The Rocks I Live On: Door County, WI

Home of the Niagara Escarpment



So... we know that there is a slight depression in the earth's crust under the state of Michigan. But what does that have to do with the Niagara Escarpment?!!

Well, unlike other rock formations, the Niagara Escarpment is not the result of a fault (a fracture in the earth's crust), but instead is a cuesta which was formed by differential erosion. Simply put, this means that underlying, soft rocks (shale) eroded away faster than the more resistant caprock (dolomite). This created a top heavy pile of rock that broke off, creating a cliff-like slope... 


Still confused?  Let me EXPLAIN...



Well, 445 million years ago, in an era known to geologists as the Ordovician era, there was a sea in the Michigan Basin. On the bottom of the Michigan Sea, layers of sediment started to accumulate (figure 1).










Figure 1


BUT, the sedimentary material varied according to the changing times and conditions, so there was some hard, durable rocks like Dolostone (Dolomite) , and some softer, flimsier rock like shale and sandstone (figure2). Picture the layers of rock like a gigantic layer cake of chocolate, white and poppy seed layers.























Figure 2



Well... this is all FINE when the rock is underwater, but as the accumulation of sediment builds up, and the rim of the giant bowl becomes bared to the elements above water, they face a harsh environment. Weather and streams take their toll on the sticking out rock layers.

However, as we learned earlier, the sticking out rock is not all the same. The Niagara Escarpment has a caprock (top layer) of dolostone which is more resistant and overlies weaker, more easily eroded shale rocks. Through time the soft rocks weather and erode away. The gradual removal of the soft rocks undercuts the resistant caprock, leaving it standing as a cliff - the escarpment.