Where did the sand & gravel come from?

"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)."

Learn more by clicking these links to the Illinois State Geological Survey's website:

Geologic Setting in McHenry County, Illinois

Surficial Geology of Wauconda Quadrangle Lake and McHenry Counties, Illinois