Friday, May 06, 2016

Double Bluff

I've always assumed that Double Bluff gets its name from its configuration which presents two distinct points to ships sailing in or out of Puget Sound. In the past, I've posted from the southeastern point (Double Bluff: 2010) and the northwestern one (Double Bluff: 2008). This time we're back near the west end again.

The lower portion of this bluff is the type-section of the Double Bluff Drift -- which pre-dates the most recent Vashon glaciers by more than 100,000 years. I believe the upper portion is Whidbey formation - interglacial fluvial sediments. The two photos above show a fossil-rich peat layer and some finely-laminated silts and clays.


A large failure earlier this spring has dumped a huge pile of material across the beach. Although waves have already cut it back tens of feet, a bump will probably remain for many years. This sediment - at least the sand and gravel fraction, is destined to travel north to the beaches of Mutiny Bay.

Twenty years ago on a field trip (led by Don Easterbrook), we talked about a reddish layer about half way up the bluff. The story was that a peat bed had caught fire and oxidized the surrounding sediments, sort of a natural brick kiln. Coal seams and peat beds, ignited by lightning, forest fires, or humans, can sometimes burn for decades.

The recent landslide had scattered brick-like lumps of peat and clay across the beach and some were already beginning their march north.

For another twist on beach sediment derived from fire-baked sediments, check out this post from the eastern side of Wyoming's Big Horn Mountains:
Lake DeSmet: 2013

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