Sunday, June 13, 2010
Travel and distractions have led me to fall behind. This post and the next date back to a Sunday in June, a low tide, and a walk on one of Seattle's most heavily used beaches.
Alki Point juts out into Puget Sound (April 2007). It is both a low point and a resistant headland. It is formed of Tertiary bedrock uplifted along the east-west strike of the Seattle Fault. These folded sedimentary rocks are exposed in the intertidal on the south side of the point and form a rich biotic community that lends itself to trampling during spring low tides by hordes of school children and strollers.
The story of longshore transport is messy here -- probably for reasons related to the seismic history. Drift is from south to north, but only a narrow band of gravel reaches the the point. There's not much loose sediment in the system south of here, at least for a few miles. There are plenty of human culprits - groins, seawalls, etc - but I suspect the main reason is geologic. If nothing else, when this shoreline was shaken and raised 1100 years ago, any beaches would have been stranded on the raised terrace, probably removing most of the sediment from the active beach (it's now beneath lawns and homes along Beach Drive). The beaches north of the point are doing better. As I pointed out in the 2007 post, I suspect Alki Beach is a fairly stable swash-aligned beach, oriented to face the northerly fetch.