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Geology of the National Parks


Glaciers: The Heavier Hand

Although rivers established the grand pattern of Olympic scenery, and running water is the underlying melody of the Olympics today, the stillness of snow and glacial ice once filled the land, and glaciers of today are still an important part of the high mountain scene.

Glacier covered south side of Mt Olympus
Fig. 32. Glacial scene reminiscent of the Ice Age. South side of Mount Olympus.

Whenever the accumulation of winter snow exceeds the amount that melts in summer and gets thick enough, it becomes ice. If the ice grows thick enough, it becomes plastic on the bottom under the weight of the snow and ice on top and begins to flow as a glacier. Many times in the geologic past glaciers grew to immense sizes. The most recent major icing of the earth occurred during the Pleistocene Epoch, beginning about 2 million years ago. During the great Ice Age, ice sheets spread from the higher latitudes into many areas inhabited by man today. At the same time smaller ice sheets and innumerable alpine glaciers formed in mountains all over the world. The Olympic Mountains not only bore their own system of extensive alpine glaciers but were nearly surrounded by the vast Cordilleran ice sheet that expanded out of western Canada (fig. 34).

Glaciers are important sculptors of the land, and they do their work in ways that differ from the ways of a river. They move very slowly but can carry great amounts of rock debris; individual blocks can be as big as houses . Debris carried by rivers is sorted by variations in the velocity of the water; mud, sand, and gravel are deposited separately. Glaciers do no sorting. When they melt away from their load of debris, it all falls together, forming an unsorted pile or moraine , unless water from the melting ice has a chance to sort it.

A river occupies only a small U-shaped channel in the bottom of the V-shaped valley it carves (fig. 33), but the glacier carves the whole valley into one great U-shaped channel. The head of a river valley generally merges imperceptibly with the ridges or highlands, whereas the head of a glacier valley is a steep-walled bowl, called a glacial cirque. Rivers abrade with sand and silt, but glaciers scrape their channels with sand plus coarse rocks, held firmly and pressed down by the weight of the overlying ice. Glacier-carved rock can be as smooth as a polished headstone or cut by deep grooves and gouges, or both. A river generally tends to be more sensitive to slight variations in rock hardness than the less selective glacier. Glaciers, like bulldozers, are not strongly influenced by the subtle differences in nature's makeup.

V- and U-shaped valleys
Fig. 33. A glacier's channel is the U-shaped valley.

Early Olympic geologists were so involved with solid rocks and somewhat obscure events of the distant geologic past that they gave little time to the more obvious records of earth history written in the loose deposits of the Ice Age. In 1913, J Harlen Bretz, who might be called the grand old man of northwestern Pleistocene geology, described the Olympic Mountains as they were during the Ice Age, surrounded on three sides by a river of ice from Canada. Since Bretz wrote, many people†University of Washington students in particular†have worked out the complicated story of the lowland glaciation.

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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