During the long inter-glacial period between the Illinois Episode and the subsequent Wisconsin Episode, loess drift filled the bedrock valleys and covered the hills to a lesser extent. It is also, most likely, during this inter-glacial period that earthquakes helped to raise this particular portion of the state, although little is known about their intensity or times of occurrence. What is known is there are several fault lines in this area, two of which are among the longest in the state.
These latter two are the Plum River and Sandwich Fault Zones. The Plum River Fault Zone starts in far north-central Ogle County and runs west into Iowa. The Sandwich Fault Zone starts somewhere near Joliet, in Will County, and runs northwest to Mount Morris, in Ogle County, and may pass through Nachusa Grasslands property.
Also, to the south, the Peru Monocline, which is a strata fold over a deeper unnamed fault, starts in far northwest Livingston County and runs north by northwest into central Lee County. This fault is probably responsible for the bedrock exposure as cliffs in Franklin Creek State Park.
And so, we have a roughly triangular area where the ideal bedrock formation is close enough to the surface, the ground sufficiently porous to allow rapid water penetration, and the climate such that an ample amount of rain and snow occur during most years to form karst terrane.
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