Thursday, April 5, 2012

Not So Hard Rock

A chunk of bedrock

I had a little field trip to the south end of the Chuckanut Range, the local name for the mountains that extend from the Northwest Cascades to the shores of the Salish Sea above the Skagit Flats.

I had been to this site before. The first time was to assess slope stability in the area. The location was within a timber harvest area and the forester had identified a break between bedrock outcrops as a fault line. Sharp hard cliffs of semi schist were located on either side of a narrow valley that headed directly up the slope. No stream was present in this valley. Good call by the forester, as soon I began heading up this little valley I encountered this mushy white to dark green to black fibrous rock. I am not entirely sure of the suite of minerals but am fairly certain of talc (soap stone) serpentine and what appears to be asbestos fibers.

Material like this is often observed along the major thrust contacts in the Northwest Cascades. And depending on the angle and slope aspects it can be a real problem for slope stability.

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