Even a casual observer cannot doubt that the Laurentian area, like the other regions of Quebec, was covered by gigantic glaciers in the Pleistocene period. The evidence of such action is given by features of glacial erosion such as U-shaped valleys, grooved and striated rocks, rounded hilltops, called roches moutonnées; by glacial deposits including terminal, recessional and ground moraines, eskers and drumlins, and other morainic deposits. Above all, proofs are shown by innumerable lakes of glacial origin, sediments in icefront lakes, sand and gravel deposited by streams which issued from melting ice, and lastly by a pattern of rivers newly organized after the retreat of the glaciers.
The Labrador ice-sheet that covered Quebec is estimated to have been more than 7,000 feet thick. It spread slowly from the center outward, until warmer weather melted away the ice from the south to the north. The enormous weight of the icecap had lowered the surface of the plateau, and as soon as the burden was removed, the surface began to rise to its former level. The process being slower than the retreat of the ice, the sea invaded the land. The Champlain sea occupied the St. Lawrence Lowlands. Hudson and James Bays expanded inland also at a slightly lager time.
Two depressions were filled by freshwater in front of the northward retreating glacier: the lake St. John basin, which still contains a large body of water, and another much larger one in Western Quebec known as Lake Barlow-Ojibway, of which Lake Abitibi is but a tiny and shallow remnant. Proofs of such a marine invasion and lake formation are of two kinds: elevated beach lines and shore features, and deep deposits of clay. Much more arable land is available in those basins than on top of plateaus where the glacial moraine is too rough for the plow.
Another result of the glaciation from which men have taken opportunity is the new pattern of drainage. The valleys could not regain their normal gradients at once. Lakes and swamps were formed on the height of the land. Rivers and streams tried to regain their former valleys, but were often deviated by the glacial deposits and forced to find new paths. Local glacial erosion also caused changes in their profiles. All the tributaries of the St. Lawrence, flowing across the Laurentians, have a great number of rapids and falls, due to glacial action. Thus water power is cheaper than it would be if artificial dams had to be built on rivers with more regular profiles.