skip to main |
skip to sidebar
I suspect this is the same beach I posted pictures from several years ago (Gravel Beach: Sept 2012), although there are several fairly similar beaches along this stretch of Going to the Sun Road. The lake seemed a little higher this year and was encroaching into the trees in a few places along the shore. I don't think the level of the lake is controlled - the outlet, McDonald Creek, appears to be free-flowing - but it makes sense flows and water levels would be higher in June than in September.
There was a neat little recurved spit - looked a response to winds blowing west down the lake.
Most of the bedrock in Glacier belongs to the Belt Supergroup (a very thick stack of very old Proterozoic sedimentary rocks). The red and green argillites are widespread and when eroded by time and glaciers, yield red and green gravel, which waves shape into these nice little beaches.
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.
AERIAL VIEW
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.
Saint Mary Lake, on the east side of Glacier National Park, has some neat beaches. Unfortunately, I didn't actually get very close to them on this trip, but that isn't stopping me from writing this post.
When I was a kid, our family had to beach our rental canoe when the winds coming down the lake prevented our rounding the point to get back to the boat launch at Rising Sun. That beach, a narrow gravel berm littered with drift wood, was just below the classic Goose Island turnoff on the Going to the Sun Road (the barrier beach is barely visible in the middle photo).
Two opposing promontories form the Narrows of Saint Mary Lake. The westerly winds that have blown down the lake for thousands of years have built swash aligned beaches on the western shores of each of these points (AERIAL VIEW). The one on the north shore is the one where I was shipwrecked as a child. The larger and more interesting beach is a beautiful barrier on the southern shore (see top photo). The best way to explore it, short of a long hike, would be by canoe or kayak and with a a big can of bear spray.
If you explore the lake with the aerial imagery, you'll find other interesting beaches, too. There are some neat beaches spreading downdrift from a stream delta on the southern shore (AERIAL). And more along the toe of the alluvial fan that forms the southeastern end of the lake (AERIAL).
