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Geology of the National Parks

GEOLOGY OF OLYMPIC NATIONAL PARK:
PART I OLYMPIC GEOLOGY

Rock ribbon of geologic time and Tertiary time scale
Fig. 11. The ribbon of time. For details of the Tertiary Period and the geologic timescale in general, click here.

Rocks and Time

Contemplation of earth history is always hindered by the mind-boggling dimensions of geologic time. We can talk about a million years or even a billion years, but we can hardly imagine the countless small events that fill such expanses of time. And it is just such small events the settling of a sand grain to the ocean bottom, the tumbling of an unsteady pebble into a creek, the death of a small snail†that add up to geologic change. If we travel down the Elwha River from Lake Mills to Lake Aldwell, we will have traversed in about 5 miles a section of rock†basalt, shale, and sandstone representing about 10 to 15 million years of accumulation. If we were to walk through time at this same rate, a few paces more would encompass all of mankind's recorded history, and a slight shuffle of an inch or so would take in one person's lifetime (fig. 11). To pace out the whole of the earth's history since its beginning about 4.5 billion years ago, we would have to hike about 1,500 miles.

Point of Arches breccias
Fig. 12. Seastacks of coarse coglomerate and breccia make up The Point of the Arches. These rocks, full of pieces of the old gabbro, overlie it.

The Oldest Rocks

Geologists are wont to ask upon what all the sedimentary and volcanic rocks of the Olympics were deposited, a question not unlike that of the Greeks who wondered where Atlas stood as he held up the earth. The answer is not entirely clear, but most of the Olympic rocks may have been laid down directly on basaltic crust under the Pacific Ocean. Some rocks, however, were deposited on the eroded surface of older igneous and sedimentary rocks †perhaps a submerged piece of the North American continent of 70 million years ago. Just south of Point of Arches, modern sea cliffs and sea stacks of old gabbro, basalt, and sandstone may represent this piece of continent. Some of the old rocks at Point of the Arches (see the geologic map) are at least 144 million years old, that is, at least 80 million years older than the oldest rocks in the Olympic Mountains (fig. 11).

Cliffs at Point of Arches, reached via Shi Shi beach on the Makah Indian Reservation, reveal Eocene conglomerates and breccias filled with large boulders eroded from the old continent (fig. 12). The intrepid explorer can reach the old gabbro a rock of white (feldspar) and black (hornblende) spots†by scrambling along the cliffs south of the point at low tide.

To approach the gabbro from the south, drive to the beach north of the Ozette River and hike north at very low tide.


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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