Imagine a broad, shallow bowl of rock buried in the ground; you know that it is there because sections of its rim poke above the surface. This would be a good model of the Michigan Basin, centered on the lower Peninsula of that State. The rock bowl is composed of Lockport Dolomite, deposited in a shallow, Silurian sea, 420 million years ago; the rim itself is known to geologists as the Niagara Escarpment. The ground inside the bowl is younger than the rim, having gradually filled in with numerous layers of sediment; the ground immediately outside the rim is older, composed of Ordovician and Cambrian deposits.
As discussed in my Niagara Falls blog (July 12), that famous cascade has been cutting through the Niagara Escarpment for the past 12,000 years. From the Falls, the Escarpment curves north-westward across Ontario, Canada, and forms the Bruce Peninsula along Georgian Bay. Continuing westward, this Silurian rim of dolomite is exposed as Manitoulin Island and forms the southern coast of the Upper Peninsula of Michigan. Curving to the southwest, it becomes the Garden Peninsula of Upper Michigan and the Door County Peninsula of eastern Wisconsin; between these two fingers of land, the Escarpment surfaces as a chain of islands in Lake Michigan.
Continuing southward, the Silurian Rim forms most of the south-western coast of the Lake, from Door County to Chicago. Up to 800 feet thick in some areas, this ancient bedrock stands out as a prominant line of cliffs (known as The Ledge) on the east side of Lake Winnebago. South of Chicago, the Escarpment curves eastward across northern Indiana and Ohio but is buried by a thick layer of glacial till in that region; only quarries and deep river valleys reveal its presence.