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POST ICE-AGE GEOGRAPHY AND ENVIRONMENT

The North American continent separated from the European continent by the process known as continental drift, about 55 million years ago. Around 10 million years ago, the North American continent joined with the South American continent but with little significant impact on the flora and fauna of the northern continent. Since that time North America remained in relative isolation until the beginning of the recent ice-age, about 1.8 million years ago. This recent ice-age was characterized by five major advances of ice sheets, each producing glaciers, which covered most of the northern extent of the continent. These glacial advances acted like giant "bull-dozers", scraping and remodelling the land. During the most recent glacial advance, beginning around 30,000 years ago, the basins of what were to become the Great Lakes were sculpted. The Oak Ridges Moraine, just north of the Toronto area, is a deep deposit of dirt mounded by the glaciers. The bulldozing action of the glaciers also obliterated much of the previous geography of southern Ontario though some traces do remain. The Dundas valley, just west of Hamilton, and Jordan's Harbour, near St. Catharines, are two examples of partially filled preglacial river valleys.

Aside from the land remodelling caused by the glaciers, two major results of glaciation are important in understanding the resulting changes in geography. The first consequence of glaciation is the amount of water locked up in the ice sheets. The vast amount of water needed to produce the glaciers caused a lowering of world-wide ocean levels, exposing vast amounts of land which are now submerged under the higher water levels.

One of these areas was the large, shallow shelf between present day Alaska and Siberia. This area, called Beringia, was a broad, low, continent sized expanse which is believed to have not been glaciated. Instead, studies of the glacial climate of this area suggests cool, wet, grasslands capable of supporting vast herds of grazing animals. This area was probably open during several, if not all, of the glacial advances and it is believed that numerous species of animals migrated across Beringia during these periods in both directions. This is also believed to have been the route of the first migrants into North America over 20,000 years ago.

The second important result of glacial advance is the depression of land under and around the ice sheets. Continental shelves, upon which land masses rest, can be compared to rafts floating on the fluid magma, or molten rock, of the earth beneath. When glacial ice sheets, often over a mile in thickness, rest on this land, they are depressed relative to surrounding land. In some areas, these depressed areas formed large lakes along the southern margins of the glaciers. In eastern Ontario, the weight of the glaciers depressed the land enough to allow salt water from the Atlantic Ocean to move well up into the St. Lawrence and Ottawa valleys. This area, known as the Champlain Sea, was an extension of salt water into areas as far west as Pembroke and Deep River in the Ottawa Valley and Brockville along the St. Lawrence River. As the weight of the glaciers was removed, the land began to slowly spring back up, a process known as isostatic rebound, changing drainage patterns and relative elevations. Ten thousand years ago, for example, both Lake Ontario and Lake Erie were much smaller than they are today and Lake Huron did not drain south through the Detroit River as it does now. Instead, the northern Great Lakes drained through a channel near present day North Bay and down the Ottawa River Valley to the Saint Lawrence. It was only after the land in this area had rebounded from the weight of the ice that the Great Lakes drained through Lakes Erie and Ontario as they now do.

Finally, the glacial conditions of much of North America resulted in much different species of plants and animals predominating in the areas near the glaciers. Mammoths and mastodons, giant species of bear, beaver and bison, and wild horses became extinct shortly after the glaciers began to recede and southern Ontario would have been like arctic tundra for the first thousand years or so of occupation by native peoples.