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Geology

The BWCAW contains some of the oldest exposed rock in the world, estimated to be as old as 2.7 billion years. It is part of the vast region known as the Canadian Shield, which underlies almost 2 million square miles of eastern Canada and the Lake Superior region of the United States. In Minnesota, this belt of ancient exposed rock extends west from the area of Saganaga Lake on the international border through Ely and International Falls to the northwestern part of the state, where the old rocks disappear beneath younger sedimentary deposits. Included in this expanse of ancient rocks are the metavolcanic Ely Greenstone formation, the metasedimentary Knife Lake Group, and great granitic masses like the Vermilion and the Saganaga batholiths.

Lunch on Basswood Lake

A mountain-building period began about 2.6 billion years ago, during which the rocks became metamorphosed and strongly deformed, and the granites were intruded from below into the older rocks. The rocks that had been formed or altered deep within the earth’s crust became exposed at the surface and were then subjected to erosion.