What is bedrock?
The earth's crust is composed of rock. But the type of rock is different in most places (click here for more information on some fundamental differences between the 3 major types of rock). Rock which occurs at any given place on the earth is called that location's bedrock. In some cases the bedrock is actually exposed, and is said to outcrop at the surface. Outcropping bedrock is great stuff, because it allows geologist to determine the local geology, and helps them put together the geologic history of the earth.
Unfortunately, in many places the bedrock is covered by water, dirt, plants, pavement, buildings, or other coverings which obscure the area's bedrock. If there is no outcrop, it is hard to determine the local geology. Often all that a geologist has to look at is loose pieces laying about on the surface. We call these disconnected pieces float. It is always a challenge to make a geologic map when all you have is float to work with, because it is often difficult to know how far the pieces have been moved since they were broken off by weathering processes.
As always, there are shades of gray between these two extremes. There are places where it is impossible to determine for sure that the exposed rock is connected to the earth (and therefore bedrock), but it sure does look like it is (and therefore probably not float). We call this "I don't think it's float, but I'm not convinced that it's attached" material subcrop. The term subcrop is also applied to material which has probably broken off, but has not been moved from it's original location.
In most cases, subcrop is better than working with float, but not nearly as desirable as bedrock which outcrops. It's important that a geologist spend some time determining which of these 3 situations he or she is facing: outcrop, subcrop, or float. How much confidence we can place on any geologic work is directly related to this issue, as well as to the geologist's overall abilities.
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