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Geology of the National Parks


drawings of common rocks in the Olympics Fig. 5. Common rocks found in the Olympic Mountains. Click on the outlined mineral names in the drawing for definitions.

The Making of the Rocks

. . . these mountains bear every indication of being of very recent formation and I fully believe they are."

  ~Lieutenant Joseph P. O'Neil 14th Infantry, 1890


Rocks of the Olympics

Fortunately for the nongeologist, the basic rock vocabulary of the Olympic Mountains is simple. The visitor can easily recognize the three most abundant rocks: sandstone, shale, and basalt. Most Olympic rocks were born in the deep waters of the Pacific Ocean, where sand and mud from the land accumulated, bed by bed, into thick, nearly horizontal layers. Adding to this pile of rock were flow after flow of basalt lava that issued from cracks or broad volcanoes on the ocean bottom. Because of this origin, almost all the rocks of the Olympics reflect the assaults and caresses of ocean currents, the buoyancy of water, and the cold and pressure of ocean deeps. The build-up of these sediments and lavas, represented now by rocks in the Olympic Mountains, spanned a period of some 40 million years. The oldest rocks are about 55 million years old, the youngest about 15 (see fig. 11). By geologic standards, these are indeed young mountains made of young rocks.

Feldspar Quartz Clays Mica Silicates Housefly Lifecycle Altered Glass Sandstone Shale Conglomerate Basalt

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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