Map of Geologic Cross Sections Across the Mahomet Bedrock Valley Now Available

Map of Geologic Cross Sections Across the Mahomet Bedrock Valley

Mahomet Bedrock Valley Cross Section
Andrew Stumpf, Associate Quaternary Geologist for the Illinois State Geological Survey (ISGS), recently completed a map product titled Geologic Cross Sections Across the Mahomet Bedrock Valley, Champaign, Ford, McLean, Piatt, and Vermilion Counties. This publication is an outcome of research conducted in east-central Illinois that was funded by a private water supply company (Illinois American Water), the Illinois Department of Natural Resources through the states’ Water Supply Planning for Illinois program, and the State of Illinois through general revenue funds to the ISGS. The objective of the study was to collect and analyze additional geologic and hydrogeologic information that could be used by the Illinois State Water Survey to further calibrate a groundwater flow model of east-central Illinois. A flow model with this level of detail is a powerful tool that can greatly aid in managing groundwater supplies. Such a tool is optimized by developing sound geologic information initially and then combining the geologic data with hydrologic data. Quantifying existing and possible future groundwater withdrawals and understanding their impacts is critical for ensuring the sustainable use of the resource.

The geologic cross sections were developed in the process of building a three-dimensional geologic model of the study area. The cross sections show the distribution of earth materials at and below the land surface across a five-county area in much more detail than was possible in the geologic model. The study area is centered over the ancient Mahomet Bedrock Valley, which has a bottom lying more than 300 feet below the land surface. Before the first glaciation in Illinois, the river running through the bedrock valley formed part of a much larger midcontinental river system that had headwaters in the Appalachian Mountains in West Virginia and Kentucky. It flowed west and then south through east-central Illinois, eventually reaching the Gulf of Mexico along a similar course as the Mississippi River today.

The earth materials shown in cross section are primarily the remnants of three main episodes of glaciation that occurred during the last 1.6 million years or so. The glacial tills were deposited under ice sheets that flowed into the area from the north and northeast. These ice sheets were centered over northern Canada. The intervening units of gravelly sand and silty clay were deposited in front of the ice sheets where meltwater drained or was ponded. The very thick deposits of sand and gravel mapped at the bottom of the Mahomet Bedrock Valley are known as the “Mahomet Sands.” These deposits, along with overlying bodies of sand and gravel (e.g., the Grigg tongue, Pearl Formation), compose the Mahomet aquifer. The aquifer is an important water resource in east-central Illinois and the sole source of groundwater for nearly 1 million people.  

The cross sections can be downloaded from