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Geology of the National Parks

GEOLOGY OF OLYMPIC NATIONAL PARK: PART Il NOTES ON THE GEOLOGY

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Dungeness River

STOP 1: Pillow basalts in the Crescent Formation

Where the Dungeness River road swings in and out of gullies along the precipitous southeast side of Maynard Peak (fig. FT 1), the traveler passes through the bottom flows of an immense pile of basalt (the Crescent Formation) deposited on the ocean floor between 40 and 55 million years ago.

The flows are tilted on edge here so that a trip up the river is a trip back in time. Between Maynard Peak and Tyler Peak is a thick sequence of shale and sandstone, but more basalt makes cliffs on the southeast side of Tyler Peak, high above the main road. These basalts are some of the earliest flows of the ancient volcanic outpouring.

A portion of the Tyler Peak geologic map
Fig. FT 1. A portion of the Geologic Map of the Tyler Peak Quadrangle showing the distibution of rocks along the Dungeness River and field trip stop 1.
basalt pillows at Lk Constance
Fig.FT 2. Vertical bed of basalt pillows on Mount Constance. The tops of the pillows are towards us.

Many of the exposures along the road display the pillows (fig, FT2) formed when the lava erupted into the sea. Because the pillows were hot and soft when formed, they tended to droop into depressions in whatever lay beneath them.

basalt pillows in x-section
Fig. FT 3. Basalt pillows sliced through by erosion. The beds are more or less perpendicular to the exposed surface, with their tops to the left.

Commonly they filled depressions between the cold. humped backs of earlier-formed pillows providing a means for the geologist to tell the original top of the lava bed (fig. FT3).

On to Stop 2. Royal Basin


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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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