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Geology of the National Parks


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Hurricane Ridge Road: Big Meadow

STOP 9: View of Mount Olympus and pencil slates

Where the Hurricane Ridge Road cuts the ridge just before the Big Meadow parking lot, slabs of brown, gray, and black slate gleam in the sun. To the west on a good day Mount Olympus gleams in the sun as well. (fig. FT 18)

Mount Olympus from Hurricane Ridge area
Fig. FT 18. Mount Olympus viewed from south of Hurricane Ridge.

The slate forms from shale subjected to great pressure, and the rock tends to break into thin splinters. Fractures develop in two ways to form the splinters (called pencils) during the folding of shale under great pressure (fig. FT 19, 20). The original shale layer is usually made up of many very thin beds and tends to break along or between these beds. The intersection of the closely spaced slate fractures and the original bedding combine to chop the rock into the splinters. In other places pressures exerted from different directions provide several sets of fractures, and their intersection produces splinters.

Formation of pencil slates
Fig. FT 19. Formation of pencil structure in slates.
Pecil slates
Fig. FT 20. Well developed and unusually large pencils in slate (The iceax is about 3 feet long).

On to Stop 10. Fold along the Hurricane Hill trail

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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