Читать книгу The Pacific Crest Trail. Hiking the PCT from Mexico to Canada онлайн

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There are three main types of glacial erosion.

 Plucking: glacial melt water freezes around lumps of cracked and broken rock. When the ice moves downhill, the rock is plucked from the back wall.

 Abrasion: rock frozen to the base and back of the glacier scrapes the bedrock.

 Freeze-thaw: melt water or rain penetrates cracks in the bedrock. At night it freezes, expands and enlarges the crack, eventually breaking the rock away.

Volcanic rocks are easily eroded but granite is very resistant. Only where it is highly fractured or has been subjected to deep weathering is it easily eroded. Such weakened rock is easily excavated by glaciers, which leave basins of resistant granite that fill with water as the glaciers retreat. These are the corrie lakes that make granite mountain landscapes so attractive. Volcanic mountains lack the resistant rocks that allow lakes to form but in places glaciers have peeled them right down to the granite beneath, allowing the formation of lakes.

There are very few sedimentary rocks in the mountains through which the PCT passes.