet in thickness was designated as Peoria Silt. There are a few areas where the Equality Formation is present and there are several bedrock exposures of the Carbondale Formation along Bailey Creek. The Peoria silt consists of sandy silt to clayey silt. The Yorkville of the Lemont Formation is composed of calcareous silty clay to sandy loam diamicton unit with lenses of gravel, sand, silt and clay. The Equality formation is a bedded silt and clay diamicton with lenses of gravel, sand and organic debris.