During the Ordovician and Silurian periods, a shallow extension of the Panthalassa ocean covered much of the continent of Laurentia, whose rocks now make up the largest part of North America. Along the shores of this ocean there was a large embayment, now called the Michigan Sea since it now forms a large bowl-shaped depression in which Michigan now sits. As time passed, the skeletons of marine creatures collected on the bottom of the Michigan Sea, forming a thick layer of limestone. The Michigan Sea was uplifted into dry land as the Earth passed into the Devonian and later periods of the Paleozoic Era, the Rheic Ocean closed up to form Pangaea, and the Appalachian Mountains were raised into the highest mountains on Earth.

During the intervening 300 million years, other sediments were deposited on top of the limestone, including large beds of coal. Also, magnesium infiltrated the limestone, turning it into a highly resistant layer of dolomite.

By the time of the Pleistocene epoch, most of the sediments on top of the dolomite had been eroded away. During the most recent ice ages, the continental ice sheets that covered the area scraped most of the rest off. The ice sheets were also able to chip away at the front of the dolomite layer, as well as gouge large depressions around the inside edges of the bowl, on top of the dolomite. After the ice sheets retreated 13000 years ago, these depressions filled in with glacial meltwater to form Lake Michigan, Lake Huron, and Lake Erie.

Now, the rim of the bowl (the northern edge of the dolomite layer) slopes gently up out of these lakes, to form a 2000 mile (3000 kilometer) cuesta starting in northwestern Iowa and snaking its way through Illinois, Wisconsin (holding up the Door Peninsula), Michigan's Upper Peninsula, and bending south into Ontario (forming Manitoulin Island, then onto the Bruce Peninsula, bending east onto the Niagara Peninsula), and into New York, ending just west of Rochester.

Along the northern face of the cuesta, where the dolomite has been broken away, lies the Niagara Escarpment. The escarpment is barely recognizable in Iowa, and has been buried by glacial till in parts of Wisconsin and Michigan, but it is dramatically visible on the western edge of the Door Peninsula and throughout southern Ontario, whose tourism industry seems to be concentrated along the escarpment.

Underneath the dolomite lies a thick layer of much weaker shale. The escarpment retreats as the face of this shale is eroded, undermining the dolomite and evenually causing parts to collapse.


_____________                    _____________           _____
             \                                \               \
 Dolomite    |                                |               |
             |                                |               |
_____________/                   _____________/          _____/
            |                           /                    |
   Shale    /                          /                     /
           |                          /                     |
           |                         /                      |
           |                        /                       |       __
____________\                   ___|                       _\      /  \oOo..
_____________) <- Sandstone     ____)                      __)oOooO\   `- .--.
            |                      /                        |oOo__oO\   //    \
   Shale    |                     /                         |oO/  \Oo\_/O\     \o
            |                     |                         |O(    )OO__OO\    |oO
            |                     |                         |Oo\__/oO(__)oo\___/OoOo

The escarpment gets its name from the most rapid (and best known) force undermining the Niagara Dolomite: the Niagara River, which drains the eastern end of Lake Erie, and flows north into Lake Ontario. Where it crosses the Niagara Escarpment, it has formed a gorge with the dramatic Niagara Falls at its southern end. Just east of Niagara Falls at Lockport, the Erie Canal leaps up the escarpment in an impressive series of locks.

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