Geology of the Boundary Waters

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The geology of the Boundary Waters Canoe Area Wilderness is a fascinating and complex story spanning billions of years and including such diverse landscapes as volcanoes, glaciers, the ocean floor, and tall mountains.

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Bedrock

The rock exposed along the lake shores and cliffs of the Boundary Waters is all a part of the very old Canadian Shield, dating from the Precambrian, or older than 540 million years ago. For reference, the dinosaurs died out 65 million years ago, animals began living on land about 350 million years ago, and complex multi-cellular life originated about 600 million years ago.[1]

Such a large area of exposed, old rock requires some explanation. The current surface expression of the Shield is one of very thin soil lying on top of the bedrock, with many bare outcrops. This arrangement was caused by severe glaciation during the last ice age, which covered the Shield and scraped the rock clean. 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 mountains and much volcanic activity, but over the millennia the area was eroded to its current topographic appearance of relatively low relief.

Mountains have deep roots and float on the denser mantle much like an iceberg at sea. As mountains erode, their roots rise and are eroded in turn. The rocks that now form the surface of the Shield were once far below the earth's surface. The high pressures and temperatures at those depths provided ideal conditions for mineralization.

The North American craton is the bedrock forming the heart of the North American continent and the Canadian Shield is the largest exposed part of the craton's bedrock.

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