Geologic map
A geologic map is a special-purpose map made for the purpose of showing subsurface geological features. In the United States, geologic maps are usually based on topographic maps, with the addition of a color mask, stratigraphic contour lines, and various other selected symbols.
The color mask denotes the exposure of the immediate bedrock, even if obscured by soil or other cover. Each area of color denotes a particular rock formation. However, in areas where the bedrock is overlain by a significantly thick unconsolidated burden of till, terrace deposits, loess deposits, or other important feature, these are shown instead.
The stratigraphic contour lines are drawn on the surface of a selected deep stratum, so that they can show the topographic trends of the strata under the ground. It is not always possible to properly show this when the strata are extremely fractured, mixed, in some discontinuities, or where they are otherwise disturbed.
Fault lines are also shown where known.
Whereas topographic maps are produced by the United States Geological Survey in conjunction with the states, geologic maps are usually produced by the states. There are almost no geologic map resources for some states, while a few states, such as Kentucky, are extensively mapped geologically.
See also
