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Rocks differ markedly with regard to their resistance to erosion by running water. Although no quantitative scales to express this difference have been developed, widespread agreement exists on certain generalities. Metamorphic rocks (those that are formed from preexisting rocks under the action of high temperatures and pressures), for example, are commonly more durable than are sedimentary rocks, and great differences can exist even among the latter because of a significant amount of variation in the degree of cementation and kinds of rock structure present in them. Thus, a quartz-rich sandstone whose constituent grains are cemented by silica tends to be ...(100 of 32811 words)