Geomorphology of the Great Lakes / St. Lawrence Region

Introduction The Wisconsin Glacial Period The Forming of the Great Lakes The Creation of the St. Lawrence Geomorphology of Montreal The Features Today Geological Time Scale Bibliography

The geomorphology of the Great Lakes-Saint Lawrence region is quite interesting even to a novice in the field given that the Saint Lawrence River did not originally connect the Great Lakes to the Atlantic Ocean. In fact, the Great Lakes did not even take their present shape until the last glacial period ended. Hence, knowing these processes enhances our understanding of how the earth functions.

Radiocarbon dating can age shells up to 20 000 years old, and wood and bones up to 70 000 years old.

Lakes forming and disappearing; rivers flowing into areas which now occupy farms are examples of the natural processes that occur on the planet and continue to occur over time. Luckily, much of our curiosity has been quenched by the availability of radiocarbon dating which is a form of radiometric dating. Radiocarbon dating can date shells up to 20,000 b.p. (before present) and wood and bones until 70,000 b.p.by measuring the radioactive decay of an organism. However on a geological time scale, 70,000 years is not much time; hence, most of the information collected pertains to the last ice age which receded approximately 10,000 years ago. The last glacial period was actually the most important period with respect to the natural topography of the region and is appropriately the first part of this section. Consequently, a better understanding of glaciers will lead into more specific explanations including the origin of the Great Lakes, St. Lawrence River, Montreal island, and some geomorphologic phenomena.

Glacial periods have been connected to the Earth's erratic rotation.

The last ice age began 75,000 years ago. The Great Lakes-Saint Lawrence region was covered by the Laurentides ice sheet.

There were many glacial periods in the history of the Earth. Some explanations have been devised to explain the phenomena. The most recent has been that our planet experiences erratic rotation on a cyclical basis which correlates well with the glacial periods of the planet. When this occurs, it changes the amount of solar radiation entering the atmosphere. This process in effect cools the Earth down by as much as 10 degrees centigrade. Such a change in temperature causes a catalyzing effect which typically lasts 20 to 50 thousand years, morphing the seascape and landscape. The most recent and well documented glacial period in the Great Lakes-St. Lawrence region is called the Wisconsin, named after the area in which most of the evidence was found.

The Wisconsin glacial period can be separated into three periods:

• Early (75,000-64,000 b.p.)
• Middle (64,000-23,000 b.p.)
• Late (23,000-10,000 b.p.)

Approximately 75,000 years ago, the temperature on the planet began to decrease dramatically; hence, ice caps on highlands began to accumulate more snow in winter than it would lose in the summer. Year after year more snow packed down until three great continental ice sheets formed and began to move south. It then molded the surface into new shapes under the tremendous pressure exerted by the ice. The three ice sheets were: the Laurentides, the glacial complex of the High Arctic, and the Cordilleran glacial complex.

The Great Lakes- St. Lawrence region was covered only by the Laurentides ice sheet which could be further broken down by the Labrador New Quebec Ice and the Labradorean Sector. Eventually the rotation of the Earth changed to allow more solar radiation to penetrate the atmosphere altering the climate once again. The powerful ice sheets then recede back towards the North bringing on our present interglacial period called the Holocene starting 10,000 b.p.

The forming of the Great Lakes was a long process of erosion and dislocation of soil.

Ice sheets gouged soils and less resistant rocks from the area.

The Great Lakes were ice free 5,000 b.p.

No lakes of unusual size existed where the Great Lakes are now located before the the first glacial advance. The forming of the Lakes has been a long and arduous task of erosion and deposition of huge masses of ice which pushed over the area several times. In fact, the Lakes owe there existence to these processes.

The area was typified as an upland of low relief on the tip of the Shield. Hence, the area was on a higher ground and could not resist the awesome pressure exerted by the ice. As ice sheets formed and migrated south, soils and less resistant rocks were chiseled out of the plain now known as Lake Superior. Moreover, the creation of the depressions was further enhanced by a morphasis of the lithosphere. The ice in central Canada caused the lithosphere to recede a little and tilted the area of the Great Lakes toward the ice advancing from the north.

Lakes formed behind dams of previously existing moraines, which are heterogeneous debris composed of gravel, sand and silt, deposited by ice. The smaller earlier lakes existed in the region during interglacial periods and probably inhabited the same area as the Great Lakes. The same process continued during the Wisconsin through a number of small advances and retreats, destroying the older moraines and other deposits. By 16,000 b.p. depressions for the Great Lakes were fully formed and the region became ice free by 5,000 b.p.

The Saint Lawrence Valley lies between two immense rock plains: the Canadian Shield and the Appalachian Plateau.

Some rivers carve their own valleys, like the Niagara, whereas the St. Lawrence flowed through the valley many years after its creation. Water entered the valley after the retreating Wisconsin. When the last ice age retreated it flooded a huge area which covered most of lower Quebec and Ontario. This was named the Champlain Sea. As water levels decreased, some overflow ran down from Georgian Bay to the Ottawa Valley to the central St. Lawrence. The Upper St. Lawrence drained Lakes Erie and Ontario until the water levels fell to their present levels. Link by link the water continually inched eastward until it reached the Atlantic to discharge its runoff.

Measurements of the age of abandoned river channels allow geomorphologists to determine the formation of the modern river system. The Ottawa and Saint Lawrence juncture is believed to have occurred approximately 9,000 b.p., whereas the channel the Great Lakes-Saint Lawrence emerged less than three thousand years ago. Hence, the river is too young to have worn deep trenches in the underlying stone. An older river would have formed a graded channel through the bedrock making the Saint Lawrence Seaway unnecessary to construct. Instead the Saint Lawrence River runs along rock always seeking the lowest course possible.

Montreal has the three main components which exist on every continent.

The city is on a seismic zone.

Some features on the island were caused following the Wisconsin.

Montreal island has three components which exist on every continent:

• a continental shield core
• thin ancient peripheral sediments covered by a structural plain
• fold mountains of different ages surrounded by offshore sediments

The Grenville fault escarpment lies to the west and north of Montreal and marks the Laurentian Shield, which is an ancient Pre-Cambrian Shield landscape. The consequence of the escarpment is that a seismic zone exists under the city causing the occasional tremor.

Mount Royal is raised over a plain of Paleozoic sediments sitting between the Canadian Shield and the Appalachian mountains. Some of the features on the island have been caused by the postglacial period following the Wisconsin.

Isostatically depressed plains from the Laurentides ice sheet were flooded by shallow waters of the Atlantic and the Champlain Sea, leaving heavy deposits of marine clay over the lowlands. In time deep sand deltas were built on the Grenville scarp front after more ice decayed. Once the water levels lowered and funneled into the Gulf of the Saint-Lawrence, sand scattered over the exposed clay. Eventually the water lowered enough whereby Mount Royal became an island with the beaches and slopes seen today.

Rapid earthflow has been documented in the Saint Lawrence Valley and the Saguenay River-Lac Saint Jean region.

Kame is usually linked to ice-contact features.

Some caves originate from a karsk.

There are many features along the St. Lawrence River which are important to discuss to understanding the geomorphology of the region today. Here are some brief examples:

• Rapid Earthflow:There are extensive deposits of highly sensitive clays in Saint Lawrence River Valleys and the Saguenay River-Lac Saint Jean region. The clays are very sensitive to liquification and cause rapid earthflows. The cause of this phenomena is linked to postglacial silt and clay deposits from 12,000 to 9,000 b.p. The deposits were later covered by sand and gravel reducing surface runoff. This in effect allows the clay deposits below to become saturated, transforming them from a brittle solid into a dense liquid. Earthflows are triggered by stream erosion on terraces, tremors or heavy traffic and can dislocate several hundred meters of soil in a few minutes.
• Kame Deltas:The term kame is simply a word to suggest a landform created through ice-contact composed of sand and gravel. Hence Kame deltas are deltas containing ice-contact sediments. Many such deltas exist near the Sept-Îles area in Quebec, and in southern Ontario.
• Caves:Most caves on the Ottawa Valley, the Saint Lawrence Valley, Lake Saint John Lowlands, Gaspé Peninsula and Anticosti Islands are generally small and young. These caves are commonly created by a karsk and a sink. In short, water will enter a whole enlarging the size of the joints and cavities in a carbonate rock such as limestone. In some cases these holes become large enough to be considered a cave such as the ones in the aforementioned areas.

We are now in the Holocene beginning 11,000 years ago.

• Marsden, M. (1981). Montreal: An island in the plain. In Montreal geographical essays, ed. D. Frost, 1-14. Montreal: Concordia University.
• Spencer, E. (1983). Physical Geography. Massachusetts: Addison-Wesley Publishing Company.
• Trenhaile, A. (1990). The geomorphology of Canada: an introduction. Toronto: Oxford University Press.

Related Web Sites

Some information about glaciers A geological time machine Canadian National Earthquake Hazards Program Natural Resources Canada: Terrain Sciences Division Photos of glaciers from around the world