Sunday, February 15, 2015

Bald Mountain - A Metamorphic Knocker Near Lake Cavanaugh

I had a nice view of Bald Mountain in the western part of the Cascades of Skagit County.

The summit looks like a bit like Devils Tower in Wyoming - a big block of rock protruding above the landscape.

The bedrock is volcanic in origin, but this is not the plumbing of an ancient volcano; it is a block of metamorphosed basalt that is surrounded by much weaker metamorphic rocks that have been eroded away. Dave Tucker has a nice write up (fieldtrips/the-high-dome-of-bald-mountain-and-big-rock-too) with references so there is no need for me to repeat. He notes that blocks of resistant rocks sticking up out of softer metamorphic rocks are referred to as "knockers".

The LiDAR of the area shows the Bald Mountain has been partially eroded out by glacial ice having over ridden the area.   

The ice direction movement can be seen by the striations marked in blue. Ice from the Puget ice lobe flowed up into the Cascade Range at this location. This ice lobe blocked the Stilliguamish River causing the formation of a lake in the Stilly valley. Sediments associated with that lake were involved in the Hazel/Oso landslide. Note too in the LiDAR the two landslides that have scarred the glacial striated slope post ice age.  

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