Sunday, September 30, 2012
Lake McDonald lies on the west side of the continental divide, the counterpart of Saint Mary Lake on the east side. Based on a quick review of Google Earth, it looks to me like the best developed beaches on the lake may be at the head end, where westerly waves rework sediment delivered by McDonald Creek.
Along the southeast side of the lake, however, small beaches occur in association with minor bumps on the shoreline. Some may correspond to small stream deltas, but not all do. This is based on very limited observations, but the shoreline between these points appears to be sediment poor. These photos are from one of these small points, where the beach has formed a small cusp at one of these subtle promontories.
Based on some Puget Sound examples, I suspect this may reflect a tendency of promontories on shorelines with highly oblique wave action to act as attractors for sediment (this is related to work done in the last decade by Ashton, Murray, and others, too). This contrasts with the classic coastal model which suggests that headlands should erode and sediment should accumulate in the areas in between. Those examples make sense on ocean coasts with big waves where incidence is more normal to the shore.
In Glacier, most of the beaches consist of green and red pebbles eroded from the Precambrian sedimentary rocks of the Belt Formation.