SURFICIAL GEOLOGIC MAP OF THE HEBRON 7.5-MINUTE QUADRANGLE, MCHENRY COUNTY, ILLINOIS

Detailed geologic mapping was conducted for the Hebron 7.5-minute Quadrangle in north-central McHenry County, Illinois. This project was part of the greater effort between the Illinois State Geological Survey, Illinois State Water Survey, and McHenry County to better understand the structure, stratigraphy, and hydrogeologic properties of the Quaternary geologic units in a larger study area. The methodology for this project included field observations, drilling, geophysical surveys, the acquisition of county wide soils data, and map production through ArcGIS. Geologic units were interpreted largely from the evaluation of NRCS soils maps and parent material interpretations.

The units exposed within the mapping area are mostly associated with the Harvard Sublobe of the Lake Michigan Lobe during the Wisconsin Episode glaciation. Haeger diamicton, deposited during the most recent readvance of the sublobe, covers the upland areas in the western and eastern portions of the mapping area. The flat lowland between these uplands is a broad deposit of fine grained lacustrine sediments of the Equality Formation. The geologic history of this deposit is poorly understood. Sand and gravel outwash deposits of the Henry Formation outcrop mainly east of the town of Hebron. More recent deposits of alluvium and peat fill modern stream valleys and bogs throughout the mapping area. This surficial map was used in part to develop a broader 3-D geologic model aimed at better understanding the hydrogeologic characterization of the local and regional sand and gravel aquifer systems.