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Geology of the National Parks


Making Mountains Out of Rocks

The story of the Olympic Mountains is long and complex. Although the material of which the mountains are made, the rock, is important in determining their nature and appearance, the forces that have sculpted the rock play the greatest role. The most interesting and varied rocks in the world uplifted as a high, uneroded plateau will not offer much to the mountain lover. It is the happy combination of rocks, uplift, and vigorous erosion that create the mountain scene.

Mount Olympus from High Divide
Fig. 28. Mount Olympus from High Divide. The mountain scene is born of the interplay of uplift, erosion, and rock hardness


Three main erosional agents have carved the Olympic Mountains: running water, glacial ice, and the direct action of gravity. They work in concert to carry the mountains back to the sea from which they were born. Also of some consequence to the mountain scene is weathering, which prepares the rock for its erosional trip Chemical weathering is particularly important in the southwestern Olympics, where hard basalt and softer sandstone and shale are all converted to clay by chemical action under the thick forest humus on low hills. Rocks so weathered are commonly stained red, red-brown, or yellow by iron oxides, which are chemically weathered from iron-bearing minerals. Roadcuts in the southwestern Olympics are characteristically colorful. In the high country of the Olympics and in the lowlands of the north and east sides, clays produced by deep chemical weathering are not noticeable because the rocks there have been scraped clean by glaciers. Furthermore, in the high country, mechanical weathering, such as the fracturing of rocks by repeated freezing and thawing of water in cracks, is much more important than it is at lower elevations.

Material in this site has been adapted from Guide to the Geology of Olympic National Park by Rowland W. Tabor, of the USGS. It is published by The Northwest Interpretive Association, Seattle.

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