Soil, rock material broken down to sand, silt, clay, may be quickly carried away by erosion, a single storm removing massive amounts of soil. Bear rock, however, does not erode quickly. Witness large rocks in streams–they remain apparently unchanged despite years sitting in running water. Weathering, chemical and biological action slowly but gradually breaking and eating into rock must occur first. Weathered tombstones and monuments attest as to rates of weathering.
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Biological weathering: Type into your browser: tree root damage images