1. n. [Geology]

A map displaying lines that include points of equal value and separate points of higher value from points of lower value. Common types of contour maps include topographic contour maps, which show the elevation of the Earth's surface; structure contour maps, which show the elevation or depth of a formation; and gross or net sand or pay maps, which show variations in the thickness of a stratigraphic unit, also called isopachs.

See: contour,  isochore,  isochron map,  isopach,  spill point,  structure map,  topographic map