Underneath everything that we see at the surface of the earth is a continuous, complex layer of solid rock that geologists call 'BEDROCK'. In most places bedrock is covered by water, ice, snow, vegetation, soil, loose sediment or structures made by people. Where it does show at the surface, it forms 'OUTCROPS'.
Bedrock is not a specific type of rock, such as granite, marble, slate or sandstone. It may be any type of rock. The particular type of rock varies with the location. For example, the bedrock face of this cliff in California is made of orange-pink granite.
Along this beach in Cornwall, England, the bedrock is a series of light and dark, tilted rock layers.
In Central Park, Brooklyn College students examine an outcrop of bedrock that consists of a gray rock traversed by two pinkish white stripes.
In most places, outcrops of bedrock are rare. But there are special circumstances that favor the exposure of bedrock. For example, on cliff faces that are too steep for loose material to adhere, bedrock outcrops may be found.
In some places along the seashore, where the waves are rough, loose material is continually swept away and the bedrock is exposed.
In areas that have recently been glaciated, outcrops of bedrock may be common. During glacial times, the moving ice removed all loose debris leaving vast areas of the underlying rock exposed.
Over about 70% of the globe, bedrock is covered by the waters of the oceans and by ocean floor sediments. Here, off the coast of the northeast Bronx, a bedrock knob protrudes from the water.
For geologists, it is important not to confuse large boulders with bedrock. Any rock that is loose and has been shifted appreciably from its original location is no longer bedrock. Geologists consider large boulders, such as these found in Big Bend National Park in Texas, to be very large grains of sediment.
On a fieldtrip to Central Park in New York City, Brooklyn College students are looking for evidence that will help them decide whether the rock in front of them is an outcrop of bedrock or merely the top of a large, partly buried boulder. Sometimes it is difficult to tell.
Geologists are not only interested in studying 'bedrock geology'. Some geologists focus their investigations upon the character, distribution and origin of the sediment that covers large parts of the earth's surface. Since the sediment overlies the bedrock and generally forms relatively shallow deposits at the surface of the earth, it is the subject of what is called 'surficial geology'. Here, stream deposits in Utah are seen.
Beach deposits along a stretch of the Pacific Coast along the Olympic Peninsula in Washington consist of both fine sediment (sand) and coarse sediment (boulders).
The floor of a lake in Norway fed by glacial meltwaters displays many thin, horizontal layers of sediment that have accumulated over the years.
Wind blown sand accumulates as dunes in the Sinai desert in northern Egypt.
As geologists study 'bedrock' and 'surficial' geology, they use the data they collect to construct geologic maps. These maps show the investigator's conclusions about the distribution of geologic materials. All geologic maps are hypotheses, and as such, are subject to revision as more data is accumulated and interpretations change.
David J. Leveson