Monday, May 18, 2015

More on Weathered Quimper Sandstone

Last week I noted a small natural bridge within the Quimper Sandstone (quimper-sandstone-notes). In my notes I speculated as to the source of the more weathered rock. Callan provides another possible and I think very reasonable explanation (spheroidal-weathering-in-sandstone).

There is plenty of spheroidal weathering within the Quimper Sandstone at the one- to two-foot scale. But as Callan's image shows, the scale can be much larger and the scale in the Callan image is very similar to the scale I observed.

Regardless, of the actual source of the preferential weathering, the deep weathering of the Quimper Sandstone at Oak Bay in presents some slope stability issues. Most bedrock lined shores of the Salish Sea are viewed as stable with very low erosion rates; however, the deeply weathered bedrock here is more readily eroded than some of the glacial sediments. In addition, the deep weathering has caused some areas of deep-seated sliding that may be related to the deep weathering.

Deep level sliding below the root zone is not unusual in deeply weathered bedrock and is indeed a problem in southwest Washington State. What is a bit unusual about this area at Oak Bay is that this area was glaciated but the glacial ice did not strip off the weak weathered rock at this spot. 


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