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 region is classical example of a deranged drainage system, caused by the watersheds of the area being disturbed by glaciation and the effect of post-glacial rebound. The Shield was originally an area of very large, very tall mountains (about ) with much volcanic activity, but the area was eroded to nearly its current topographic appearance of relatively low relief over 500 million years ago. Erosion has exposed the roots of the mountains, which take the form of greenstone belts in which belts of volcanic rock that has been altered by metamorphism are surrounded by granitic rock. These belts range in age from 3600 to 2680 million years old. Much of the granitic rock belongs to the distinctive tonalite–trondhjemite–granodiorite family of rocks, which are characteristic of Archean continental crust. Many of Canada's major ore deposits are associated with greenstone belts.