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Geology of the National Parks


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Obstruction Peak Road

STOP 11: Collapsed pinnacle

Most geologic processes are of barely comprehensible slowness, but a few are rapid and even catastrophic. Where the Obstruction Peak Road emerges from the forest to balance precariously on a knife-edge saddle before the long hill up to the base of Steeple Rock (about 1 mile from the turnoff), look to the west to see some large blocks of dark rock among the trees on the bumpy terrain (fig. FT 23).

Collapsed pinnacle
Fig. FT 23. Collapsed basalt pinnacle along the Obstruction Peak Road.

The explorer who wanders down among these monoliths of pillow basalt will find a fantastic jumble of rock wails, blocks, and rubble piles. The scene resembles the ruins of some gigantic city, overgrown and partly hidden by trees At the base of some huge mountain escarpment we would expect to find such debris fallen from the heights, but there is no such escarpment near here. Up the road, however, is a clue to the ruin in the form of Steeple Rock, an erosion-resistant pinnacle of basalt standing high above the rolling meadows of slate and sandstone. It is part of an uptilted zone of basalt layers, the inner basalt ring, extending many miles to the southeast (see figs. 16 and geologic map). The rocky ruin lies along a projection of the same zone to the northwest The rubble probably once stood as a pinnacle like Steeple Rock. From the amount of debris, we can surmise that the pinnacle was even larger. The cause of its collapse appears to be landsliding. Slippage of the surface layers of rock, especially in areas of weak slate or shale, is very common in the Olympics.

Side hill creep trough
Fig. FT 24. Sketch of depression along the Obstruction Peak Road.

Along the Obstruction Peak Road are shallow swales paralleling the contour of the hillside; the road follows some of these features (fig FT 24). Similar to ridgetop depressions, they are caused by downhill creep of surface rubble and in some places of the bedrock itself.

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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