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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 superimposed over a topographic map (and at times over other base maps) with the addition of a color mask with letter symbols to represent the kind of geologic unit, stratigraphic contour lines, fault lines, strike and dip symbols, and various additional symbols as indicated by the map key.
The color mask denotes the exposure of the immediate bedrock, even if obscured by soil or other cover. Each area of color denotes a geologic unit or particular rock formation (as more information is gathered new geologic units may be defined). 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.
Strike and dip symbols consist of (at minimum) a long line, a number, and a short line which are used to indicate tilted beds. The long line is the strike line, which shows the true horizontal direction along the bed, the number is the dip or number of degrees of tilt above horizontal, and the short line is the dip line, which shows the direction of tilt.
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.
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