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Geology of the National Parks

GEOLOGY OF OLYMPIC NATIONAL PARK:
PART I OLYMPIC GEOLOGY

Rising Land and the Olympic Scene

Fortunately for Olympic visitors, the restless crust of the earth has not yet allowed gravity and its cohorts, water and ice, to level the land. There is considerable evidence that the Olympics today are less gentle than they once were. As indicated earlier, when a mass of the earth's crust is uplifted high above the ocean, streams are born and begin gnawing away at the elevated land. They work rapidly, creating steep, V-shaped valleys and high, jagged ridge crests. Eventually the valleys reach a maximum depth, and the crests a maximum sharpness; the streams cannot cut deeper than the level of the sea to which they flow. Then the slower erosive processes such as creep, landsliding, and the washing of rain round off the sharp ridges and flatten the steep valley sides.

Upland surface and glaciated peaks
Fig. 38. Remnant of gentle upland surface with steep, glaciated terrane in background.

At some time in the past, the ridges of the Olympics were smoothed into rolling meadowlands, such as are now seen on Hurricane Ridge (Field Trip Stop 12) and on a few ridges along the eastern side of the park. But before the agents of rounding and smoothing had a chance to completely reduce the mountains to lowland hills, the range was glaciated and uplifted again. In general, glaciers are even more ravenous than young streams in the way they eat at mountainsides, leaving their steep-sided cirques (fig. 38). They have also been effective in removing all traces of the old, gentle upland in the central part of the range and on the north sides of ridges. The uplift, however, gave the streams new appetite by increasing their gradients and they once again began steepening the valleys and sharpening the ridges. The gorges along many of the major rivers are testimony to the steepened gradient. The river erosion has been so rapid that slower gravitational processes, which would otherwise open the gorges into V shapes, have not yet caught up. The story might be further complicated had sea level changed: lowering the sea level has the same effect on streams as raising the land. But most geologists feel that the oceans are as full today as they have been for many years.

South Fork of the Hoh River
Fig. 39. Deeply glacial-carved valley of the Hoh River South Fork. The modern river is cutting down into the glacial valley.

Considerable uplift of Olympic rocks results from their tendency to float on the deeper and denser oceanic crust; but much recent uplift may have been caused by the removal of the Cordilleran ice sheet, which once depressed the land by its great weight. The high outwash terraces of the south and west sides of the mountains may be evidence of a time when the land was depressed and streams running slowly to the ocean could not carry all the debris delivered to them by the melting glaciers. The ice sheet does not provide the complete answer, however, for what goes down must have been up; the land must have also been higher before the coming of the ice sheet. In fact, when we consider all the advances and retreats of the ice sheet, it seems that the land must have risen and fallen numerous times.

The sum total of the movements, however, seems to be up, since the mountains are still high and the streams are aggressively eroding them. The mountains may still be rising, but whether they can gain elevation over the effect of erosive processes will be known only by surveyors of the future, provided that man continues to be concerned with such things.


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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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