Monday, December 17, 2007

Mount Talbert and Landslides

Built by volcanic outpourings, carved by ice age floods, Mount Talbert has some very steep slopes. Naturally enough some landslide potential exists. This mountain seems to consist mostly of grey andesitic flows which are quite resistant to erosion. The rock bones of the mountain are mostly near the surface. Only where deep soils have accumulated is land slippage likely.

The mechanics of most landslides are fairly simple. The section of ground destined to slide cracks loose from the remaining slope along a nearly vertical but curved surface and gravity tugs until the slide block rotates downward. The falling mass moves most quickly along the curved surface next to the remaining slope, so the top angles back towards the slip surface. The rotating block begins to break up from the bottom. This process often continues until the block disintegrates, but an equilibrium can be reached, stopping the process - part of the block stops in place, freeze-framing the process at any point. I recently located such a slide on the west side, above some apartments.

Just below the Loop Trail is the sudden drop characteristic of a slip surface. The drop is some thirty feet. A game trail tracked by deer passes over a seepage area onto the hummocky top of the frozen-in-place slide. Many hummocks are made as the slide block breaks up. Some firs rode the slide and survived, slanting strongly towards the mountain. Adjacent trees on different hummocks slant at different angles while those on a single block angle the same. The trees in the decades since this land movement grew upright, bending the trunk.

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