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Geology of the National Parks


Running Water: The Persistant Sculptor (cont.)

With the major drainage pattern set, the erosional forces could begin shaping the mountains in a variety of interesting and subtle ways. Of utmost importance to erosion by running water are the differences in rock hardness.

Piros Spire, erosional resistant basalt
Fig. 30. Erosional resistant basalt holds up Piros Spire.

The basalt of the Olympics is generally harder than the surrounding shale and sandstone; it resists erosion and stands out in bolder relief, making sharper peaks and pinnacles (fig. 30). The sandstone is harder than the shale or slate, so it too becomes etched out. When interbedded with shale or slate. it commonly forms ribbed cliffs. This differential erosion has modified the original radial drainage pattern. Because they cut faster in soft rocks than harder ones, some tributaries of the main radial streams developed deeper valleys in the softer rocks between hard basalt and sandstone layers than did the small streams crossing the basalt. Eventually the streams in the deeper valleys between the hard rocks captured the drainage of the smaller streams. Thus, the drainage pattern in some areas of the Olympics was modified from radial to radial and concentric (fig. 31).

Radial and concentric drainage
Fig. 31. Development of radial and concentrc drainage.

In the core rocks, control of erosion by rock hardness is much more complex than around the periphery and of course has been considerably modified by glaciers. Rocks of the core have a strong northwest structural grain. Many factors contribute to this grain, but predominating are aligned beds of sandstone and basalt, mostly steeply tilted, and cleavage in slates.

The drainage pattern clearly parallels the northwest grain, but other structures in the rocks such as joints and faults are also influential. Running water finds all these subtle differences and carves the softest zones, layers, and areas into rills, gullies, and valleys. The harder areas stand out as ridges and peaks.

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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