Source: Laboratory of Alan Lester - University of Colorado Boulder

Geologic maps were first made and utilized in Europe, in the mid-to-late 18^th century. Ever since, they have been an important part of geological investigations all around the world that strive to understand rock distributions on the surface of the earth, in the subsurface, and their modification through time. A modern geologic map is a data-rich representation of rocks and rock-structures in a two-dimensional plan view. The base for most geologic maps is a topographic map, onto which color variations have been placed to represent specific rock units. The boundaries between the rock units are called contacts. In addition to the contact lines, geologic maps contain symbols that represent key features, such as the dip and strike of the rock units, anticlines and synclines, and the traces of fault surfaces.

Although the two-dimensional map-view is useful, one of a geologist's key tasks is to infer the type and orientation of rocks in the subsurface. This is done using geologic rules, inferences, and projections downward from the surface. The result is a geologic cross section, a view that essentially provides a cutaway image, much like one would see on a canyon wall or in a roadcut.

This hypothetical slice into the earth, providing a third dimension (depth), is the key to a host of geological applications. Cross sections are used to assess temporal models of rock formation through time. In other words, the goal is to recreate a step-by-step sequence of which rocks and structures came first, last, and in between. They're also used to determine specific modes of deformation - whether rocks have undergone compressional, extensional, or other stresses.

Geologic cross sections help to identify regions of groundwater movement, evaluate potential sites for economic mineral deposits, and locate oil and gas reservoirs.