Over 300 million years ago, an inland sea covered most of the Midwest. This sea teemed with billions of microscopic creatures, shell fish, and other denizens of the deep. When they died their calcium-rich carcasses settled into the mud at the bottom.

After a while, (say 40-60 million years or so later) the sea dried up, the mud and sea creatures fossilized. Eventually (that would be another 40-50 million years) the mud and sea creatures were covered over by the earth. During the recent ice age (in geological terms the last ice age is recent) huge glaciers slid south over Southwestern Indiana exposing a small area of ancient bedrock between Bloomington and Bedford, Indiana.