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4.2.1.b Niagara Escarpment Glaciation
Michigan Basin
In the Great Lakes region, there is a vast depression in the earth’s crust, centered under the state of Michigan. Four hundred and forty-five million years ago, in an era known to geologists as the Ordovician, there was a sea in the Michigan Basin.
In the Great Lakes region, there is a vast depression in the earth’s crust, centered under the state of Michigan. Four hundred and forty-five million years ago, in an era known to geologists as the Ordovician, there was a sea in the Michigan Basin.At that time, North America was in the tropical zone. Land plants and animals did not yet exist, but the Michigan Sea was full of life. There were trilobites, crinoids (animals which look like plants), shellfish and corals.
On the bottom of the Michigan Sea, layers of sediment started to accumulate. The sedimentary material varied according to the changing times and conditions. One source of sediment was rivers which flowed into the sea, carrying sand and clay from nearby mountains. Another source of sediment was the hard shells or skeletons of calcium carbonate of the sea creatures. Shellfish and corals and the less known foraminifes, single celled organisms, are examples.
When their shells sank to the bottom and formed thick sediments of calcium carbonate and after a time of millions of years and through pressure, heat and chemical reactions, the sedimentary layers of sand, clay and calcium carbonate became stone. Sand became sandstone. Clay became shale. Calcium carbonate became limestone, and, when there was magnesium in the sediment, dolostone.
The Michigan Sea formed and disappeared several times during Earth’s history. Other layers of sediment accumulated, but after the Silurian era, layers of limestone and dolostone never again formed. Two hundred and fifty million years ago, the sea disappeared for the last time, and a long period of erosion began.
The Niagara Escarpment is the most prominent of several escarpments formed in the bedrock of southern Ontario. It is traceable from the Niagara River to northern Michigan, forming the spine of the Bruce Peninsula and Manitoulin and other islands in northern Lake Huron. It also extends into New York State and Wisconsin, roughly encircling the Michigan structural basin in the bedrock.
The rock layers of the Niagara Escarpment date from the upper Ordovician (445 million year ago) to the lower Silurian era (420 million years ago); a period of 25 million years. The rock below, and east of the escarpment is mostly Ordovician limestone. The rock on, and west of the escarpment is mostly Silurian dolostone. Because of this, the escarpment was once called the Silurian Escarpment.
The Niagara Escarpment is not formed by faulting. There is no displacement of the rock layers at the Escarpment, as shown by study or rock exposures and drillholes.
Erosion
When the Michigan Seas finally left about 100 million years ago, during the time of the dinosaurs, the Escarpment was exposed and was then eroded by weather, ice and streams. The Niagara Escarpment has a caprock of dolostone which is more resistant and overlies weaker, more easily eroded shale rocks. Through time the soft rocks weather and erode away by the action of streams. The gradual removal of the soft rocks undercuts the resistant caprock, leaving it standing as a cliff – the escarpment.
When weathering and erosion exposed the edge of the rock layer cake, the weak layers were broken down and carried away more quickly than the strong layers. When a thick strong layer has weak layers of rock beneath it, the weak layers will be removed by water, ice and wind leaving the strong layer unsupported as in Figure b. Eventually the overhanging strong layer breaks off and falls, forming a cliff or escarpment as in Figure c. Over time the pieces of the strong rock will be broken up and carried away. More of the weak layers will then be removed until the strong layer collapsed again. This cycle of erosion and collapses is repeated over and over and the escarpment gradually retreats in the direction of the erosion.
A series of sketches showing escarpment retreat from a to c.
This process has been going on since the time of the dinosaurs. The process was very slow; taking place in environments that gradually changed from quite warm too much colder as North America slowly drifted north. Glaciers were formed, advanced and retreated. The last glacier retreated from the Niagara Escarpment about 12,000 years ago.
Glacier movements and their melt waters leave clues on the land – the lithosphere – that can be used to reconstruct ice age events. For example, spillways and outliers can be seen on the landscape. See 4.2.1c and 4.2.1d for evidence you can find on the land today.
Impacts of climate change on water – the hydrosphere, and life forms – the biosphere – are seen in sections 4.2.2 and 4.2.3
ACTIVITY 1
- Find a contour map that includes the Great Lakes Basin in US and Canada. Look at elevations and find the elevation of the edge of the Michigan Basin. Hint check out the Niagara Escarpment first.
- a) When was it a sea?
b) How do we know that this was a tropical zone then? - What do we see now when hiking or driving about the area? Check the first photo in this section.
- a) Define escarpment.
b) Name the Ontario trail that runs 750 kms along the escarpment. - What role does erosion play?
- Why does talus always form at the bottom of these slopes?
ACTIVITY 2 – Research:
There are ancient cedars, over a thousand years old, living in the cracks along the Bruce Peninsula – check out the science behind these done by Doug Larson and Cliff Ecology Research Group (CERG) www.uoguelph.ca/botany/research/cerg/index.html
Look for other clues to discover the history of geological and glacial activity both on the ground and in historic maps and diaries. See Roger Chittenden’s reconstruction of the events following the retreat of the last glacier from the Crawford Lake, Bronte Creek and Hamilton area by using air photos, contour maps and historic documents.
Pollen core science documents the succession of vegetation since the retreat of the glaciers See 4.1.e for the pollen core results from Mono Cliffs Provincial Park, McCarston’s Lake. Jock McAndrews team from University of Toronto have been carrying out this research across southern Ontario.
Written works by Walter Tovell are the key to understanding what you see when touring the area eg The Niagara Gorge.