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Geology of the National Parks

GEOLOGY OF OLYMPIC NATIONAL PARK
Introduction (cont.)

camp on skyline trail
Fig. 3. Geologist camp on Skyline divide above deep valleys filled with fog.

How the Geologist Explores: The Geologic Map

The organization of geologic knowledge begins with the geologic map, which shows the distribution of different kinds of rock. To make a geologic map of an area like Olympic National Park, which has only 128 miles of road in 1,400 square miles of rugged mountainous terrain, the geologist must, first of all, walk. He must hike the trails, climb the trailless ridges, and wade down the log-choked creeks to find outcrops, that is, exposures of bedrock. At times a pack train may carry his duffle, or a helicopter may whisk him from one range to another; but to see a significant amount of rock, he must travel on the ground, as close to the rock as his legs will carry him (figs. 3 and 4). At each outcrop, the geologist notes the kind of rock, its characteristics, and its geometry, and marks its position on the map. For instance, he may find an outcrop of sandstone. He notes the size of its grains, its color, and the characteristics of the bedding. He measures the position of the bedding, which is especially important if the rock has been folded. He looks for fossils and collects a specimen of the rock to take back to the laboratory where he can study it with a microscope or analyze its chemical components.

Wally Cady on Delmonte Ridge
Fig. 3a. Wally Cady taking notes on Delmonte Ridge, looking southeast, with The Brothers on the right skyline.

As the number of notes on the map grows, he develops a hypothesis to explain what he sees: the green sandstone is older than the black shale; both have been folded and eroded before a younger conglomerate was deposited on top of them. To test this hypothesis, he makes more observations, then modifies or rejects it accordingly. The pattern of rock distribution on his map becomes a geologic map. In his head and in his notes, the geologist develops a geologic history. However, the story is not always simple, and there may be several reasonable versions of how things happened. see the Preface for discussion of some recent ideas.

Using This Guide

Thick beds of sandstone, Valhallas
Fig. 4. Thick beds of sandstone in the Valhallas and geologists at work.

The Guide to Geology of Olympic National Park is divided into two parts. The first part summarizes the geology of the Olympic Mountains and relates their geologic history. The second part guides the reader to places where he can observe geologic features. Many of the more fundamental geologic principles are explained in the second part, on the outcrop, so to speak. The geologic story is keyed to these points of interest, allowing readers to expand their knowledge of topics or terms. A glossary helps readers find their way through a welter of geologic terms and concepts. The guide to points of interest (Part II) is organized around major drainages.

Here I must add a note of caution. Olympic National Park is a special place to enjoy rocks. Find them, admire them, learn from them; but do not collect them. Collecting is taboo without a special permit; if it were not, an outcrop of unusual rock might virtually disappear under the onslaught of rock hounds and geologists.

This web version offers a sampling of geologic points of interest from the book in the Virtual Field Trips part of the site.


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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