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Geology of the National Parks


Predominent geologic terranes
Fig. 16. Predominent geologic terranes of the Olympic Peninsula.

Theories for the Origin of Olympic Structure

Looking at and recognizing the rocks of an area and how they formed is only the first step in interpreting the geologic story. We would like to see how all the different rocks are related to each other. The pattern of relationships, portrayed by a geologic map, helps decipher the geologic structure; from this we can begin to reconstruct the geologic history. The ultimate goal is to fit the local geologic history into a total earth history.

The Earliest Ideas.

Some of the observers in the Olympics thought that the bedrock of the mountainous core, sometimes called the inner Olympics (fig 16), must be composed of gneiss and granite, rocks known to be common to the cores of many mountain ranges. The early travelers were fooled, no doubt, by the gneiss and granite occurring as pebbles and boulders in the gravels of the major streams on the north and east flanks of the range We now know that these exotic rocks do not occur as bedrock in the Olympics but instead were carried and dumped there by the Cordilleran ice sheet as it flowed agamst and around the mountains.

Formation of an Anticline
Fig. 17. Formation of anticline.

One of the earliest expeditions to penetrate the mountainous interior of the Olympics, the Press party of 1889-90, correctly determined that at least some of the core was composed of slate and sandstone; but their scientific observations were not taken very seriously and they lost all of their specimens when their raft capsized on the Quinault River.

By the early 1900s geologists were hammering on rocks around the periphery of the range and venturing up trails along the major drainages. Among these early workers was Albert B. Reagan, an Indian agent and writer with an insatiable curiosity. Reagan published not only on the geology of the area but also on the flora, the fauna, and, of course, the Indians. He collected numerous Indian legends, a few of which explained, with some basis of truth, geologic phenomena in the Olympics .

A more modern approach to the geology was taken by Charles Weaver, a geologist-paleontologist from the University of Washington. Weaver studied an incredible amount of Tertiary sedimentary rock along the western coast of the United States. In the course of this monumental study, he examined most of the rocks on the north side of the Olympic Peninsula and outlined the horseshoe pattern of basalt (figs. 16 and geologic map). Today the basalt is called the Crescent Formation (named for exposures near Port Crescent now Crescent Bay not for its crescent outcrop pattern). In 1937, Weaver proposed that the Olympics were part of a giant plunging arched fold, the top of which had been eroded off. If layers of sediment are folded up in the middle, they form an anticline, and if the top of the anticline is eroded off, the older layers will then appear at the surface in the center of the fold (fig. 17).

Weaver believed that the rocks of the Olympic core were older and therefore represented the center of the anticline. This was a reasonable guess, since most of the core rocks looked older; they were more highly deformed and certainly harder than the peripheral rocks. He could not prove that all the core was older, however, for although he and others had found Eocene and younger fossils in the peripheral rocks, they found no fossils in the mountainous part of the core.

Fig. 18. Foraminifera shells.

Meanwhile, oil company geologists, prospecting along the western part of the peninsula, found fossil Foraminifera, or "forams" as they are fondly called (fig. 18). The location of these fossils indicated that at least some of the core rocks encircled by the arms of the basaltic horseshoe were younger than the basalt, thus casting some doubt on Weaver's theory. In spite of this, his anticline hypothesis has persisted into modern times.

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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