Читать книгу Sierra South. Backcountry Trips in California's Sierra Nevada онлайн

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Now far from the main creek, the trail climbs up steep slabs, following the curve in the canyon as it turns east, steepens, and narrows. Downstream, the canyon is wide and gently sloping, and beyond the narrow gorge, the canyon floor again becomes a meadow, wide and gently sloping. This variation in form is due to differences in the structure of the bedrock.

EFFECTS OF GLACIAL EROSION

Although stream erosion did most of the difficult work to carve this canyon, glaciers have also modified its shape. Before glaciers come, stream canyons usually have a V-shaped cross-section. After the passing of glaciers, the canyons usually have a pronounced U-shaped cross section.

Glaciers, then, tend to do most of their erosion horizontally for several reasons. Primarily, Sierra glaciers are wet based: They ride along on a thin layer of water between the ice and the rock. (Water occurs there due to heat and pressure trapped under the heavy, insulating river of ice.) With this kind of lubrication, the ice tends to slide over, rather than dig deeply into, the bedrock. However, out at the margins of the glacier, colder air and thinner ice allow the glacier to freeze to the ground. Being “glued” to the sides of the valley, the passing glacier tears out pieces of bedrock and carries them away. This action is called glacial quarrying.