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

The Merna 7.5 Minute Quadrangle is located in McLean County, central Illinois. The surficial geology of the Quadrangle is dominated by Quaternary glacial sediment units. Surface deposits are a result of the geologically recent Wisconsin glacial episode. The Quaternary units overly Pennsylvanian bedrock strata. The glacial drift thickness exceeds 100 feet everywhere in the quadrangle. Quaternary sediment units include diamicts of the Wedron Group, Sand and Gravel of the Henry Formation, Silt and Clay of the Peoria Formation, Colluvium of the Peyton Formation, and gravel sand and silt of the Cahokia Formation. Units that are present with less than five feet in thickness were not considered mappable based on the contour interval of the map. The Peoria and Wedron Formations comprise most of the surficial geology of the Quadrangle. The Henry Formation is found only where outwash deposits are thick enough to map. The Cahokia Alluvium is present in mappable abundances in few areas, which are flood plains. The Peyton Colluvium is only found in one small area in a closed depression. Formations were interpreted and mapped based on lithological and thickness descriptions of well data received from the McLean County Soil Survey. The well data was also used in generation of cross sections of the quadrangle by correlating data from multiple described wells. Soil maps from the McLean County Soil Survey were also used in the generation of the final surficial geologic map. Once mapped based on the provided data, the quadrangle was then field checked to correct and perfect any formation contacts.