Saturday, March 06, 2010
Possession Point marks the southernmost tip of Whidbey Island and is built of a tall stack of sediments that record a series of late Pleistocene glaciations. The 350' bluff has a propensity to collapse onto the beach in big chunks, but they don't stay long. Southerly waves rapidly move the sediment away, either east or west, depending on the storm and the orientation of the particular stretch of beach. On Puget Sound, we call beaches where the net direction of longshore transport is equivocal divergent zones. Once the sediment moves around the corner, it can pretty much only move north (sure, occasional northerlies might reverse this, but the odds are against it in the long run).
Possession Point feeds two drift cells. Sediment that moves east gets transported up the eastern side of the island, past Possession, Glendale, Columbia Beach, under the Clinton ferry dock, and on up to Sandy Point. The abundance of sediment and the orientation of the coast give rise to a series of extended spits that run along the base of the bluffs - beautiful wetlands now turned into long lines of houses.
Sand and gravel transported westward from Possession Point ends up in a spit on the east side of Cultus Bay now known as Sandy Hook - although its journey has been complicated by groins and parking lots built out across the beach.
The foreshore of the point is mainly coarse gravel - but like so many places on the Sound, if you remove the top layer of pebbles, what lies underneath is often sand and broken shell.