<P> The Canadian Shield is a collage of Archean plates and accreted juvenile arc terranes and sedimentary basins of the Proterozoic Eon that were progressively amalgamated during the interval 2.45 to 1.24 Ga, with the most substantial growth period occurring during the Trans - Hudson orogeny, between ca . 1.90 to 1.80 Ga . The Canadian Shield was the first part of North America to be permanently elevated above sea level and has remained almost wholly untouched by successive encroachments of the sea upon the continent . It is the Earth's greatest area of exposed Archean rock . The metamorphic base rocks are mostly from the Precambrian Supereon (between 4.5 billion and 540 million years ago), and have been repeatedly uplifted and eroded . Today it consists largely of an area of low relief 300 to 610 m (980 to 2,000 ft) above sea level with a few monadnocks and low mountain ranges (including the Torngat and Laurentian Mountains) probably eroded from the plateau during the Cenozoic Era . During the Pleistocene Epoch, continental ice sheets depressed the land surface (see Hudson Bay), scooped out thousands of lake basins, and carried away much of the region's soil . </P> <P> When the Greenland section is included, the Shield is approximately circular, bounded on the northeast by the northeast edge of Greenland, with Hudson Bay in the middle . It covers much of Greenland, Labrador, most of Quebec north of the St. Lawrence River, much of Ontario including northern sections of the southern peninsula between the Great Lakes, the Adirondack Mountains of New York, the northernmost part of Lower Michigan and all of Upper Michigan, northern Wisconsin, northeastern Minnesota, the central / northern portions of Manitoba away from Hudson Bay, northern Saskatchewan, a small portion of northeastern Alberta, and the mainland northern Canadian territories to the east of a line extended north from the Saskatchewan / Alberta border (Northwest Territories and Nunavut). In total, the exposed area of the Shield covers approximately 8,000,000 km (3,088,817 sq mi). The true extent of the Shield is greater still and stretches from the Western Cordillera in the west to the Appalachians in the east and as far south as Texas, but these regions are overlaid with much younger rocks and sediment . The underlying rock structure also includes Hudson Bay . </P> <P> The Canadian Shield is among the oldest on earth, with regions dating from 2.5 to 4.2 billion years . The multitude of rivers and lakes in the entire region is caused by the watersheds of the area being so young and in a state of sorting themselves out with the added effect of post-glacial rebound . The Shield was originally an area of very large, very tall mountains (about 12,000 metres or 39,000 feet) with much volcanic activity, but over hundreds of millions of years, the area has been eroded to its current topographic appearance of relatively low relief . It has some of the oldest (extinct) volcanoes on the planet . It has over 150 volcanic belts (now deformed and eroded down to nearly flat plains) whose bedrock ranges from 600 to 1200 million years old . </P> <P> Each belt probably grew by the coalescence of accumulations erupted from numerous vents, making the tally of volcanoes reach the hundreds . Many of Canada's major ore deposits are associated with Precambrian volcanoes . </P>

When did the glacial erosion that exposed the canadian shield begin
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