Grus
(c) 2004 Andrew Alden, licensed to About.com, Inc. (fair use policy)
Grus ("groos") is crumbled granite that forms by physical weathering. What causes that is the hot-and-cold cycling of the daily temperatures, repeated hundreds and thousands of times, especially on rock that is already weakened from chemical weathering by groundwater. The quartz and feldspar that make up this white granite separate into clean individual grains, without any clay or fine sediment. It has the same makeup and consistency of the finely crushed granite you would spread on a path. Granite is not always safe for rock climbing because a thin layer of grus can make it slippery. This pile of grus has accumulated along a roadcut near King City, California, where the basement granite of the Salinian block is exposed to dry, hot summer days and cool, dry nights.
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