4.4: Bedrock
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By bedrock I mean solid rock, exposed at the Earth’s surface or buried at shallow depths, that is connected continuously with rock at greater depths in the crust (Figure 2-4). A great many geologists spend most of their professional lives studying the bedrock of the Earth’s continents.
In some places on the continents, bedrock is exposed at the surface, in the form of what are called outcrops or exposures . There are natural exposures (to which the term outcrop is typically applied) and artificial exposures. Natural outcrops range in area from small, human-scale patches peeping out from the surrounding blanket of regolith, to whole mountainsides. Favorite places to find good outcrops are stream beds, hill slopes, mountain tops, and sea cliffs. These days, highway road cuts make the best artificial exposures; in times past, railroad cuts were prime exposures. Building foundations are good but temporary.
The percentage of continental surface where bedrock is exposed varies greatly, from nearly one hundred percent, in regions with rigorous climate and steep slopes, to zero, in regions with warm humid climates and little topographic relief. (The term relief is used for the magnitude of differences in elevation of the land surface in a given region .) The extent of exposure of bedrock depends mainly on the competition between two opposing processes: the rate of production of new regolith by weathering of exposed bedrock, and the rate of removal of existing regolith by the erosive action of running water or moving glacier ice or by downslope movement of regolith by the pull of gravity. Climate is important because the rate of production of regolith by weathering is much greater in warm and humid climates than in cold and arid climates. In some regions with a warm, humid climate and low relief, one needs to dig or drill tens or even hundreds of meters down to find fresh bedrock!