Geologic Setting in McHenry County, Illinois

The surficial geology of McHenry County has been developed almost completely as a function of pre-Wisconsin and Wisconsin Episode glaciations, which specifically include ice marginal/subglacial and proglacial deposits (moraines/till and outwash, respectively). The morphology of the landscape includes some of the most prominent preserved moraines and outwash fans in the Midwest. The sediments include at least 2 sequences of pre-Wisconsin Episode deposits and 3 Wisconsin Episode deposits, which cumulatively range in thickness between 3 and 150 meters thick. These deposits generally lie atop a regionally extensive fractured carbonate bedrock unit. However, that unit is sometimes fully incised by steep bedrock valleys, which may have as much as 100 meters of relief. These bedrock valleys are largely filled with sequences of pre-Wisconsin Episode proglacial outwash and lacustrine deposits.

Shaded relief of northeast Illinois

Shaded relief of northeast Illinois

Three major aquifer systems in the Quaternary deposits are utilized in McHenry County and have been the focus of this study and previous studies (Curry et al., 1997). These deposits include a lowermost sand/gravel unit most often in contact with fractured carbonate bedrock. This unit is likely associated with Pre-Wisconsin Episode glaciations and interglacial periods of erosion. A second aquifer system is associated with the first advance of the Wisconsin Episode glaciation in McHenry County, but also often includes proglacial deposits of the Pre-Wisconsin Episode retreat. These two deposits are often unconformably separated by a preserved paleosol. Lastly, an extensive surficial aquifer includes late stage outwash deposits of the last Wisconsin Episode glacial retreat and other modern alluvial deposits. In McHenry County, there are several major glacial meltwater discharge paths that were likely reoccupied by multiple glacial events. Thus, in these areas all three major aquifers are often interconnected.

Sample cross section of glacial deposits from south-central McHenry County

Sample cross section of glacial deposits from south-central McHenry County