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Home Archived Aug 4, 2016

USGS Geology in the Parks

Field trip 2 map

SIDE TRIP B - South end of Ross Lake, off the North Cascade Highway (State Route 20)

Ross Lake Boat Landing

Ross Lake view
Looking south down Ross Lake

Finding marble in the Skagit Gneiss

Much of the Skagit Gneiss Complex that holds up the mountains at the south end of Ross Lake is so thoroughly metamorphosed, and has been so thoroughly injected by magma, that even the experienced geologist cannot always be sure how the rocks began. Blasted roadcuts at the edge of Ross Lake, next to the Ross Lake Resort boat dock, reveal gray marble, a metamorphic rock derived from limestone. Although the marble is lighter-colored on broken surfaces, the dark layers reveal the layering of the original sedimentary beds. Most limestone begins life in a body of water at the Earth’s surface, probably as accumulated shells (made of calcium carbonate) of marine animals. We think that the schists of this outcrop were also derived from sedimentary rocks, but there are also plenty of white granitic dikes cross-cutting the marble and schist. How can the lighter-colored marble be distinguished from the light-colored igneous dikes? The marble scratches readily with a knife or ice-ax; the feldspar and quartz of the dikes do not.

On to Ruby Mountain

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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