Steven Dutch University of Wisconsin - Green Bay
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The Niagara Escarpment is the edge of a thick series of dolomite layers of Silurian age. The rocks are resistant to erosion and stand up in relief as a prominent line of bluffs. On the map below, Precambrian and Cambrian rocks are shown in brown, Ordovician rocks in yellow, Silurian rocks in purple, Devonian rocks in light green and younger rocks in dark green. The Niagara Escarpment is shown in red. South of Lake Winnebago (LW) the escarpment is often concealed by glacial deposits, as it also is in upper Michigan. The term "Niagara Escarpment" is sometimes used in other places where the edge of the Silurian rocks form bluffs, especially in Iowa.
The Niagara Escarpment is a landform called a cuesta, where a gently-sloping layer of rock forms a ridge. One side of the ridge has a gentle slope, a so-called dip slope that is essentially the surface of the rock layer. The other side is a steep bluff. The Niagara Escarpment owes its prominence to both the resistance of the Silurian dolomite layers and the relative softness of the Ordovician and Devonian rocks on either side. On the map below, note that the main basins of all the Great Lakes except Superior are defined by belts of soft Devonian rocks. Also, the Ordovician rocks have also been eroded away to create lake basins or lowlands, notably Lake Winnebago (LW), Green Bay (GB), and Georgian Bay in Lake Huron. The Silurian rocks stand up as a ridge that forms Door County (DC), the Garden Peninsula (GP), part of Manitoulin Island (MI) and the Bruce Peninsula (BP). The Escarpment swings southeast across Ontario and into New York, where it is cut by Niagara Falls (NF).
The diagram below shows the escarpment in three dimensions, looking southeast. An imaginary trench has been dug across Michigan, showing the structure of the underlying rocks. The rocks sag downward beneath Michigan, a structure called the Michigan Basin. The Silurian rocks dip gently in toward Michigan, and their outer edge forms the Niagara Escarpment.
In this view looking north along the escarpment, we see the partially submerged cuesta of silurian rocks (yellow). The soft underlying Maquoketa Formation (blue-gray) has been largely scraped away by glaciation. The resistant Sinnipee Group dolomites (green) are partially flooded by Lake Winnebago and Green Bay.
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In the geologic map at left, Devonian rocks are blue, Silurian yellow and Ordovician rocks are brown and green. We can see that south of Fond du Lac the Silurian escarpment is deeply embayed and trends nearly north-south. North of Fond du Lac, the escarpment is much smoother and trends NNE, and the same is true of the underlying Ordovician rocks as well. |
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Created 18 June 1999, Last Update 18 June 1999
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