Geological map
Stratigraphic contour lines may be used to illustrate the surface of a selected strata illustrating the subsurface topographic trends of the strata. Isopach maps detail the variations in thickness of stratigraphic units. It is not always possible to show this properly: the strata may be extremely fractured, mixed, or otherwise disturbed. There are often gaps (discontinuities) where erosion has removed strata.