Saturday, December 22, 2012

Kettles and Dunes on Whidbey

Blue sky and even sun was a nice treat earlier this week - its been wet in western Washington this fall. I had a nice view of a section of the west shore of Whidbey Island during the return trip from Port Townsend.
West shore of Whidbey Island - note dip on the left center of the bluff slope
This particular reach of Whidbey shore stands out as a usually tan to brown streak between the waters of the Salish Sea and the uplands covered with evergreen forest. For a brief period the slope will turn green in the spring, but otherwise it is like an out of place swath of high plains prairie on the shores of western Washington. There are even cactus on this slope!
There are several reasons why this area is brown. For one thing this area is located within the rain shadow of the Olympics and has rain fall totals a bit under 20 inches. Second the slope is a southwest facing slope so it tends to dry out. It is also windy another drying factor. But another factor is the geology. The slope is underlain by sand and gravel and hence another factor in the dry condition. The slope is also an eroding slope. Waves routinely erode the base of the slope and with the thin soils and active erosion trees are not able to get established.
Toward the end of last glacial period this location marked the southern terminus of the Puget ice lobe. The ice had retreated out of the main portion of Puget Sound and for a period the ice margin lingered near this location on Whidbey Island.
The Washington State Department of Natural Resources produced a map of this area that provides an excellent explanation of some of the features on the quadrant (Poelnz, Slaughter and Thorsen, 2005, The map include an inset figure with LiDAR reproduced below showing some of the ice margin features and a photograph of the same bluff shown in my picture above.

LiDAR of central Whidbey Island in Coupeville area
Red circle corresponds with dip in bluff shown in my picture
above and in the DNR map inset picture below 

Photo inset from DNR map
Qd - dune sediments, Qs - soils, Qgome - glacial marine drift 
The dip in the slope is a deep kettle. A spot where a large block of ice melted leaving a big pit. This particular kettle was partially filled by sand - note the Qd (dune sediments) on the picture and the sand dunes that show up in the LiDAR.
There are a number of locations above west facing bluffs on Whidbey Island as well as few other bluff sites on the Salish Sea where there are sand dunes or sand accumulation areas. After a large slide exposes a lot of sandy bluff soil, the strong winds blow the sand up the bluff to form dunes on the top of the bluff.
Given the dry conditions in this area, the forest currently at the top of the bluff may not have always been present. Indeed a few miles to the south of here, there were open prairie areas when the first European explorers arrived in the late 1700s. 


B and G said...

Dan, do you think the Coupeville moraine is a lateral or terminal moraine. There are drumlins indicating southerly ice flow to the south and drumlins indicating westerly ice flow to the north, I've always said the glacier stopped right at my house, (more or less)as the Partridge gravel hill at the east end of the moraine is in my back yard. (Qgome)

Dan McShane said...

B and G: I think I'd go with the term terminal as it marks the extend of ice coverage at least for a period of time. Might have to run this by a glacial linguist though.