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Escarpment Geology


The single factor that makes the Niagara escarpment so unique is its
geology, yet many people who live near it do not know how it was formed,
or even what an escarpment is. The following is a brief explanation of
this fascinating landform.

* The Escarpment: Part of a Bigger Picture*

It is much easier to understand what the escarpment is when you
understand the formation that it is a part of. The Niagara Escarpment is
the cliff face of a ridge that is at the edge of a much larger formation
called the Michigan Basin. The Michigan Basin is a roughly circular
depression in the earth's crust centered under the state of Michigan. It
is formed of layers of sedimentary rock that were created over 420
million years

 

* How Sedimentary Rock is Formed*

The Michigan Basin used to be the location of an inland sea called the
Michigan Sea. This sea has shrunk and grown in area several times
between 445 million and 420 million years ago (a period of 25 million
years).

In the times that this area was under water, the sea was full of life.
The creatures that lived there grew, reproduced, and died, like modern
sea creatures do, and like those modern creatures, their bodies were
left on the floor of the sea after death. Many of these animals were
invertebrates ? animals without a skeleton. They may have lacked an
inner bone structure, but like most modern invertebrates, many of them
did have hard outer coverings. These were things like corals, shellfish,
trilobites and foraminifers, the descendants of which can still be found
today. Their hard outer coverings were made of a material called calcium
carbonate.

When material like this falls to the sea floor, is worn into particles
and forms layers it is called biological sediment. This material mixed
on the sea floor with non-biological sediment: fine material washed into
the sea from rivers, made up of clay, sand and silt. These layers
hardened over time, through the downward pressure of the overlying
material and water, and through both warming and chemical changes in
composition.

Layers with a lot of sand in them became sandstone, which is fairly
weak. Layers with a lot of clay in them became shale, which is also
weak. Layers with a lot of calcium carbonate in them became limestone,
which is strong, or dolostone when magnesium was also present, which is
even stronger.

Because these layers were laid down along the bottom of the sea floor,
these layers gently tilt upwards at the edges where the old seashore
would have been. The last layers that formed underwater cover a smaller
area than those below them because the sea was shrinking for the last
time. These last layers were formed mainly from clay and sand, and so
are primarily shale and sandstone.


Fossils found in escarpment rock. Photo by Madeleine Ernst.

*The Escarpment is Part of a Cuesta*

Around the perimeter of the Michigan Basin, the edge of the sedimentary
rock layers is tilted upwards. Where this edge is exposed it has become
a ridge formation, also known as a cuesta. Cuestas are ridges formed by
gently tilted rock layers. Every cuesta has a steep slope where the rock
layers are exposed on their edges, called an escarpment. They also have
a more gentle slope on the other side of the ridge called a 'dip slope'.
When you are driving across the Niagara Escarpment from west to east
(try it on Hwy 5 for example) you drive up the dip slope, and down the
escarpment.

Cuestas are formed for two reasons. One reason is that there are gently
tilted rock layers present. The other is that the layers are of
different hardnesses and so they erode at different rates.


Red Hill Creek below Albion Falls. Photo by Alan Ernst.

The area around the escarpment, where most of us live, has been eroded
by water, wind, frost and glaciation. The cuesta has also been subject
to the forces of erosion, but it is made up of layers of rock that are
more resistant to erosion than the land around them. In our Niagara
Escarpment, the low layers are made up of sandstone and shale, while the
top layers are limestone and dolostone.

Because of this, the lower weaker layers erode from under top layer,
until the top layer has no support and falls off in large pieces,
forming a cliff face with jumbles of rock at the base. /This cliff face
is the Niagara Escarpment!/


Habitats of Hamilton and Halton © Hamilton Naturalists' Club