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Retreat of Pleistocene Glaciers from North America



During the last 1.6 million years known as the Pleistocene, huge glaciers advanced and retreated several times over North America. The last ice age, the Wisconsin ice age, began 23 thousand years ago and covered Canada and the northern United States with a layer of ice 2 or 3 kilometers thick. The ice melted away about 12 thousand years ago.

The glaciers of North America scraped material of all sizes, from sand to boulders, carried it long distances, and dumped it where they melted. The effects are visible everywhere in Ontario. In some places the ground is shallow, because the glacier scraped away the cover. There are furrows on rock surfaces, where stones in the glacier gouged them. In other places, the glacier unloaded its burden, and covered the land with a huge mass of sand, clay and stones. Because of this, the Niagara Escarpment is not visible everywhere. In such places, only steep slopes can be seen.

When the last glacier melted, it created lakes and rivers that helped to sculpt the escarpment. The Niagara Escarpment doesn’t follow a straight line, but zigzags, because large rivers cut out valleys called reentrant valleys. On both sides of these valleys are high slopes and cliffs.

The glaciers also broke down and carried away large amounts of bedrock and had the general effect of softening the appearance of the Niagara Escarpment. They widened, deepened and rounded the outlines of existing river valleys such as the softened “U” shape of the Nassagaweya Canyon. Along the Niagara Escarpment the outcrops are frequently smoothed, furrowed and scratched. These markings indicate the direction in which the ice flowed.

Meltwater was released in large quantities especially as a glacier retreats, in channels within and at the base of the glacier. The course of some of these rivers can be traced over distances and are referred to as meltwater discharge channels. During the great melting of the glaciers, water levels of the surrounding lakes rose and covered some lower-lying regions. Stream recapture also occurred. Clues on today’s landscape included remnants of lakes and recaptured streams.

See 4.2.1d Crawford Lake – The text, maps and photos help to find the clues to its the glacial history of glacial advance and retreat, plunge pool formation, stream recapture and braided streams. A sample map of the area is below. The red square is the key area of the Crawford Lake study by Roger Chittendon.