Sunday, June 27, 2010
One of the neatest places to find coastal sand dunes is high on the top of bluffs. The Grand Sable Dunes, west of the town of Grand Marais, on the southern shore of Lake Superior are one of the best examples of perched dunes anywhere. Wind blowing across the lake accelerates up the bluff face, eroding the sandy sediments and carrying the sand over the crest where it is deposited to form dunes.
Our visit, and these pictures, captured very little of the actual dunes. The dunes themselves are on top of the bluffs, which consist of sandy non-aeolian deposits. The Google Maps aerial image linked to the title of the shows the dunes better. The bluffs are still very dune-like and I'm sure when D ran down and slogged back up the 300' vertical log slide, he didn't really care that it was a bluff and not a sand dune.
My post-visit homework was fascinating. Like dunes in many places, these have built in discrete episodes, the most recent 300 years ago. The episodes are driven by sand availability, which in turn is driven by the condition of the bluffs. High water levels lead to more rapid toe erosion. More rapid toe erosion leads to active, exposed bluffs. Active, exposed bluffs make more sand available for the wind to blow up the face of the bluff. More sand makes bigger dunes. And when the dunes are growing, they block streams, raising base level and creating ponds, which can create a stratigraphic record of dune building events and of variations in the level of Lake Superior.
We have perched dunes on Puget Sound, too. One set are high on the northwestern shore of Protection Island, north of Port Townsend. My favorite ones on Puget Sound would have been the dunes perched on the high bluffs of northwest Whidbey Island, overlooking the Strait of Juan de Fuca. Unfortunately, these dunes have been pretty much eliminated - leveled and buried under view homes on West Beach Road. They were not as large or extensive as these in Michigan, but they still would have been cool.