
Looking south down Ross Lake
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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.
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