Geology
The cave was, and in fact still is being formed in the Mississippian limestone (CaCO3) and dolomite (CaMg(CO3)2) (or karst common to the region) by water dissolution, and includes cave formations such as stalactites, stalagmites, rimstone dams, flowstone, and soda straws. The cavern is a branchwork type cave, which generally consists of a central cavern and stream with smaller passages and tributaries branching off. Numerous circular, crater-like depressions are evident in the area surrounding the entrance, easily visible from the air or space. These are typical of both the area and karst topography in general
Read more about this topic: Illinois Caverns State Natural Area