Monday, February 13, 2012

West Beach

The Hastie Lake boat launch is built on a depositional beach - in this case, a small spit that interrupts a long stretch of high bluffs.  Of course, it's hard to tell it's a spit beneath the asphalt, the riprap, and the houses.  There are more photos of the area in the preceding post and at:  Hastie Lake: Nov 2011.

The spit terminates about a quarter mile north of the boat ramp and the bluff begins to rise again.  The till gradually emerges above beach level and the bluffs continue to get higher.  Continuing north, the bottom of the till finally appears and the underlying stratified sediments emerge.  A few hundred feet farther north and the entire lower bluff consists of Whidbey Formation and the till, if it remains at all, is much thinner and much higher in the sequence.

Gerry Thorsen first called my attention to the distinct dogleg in the bluffs at the point where the till rises above the beach.  The till resists erosion south of the point much better than the Whidbey Formation to the north.  This is a nice example of how lithology at beach level can influence erosion rates.  It is not the only factor - wave exposure and beach character are both important, too.

The Whidbey Formation - imagine rivers flowing north across a broad floodplain towards a long-gone delta somewhere in the Strait of Juan de Fuca about 100,000 years ago - makes up a bulk of the bluffs to the north all the way to Swan Lake.  The bluffs rise to upwards of 250', higher if you include the perched dunes along the top edge of these bluffs (most now bulldozed for home sites).  Perched dunes, in case you weren't paying attention two years ago in Michigan (Grand Sable Dunes:  June 2010), are dunes formed by sand blown up and over the top of sandy bluffs.


Air Photo Note.  The Department of Ecology has just upgraded it's online coastal Atlas and air photo site.  You can find it at:
Washington State Coastal Atlas

It takes a little learning to navigate, but it is an incredibly powerful site, with a number of useful tools.  One feature, that I have long wanted, is the ability to link to individual photos.  I'll be looking for new ways to link between the blog and the Atlas in coming months.

Here's a link to a photo of the dogleg north of Hastie Lake:  2006 PHOTO

No comments: