It was primarily in this third layer that Missouri caves were formed. Before the first big upheaval 42 to 46 million years ago, this shallow sea consisted of 60 feet of water over about 200 feet of silt. Caves began to form after this upheaval occurred. In all, six ice ages have occurred over North America at widely spaced intervals of geologic time. Since the first big upheaval, there have been three tremendous earthquakes.
Geological evidence indicates that each quake was lower in magnitude than the previous quake.
We had our first earth upheaval 42 to 46 million years ago and our second upheaval 16 to 17 million years ago. The second quake predated the last ice age by several hundred years. The last, or third of these tremendous quakes was the New Madrid Earthquake in 1811-1812. It was the strongest recorded earthquake in the history of the North American continent but geological evidence indicates that it was lower in magnitude than the previous two quakes. Two hundred damaging aftershocks followed.