when did the glacial erosion that exposed the canadian shield begin
Canadian Shield The Canadian Shield is among the oldest on earth, with regions dating from 2.5 to 4.2 billion years.[9] 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)[10] 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.[citation needed] It has some of the oldest (extinct) volcanoes on the planet.[citation needed] 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.[citation needed]