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Home Archived Aug 4, 2016

USGS Geology in the Parks

Domains of the North Cascades

SIDE TRIP D - off North Cascade Highway (State Route 20)

Easy Pass

Easy Pass
On the east side of Easy Pass. The fault comes down the draw on the left.

A groovy fault

Easy Pass was eroded where the rocks of Ragged Ridge are crushed along an east-west fault. Rocks on the south side of the pass are speckled granitic rock of the Black Peak batholith. To the north are metamorphosed siltstone (now dense, fine-grained brownish-purple rock) and conglomerate of the Methow Domain, here intruded by numerous dark dikes. In places these rocks were stretched during metamorphism, and the conglomerate pebbles are now cigar-shaped. A little way to the northeast is light-colored Golden Horn granite.

To see crushed and grooved rock in the fault zone, leave the trail about a quarter-mile east of the pass and walk across to the south wall of the gully. Not every part of the rock wall has grooves. It may take some looking. The grooves were gouged along the fault as the block on one side scraped along the block on the other side. That the grooves are horizontal indicates the fault movement was also horizontal. A comparison of the offset of rock units on either side reveals that the south side of the fault moved eastward relative to the north side.

On to Maple Pass

Material in this site has been adapted from a book, Geology of the North Cascades: A Mountain Mosaic by R. Tabor and R. Haugerud, of the USGS, with drawings by Anne Crowder. It is published by The Mountaineers, Seattle.

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