Bedrock

A surface exposure of bedrock is called an outcrop. Bedrock is only rarely exposed, or crops out, where sediment accumulates rapidly, for example, in the bottom of stream valleys and at the base of hills or mountains. Outcrops are common where erosion is rapid, for example, along the sides of steep stream channels and on steep hill or mountain slopes. Deserts and mountain tops above the treeline also host good bedrock exposures due to the scarcity of vegetation, and resulting rapid erosion. Man-made outcrops are common where roadways cut through mountains or hilltops, in quarries, and in mines.

Generally, the more rock resists erosion, the more likely it is to crop out. Granite and sandstone commonly form well-exposed outcrops. Natural exposures of shale and claystone, both soft, fine-grained rocks, are rare—especially in humid climates.