Tuesday, February 5, 2013

Glacial Drift Slab Failures in Port Townsend

A cliff of vertical glacial drift rises above the Port Townsend, Washington waterfront. The drift was deposited directly by glacial ice. The approximately 2,000 to 3,000 feet of glacial ice that was piled on top of the drift has greatly compacted the drift such that it is very hard and stands like a wall of concrete. However, some moisture seeps in, and after a bit of freeze and thaw weather occasionally a slab will peel off the face. On my last trip through PT, several slabs had calved off the cliff face. This gave me a chance to assess the size and mechanics of the recent failures - not an idle issue for me as I encounter slopes like this doing geology hazard assessments. 

Small slab failure

This failures was a bit bigger and appears to have initiated further up the slope and over topped the road side barrier

Recent slab failure next to a slope that has recessed back a bit more and has a lower vertical cliff

Closer view of the slab failure, note the few boulders embedded in the drift

View looking north along Water Street or Washington State Highway 20

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