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.

1. Identify two points that define a cross-section profile, e.g. A-A'.These points are chosen such that the line between them is approximately perpendicular to the strike directions of the intervening rock units.
2. A topographic profile is drawn between the two points, A-A'. Directions for how to generate a topographic profile are provided in a different video.
3. Take a strip of paper and align it along the line, carefully marking the contacts between the different rock units.
4. Transfer the conta

For this demonstration, a portion of the Carter Lake, Colorado, USGS 7.5-minute Quadrangle Map was used. This notation means that 7.5 minutes of longitude and 7.5 minutes of latitude define the E-W and N-S boundaries on the map. On the east side of the cross section line A-A', the rock layers dip to the west; in contrast, on the west side, the layers dip to the east. It can be inferred that these layers meet in the subsurface to form a bowl-shaped fold-structure, known as syncline. Ultima

Cross-sections provide a means to analyze and assess the subsurface orientation of rock units. Geologists use the relative dating rules of cross-cutting and superposition to determine the timing of deposition and deformation. For example, when one layer sits above another, it can be inferred that the top layer is most likely younger than the layer below. Furthermore, if a fault cuts across a particular rock unit, then the fault is most likely younger than the rock unit it offsets.

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