![]() The sea withdrew 9,500 years ago and for a short time a lake with a surface at 40 m (present elevation) occupied the central part of the basin. The highest relict strand lines of the Champlain Sea are now 230 m above sea level on the north side of the lowlands and 75 m lower on the south side. Lawrence Lowland rose rapidly (as much as 20 m per century) in response to the disappearance of the ice mass. Lawrence Valley near Québec disintegrated 13,000 years ago and the sea flooded the region, forming a body of water known as the Champlain Sea.įrom 13,000 to 10,000 years ago the St. This ice sheet eventually retreated with minor re-advances, intermittently uncovering the south and southwest parts of the lowlands, which were then inundated by pro-glacial lakes. The last major glacial advance covered the region prior to 18,000 years ago. This interval lasted from about 70,000 to possibly 34,000 years ago. These were separated by a non-glacial interval during which peat and lake sediments accumulated. Evidence of early glaciation has been obliterated by later ones, but deposits exposed in valleys near Lac Saint-Pierre indicate that an early interval of weathering and deposition of river gravels was followed by at least two episodes of glaciation. ![]() The details of the lowland's present landscape are the result of the last continental glaciation, followed by marine submergence, emergence and, finally, river erosion and deposition. In addition, an arm extends 130 km south into the Lake Champlain valley. It is 450 km long west to east and 100 km wide in the western half, narrowing to 35 km at Québec. The lowland’s total area is 46,000 km 2 (5,000 km 2 of which is in the United States). ![]() ![]() Lawrence River between Québec City in the east and Brockville, Ontario, in the west, including the Ottawa River valley west to Renfrew, Ontario. Lawrence Lowland is a plain along the St. The topography of the Lowlands is the result of weathering and erosion by rivers of the nearby flat-lying early sedimentary rock (photo by Thomas Kitchin). ![]()
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