The Canadian Encyclopedia Great Lakes-St. Lawrence Lowlands
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There is a plain along the St. Lawrence River between Québec City and Brockville, Ontario.The United States has 5,000 km2 of the lowland's total area.It is 450 km west to east and 100 km wide in the western half.An arm is 130 km south into the Lake Champlain valley.
The lowland's present landscape is the result of the last continental glaciation, followed by submergence, emergence and river erosion and deposition.Evidence of early glaciation has been destroyed 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.They were separated by a non-glacial interval.The interval could have been as long as 34,000 years ago.
Prior to 18,000 years ago, the last major advance covered the region.The south and southwest parts of the lowlands were flooded by pro-glacial lakes after the ice sheet retreated.A body of water known as the Champlain Sea was formed after the sea flooded the St. Lawrence Valley.
The St. Lawrence Lowland rose rapidly in response to the disappearance of the ice mass.On the north side of the lowlands, the highest relict strand lines are 230 m above sea level, while on the south side, they are 75 m lower.
The central part of the basin was occupied for a short time by a lake with a surface of 40 m.The current course of the river was established around 6,500 years ago when the St. Lawrence River eroded its channel past Québec deeper.
The altitudes range from 15 m above sea level along the St. Lawrence River northeast of Montréal to 150 m with the exception of the Adirondacks to the south and the Precambrian Shield of Ontario in the southeast.
The Tributaries of the St. Lawrence drain the lowland from the south.The South Nation, Rideau, Mississippi, Madawaska and Bonnechere rivers are in the lowland.
The large-scale topographic features are the result of subaerial weathering and erosion by rivers over the last 100 million years.The oldest (lowest) to youngest (uppermost) rocks range from 520 to 480 million years old.The basin is surrounded by older, more resistant rocks.The ancient surface of moderate relief was eroded on the older Precambrian rocks.
The lowland is dominated by the Monteregian Hills, a series of isolated mountains in a belt about 20 km wide.They are from the west to the east.The erosional remnants of the early Cretaceous age are here.There is no direct evidence that any volcanoes existed above the intrusions.
The Monteregian Hills and some terraces around the mountains form parts of the Lachine Rapids in the St. Lawrence River.The Hills of Precambrian project through the rocks of the lowland west of Montréal at the Oka Hills.The rock has been eroded from the lowland in the last 100 million years.In many places, the north boundary is an eroded fault line.
The lowland is part of a rift valley that has been the site of 10 major earthquakes in the last century.
Much of the St. Lawrence Lowland is underlain by clay.It is as thick as 60 m along the north side near the former glacier margin.The clay becomes unstable when it is subjected to excessive water percolation from rain or snow, which can cause a lot of loss of life and property.
The west and south parts of the lowland are underlain by glaciers.Beach deposits are common on the hills in this region because of the wave action that removed silt and clay from the till.There are many sand and gravel beaches and spits around the lowland.The waters of the present Labrador coast and Gulf of St. Lawrence are similar to the deposits in the Champlain Sea.
There are areas of sand north and east of Montréal that were once part of the St. Lawrence and Ottawa rivers.The east part of the lowland has low terraces covered by river sands.Some of the sand has been formed into dunes and these are called "crtes de coq."The vegetation protects the aeolian sand.There are abandoned river channels as wide as 2 km and banks as high as 10 m along the St. Lawrence river from Lac Saint-Pierre to Ottawa.
There are ridges of bouldery sand 1.5– 4.5 m high and 30 m wide, which appear to be material transported and pushed up by floating river ice during the river's early stages.Many rural communities are built on the ridges.
There is a good amount of agricultural land in the lowland.Sand and gravel deposits are exploited for road metal and construction material, and its clay has been used for brick and tile manufacture.Building stone, cement, lime, crushed stone and concrete aggregate, and brick and tile have been made from the rocks of the lowland.
Jacques Cartier arrived in 1535 and the lowland was occupied by Iroquoian-speaking people.The farmlands were settled in a pattern of narrow strips fronting the river.The industry began near Trois-Rivires.The largest part of Québec is located in the St. Lawrence Lowland.
There is a technical report about the potential of the St. Lawrence Lowlands basin.A description of area geology, geological maps, charts, and more are included.From the national de la recherche scientifique.
There is a brief overview of various archaeological sites found in the St. Lawrence Lowlands region of Québec.There is a Virtual Museum of Canada.
The major geologic features of the St. Lawrence Lowland are described in "The Geology of North America."