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Geology of the National Parks

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

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Hurricane Ridge Road

STOP 8: Klahhane Ridge

Breccia beds on Zig-Zag trail
Fig. FT 16. Tilted lava and breccia beds on Mount Angeles, viewed from the Klahhane Ridge (Zig-Zag) trail.

On the trail to Klabhane Ridge, view cliffs of basalt and breccia rubble on the left in the crags of Mount Angeles (fig. FT 16) and to the right find meadowed and hill-covered slopes of sandstone and shale.

The hiker here traverses the edge of the upturned lava field where volcanic material was deposited next to quiet accumulations of sand and mud. In one place the trail draws close to cliffs of volcanic breccia and a contorted bed of red limestone. The contortions in these beds may not have resulted from folding during major upheavals in the earth's crust but from sliding and slumping of the partly consolidated strata on the ocean bottom soon after deposition.

Overturned beds of volcanic breccia, Klahhane Ridge
Fig. FT 17. Coarse volcanic breccia on Klahhane Ridge grades upward (to right) in bed showing strata has been tilted over.

The ridge crest and trail junction overlooking the Strait of Juan de Fuca provide the most spectacular views of the volcanic rocks of the Mount Angeles massif. To the northwest on the shoulder of Mount Angeles, colorful beds of sedimentary rocks, rich in fragments of lava and alternating with beds of volcanic breccia, lean toward the strait (fig. FT 16). The difference in resistance to weathering of each bed has produced startling ribs and flutes: thick beds of hard volcanic breccia stand up in straight walls; the soft beds of red shale make deep alleys. When sediments are dumped into the ocean, the largest, heaviest particles or chunks tend to settle to the bottom first, and the smallest last. The result of this simple process can be seen in many outcrops of graded breccia (fig. FT 17) and tells us which side of a particular bed was originally up. These graded beds show that the lava field of Mount Angeles has not only been tilted up from its original horizontal position but also tipped over.


On to Stop 9. Mount Olympus view and pencil slates

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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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