I had a work venture up the Skagit Valley near Marblemount. A major tectonic structure is located near Marblemout, the Straight Creek-Fraser River Fault. The fault is a strike slip fault with the west side having moved about 60 miles to the north. Movement took place about during the Eocene. The fault divides the Northwest Cascades to the west from the Cascade Crystalline Core to the east. Near Marblemount the rocks on the west side of the fault consist of the Shuksan Greenschist.
There is a good chance that you have seen pictures of Mount Shuksan as viewed from Picture Lake. Ned Brown noted in his book Mountain Building Geology in the Pacific Northwest that he saw a mural of Mount Shuksan in a coffee house in Japan. The back of my 4th grade class located nearly 1,000 miles from Shuksan had a mural of Mount Shuksan and Picture Lake and I saw the same mural image in a taverna in Greece.
Shuksan Greenschist has a high density and in places where the joint sets in the rock are widely spaced, the rock breaks into large heavy blocks that make for good ocean jetty rocks, in particular at the mouth of the Columbia River (skagit-county-greenschist). This possible use has generated some interest in mining into a mountain side a bit west of Marblemount. My venture took me past the potential mine site.