Guide to the Geologic Map of Illinois

Illinois’ surface may look flat today, but it is a relatively recent masterpiece painted by massive Ice Age glaciers. As the ice advanced and retreated, it ground up distant rock, dumped it across the state, and left behind ridges of till, sand‑filled meltwater channels, and wind‑blown loess blankets. Those deposits—collectively called glacial drift—masked the older landscape, carving new valleys and forming the gentle hills that define much of the prairie today.

This guide lifts that blanket away with a detailed geologic map, revealing the hidden layers of limestone, shale, sandstone, and ancient crystalline rock beneath. By tracing the sequence from the youngest Pleistocene drift through Tertiary and Cretaceous sands to the coal‑rich Pennsylvanian formations, listeners gain a clear picture of how millions of years of Earth’s history are stacked beneath Illinois. The result is an accessible tour of the state’s deep past, perfect for anyone curious about the forces that shaped the land beneath their feet.