Sunday, August 16, 2009

Centennial Beach

Lily Point, at the southeast corner of Point Roberts, has been feeding sediment to the beach for millenia and waves have been sending it north to form the beaches that extend from Maple Beach
on the American side into Beach Grove on the Canadian side. Yes, several thousand cubic yards of American dirt going to Canada every year -- no charge, no duty, no intention to return. The sediment has accumulated in a series of prograding spits in the northwest corner of Boundary Bay, where they created the large marsh behind and beneath Beach Grove - now diked and managed to accommodate the growth of suburban Vancouver (this is all part of Tssawwassen).

The long history of accretion means there's a broad sandy backshore, and even some semblance of low dunes. The upper foreshore is a narrow band of gravelly sand, running along the edge of the broad flats of Boundary Bay. The transition between the upper foreshore and the flats occurs at mid-tide or above. The lower beach appeared to be a lag surface of small gravel.

What I found intiriguing was the strong southeast-northwest fabric to the lower beach - a pattern seen both in gravel ridges (amplitude of ten cms or so and wavelength of several meters) and in the sand between them (amplitude of mms and wavelength of several cms). Strange. These did not look like ripples or cusps or any sort of depositional feature - they looked more like erosional features (I'm not sure why I felt that). And their orientation was parallel to the large fetch from the southeast - where I might have expected just the opposite. It looked like a surface that had been scoured by strong southeast winds and waves. As with every beach, there is a story here. I just don't have much clue as to what it is.

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