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.