 Visitors to the park can view the ancient geological history of Minnesota as they walk the path leading from the falls down to the Mississippi River . The uppermost layer of soils and gravels of Minnesota were deposited by the most recent Ice Age , about 10 @,@ 000 years ago . Four great ice ages have swept away all traces of the more recent Mesozoic and Cenozoic eras in the Twin Cities area , however standing at the upper falls , one is standing directly on the Platteville Limestone Formation laid down during the Ordovician Period of the Paleozoic Era . During the Ordovician Period , about 450 million years ago , the North American continent was situated along the equator and a warm shallow sea covered much of Minnesota . Sea life was abundant and a large number of marine fossils including corals , bryozoans , brachiopods , clams , snails , cephalopods , and trilobites can be found in the limestone and shale sediments at several areas in the Twin Cities and along the Mississippi River in the park . 
