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USGS Geology in the Parks

North Cascades Geology

Granite polished by glaciers
Granite polished by glaciers.

The Constant Levelers: Water, Ice, and Gravity

In spite of the romantic notion that mountains are thrust up into the sky, or even geologists' chatter about mountain building, most mountain scenery is the product of erosion gradually reducing the mountains back to sea level. As a sizable bit of the Earth’s crust is elevated significantly above sea level, the main agents of erosion gravity, running water, and moving ice get to work, carving it into interesting topography. The overall driving force is gravity and, in the long run, elevated parts of the Earth’s crust are reduced because the force of gravity tries to reform the Earth’s irregular surface back into a perfect spheroid.

When looking back in geologic time at the erosional history of the North Cascades, geologists have to decide at what point in the geologic past they want to call this collection of rocks the North Cascades. Much of the range is made up of exotic terranes that probably did not evolve on the same spot on the Earth as the present North Cascades. Many of the pieces had their own erosional history histories before they became part of the North Cascade mosaic, and that early history is obscure. But if the geologists confine their view to some time since the earliest Tertiary, that is about 65 million years ago, they can speculatively recreate the North Cascade scene and ponder its erosional history.

By earliest Tertiary time, most of the terranes of the North Cascades were in place, and stacking by overthrusting and general squeezing of the terranes had thickened the crust significantly. A thick crust tends to float high on the denser mantle, like an iceberg in the sea, so it is safe to assume that by this time the mountains were elevated and erosion had begun its work. Even today, as erosion removes the top of the stack, the mountain root continues to rise.

On to The Work of Running Water

Material in this site has been adapted from a book, Geology of the North Cascades: A Mountain Mosaic by R. Tabor and R. Haugerud, of the USGS, with drawings by Anne Crowder. It is published by The Mountaineers, Seattle.

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