Thursday, June 07, 2007
I hadn't walked the beach east from Tolmie State Park in over a decade, so I was a bit apprehensive about doing it with a class. As it turned out, it not only went well, but provided me an opportunity to revisit a site that I had wondered about for years.
In the mid-1990s, a brand new bulkhead was built at the base of the near-vertical bluff just west of Butterball Cove. Shortly afterwards, the bluff collapsed on top of the new wall. I've used a picture of that partially buried seawall for years to show the limitations of a narrow-minded focus on the bluff toe. It turns out that more failures have occurred since then and folks had to take my word that there was indeed a concrete seawall buried deep beneath the dirt and broken trees. The bluff itself has now been sprayed with gunnite (or shotcrete - are they the same?) and flowers neatly planted across the top. Someday, I suppose the bulkhead will be exhumed by waves - probably much to the surprise of the future owners of the property!
On very rare occasions on Puget Sound, we find Pleistocene sediments so resistant to erosion that they persist as headlands while the adjacent bluff recedes. This prow on the beach at Butterball Cove is one such example - the older iron-stained conglomerate is a natural bulkhead, extending out to a mid-tide. Interesting that on the beach to the west we saw what appeared to be this unit and possibly an underlying one outcropping in shore parallel bands on the beach. Again, not something we observe very often, since it probably requires resistant rock units, limited wave energy, and a minimal veneer of beach sediment.