The areas that geologists want to study are often covered by vegetation, cities, or other surface features that obscure the underlying geologic materials.
Ideally, the geologist would like the surface cover removed to reveal the geologic materials, as in this three-dimensional drawing, called a block diagram. Here, the geologic materials are shown at the surface and on the sides of the block.
Geologists divide geologic materials into units called formations. A geologic formation is a body of rock or sediment that formed more or less in the same manner at approximately the same time. Formations are separated from each other by surfaces called boundaries or 'contacts'.
The top of the block diagram is a geologic map. A geologic map shows the shapes, sizes and distribution of the formations and their contacts as they would appear if you were looking directly down at the top part of the block diagram.
But the geologist doesn't actually see the complete arrangement of the formations and their contacts. Instead, what he or she sees at the surface of the earth are a few scattered exposures or outcrops of the formations: places where the geologic materials are not covered up.
At each outcrop, the geologist identifies the formations and determines if any contacts between formations can be seen.
The next and often trickiest task is to judge the size, shape and distribution of the formations and to infer where the hidden contacts lie. The geologist may note that the contacts that show in outcrops A and B lie along a straight line. An inferred contact is drawn connecting them and extended to the edges of the map.
The geologist theorizes that the red and green formations seen on each side of the observed contact extend, on either side, along the length of the inferred contact. On this map, that is indicated by the transparent red and green bands of color.
The geologist notes that the observed contacts at outcrops C and D parallel those at A and B. The simplest assumption is that beyond where they can be observed, the contacts at C and D will, like those at A and B, run straight north and south to the edges of the map. Like before, the red and green formations can be assumed to extend on either side along the lengths of the inferred contacts. These ideas are sketched on the map.
Somewhere between the green of outcrop E and the red of outcrops F and A there must be a contact between formations 2 and 3. There is no outcrop that shows that contact. Its position, shape, and orientation must be inferred. The simplest assumption is that this contact will parallel the other contacts and be a straight, north-south line. If that is correct, its position must be to the west (left) of E and to the east (right) of F.
No more contacts are required. All the observed outcrops of formation 2 (red) are separated from observed outcrops of formation 3 (green) by contacts. Now the colors representing the inferred parts of the formations may be extended to fill the areas between contacts. Every part of the map may be colored in.
The end result is a geologic map. It represents visually the geologist's idea of how geologic materials are arranged in this area.
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David J. Leveson