Mapping in the Field

The ease with which structural geology can be understood is largely dependent on how much of the bedrock is available for study. In areas such as northern Canada, where much of the bedrock has been exposed by glaciation, as much as 75 percent of the bedrock can be walked on and studied. Alternatively, in the southeastern United States, often less than 10 percent of the bedrock is exposed because of abundant weathering, soil cover, and vegetation. Reconstructing the geologic history of an area can be especially challenging (and creative) if little rock is exposed.

Geologists try to find all the bedrock exposures, or outcrops, in an area to construct a geologic map. They identify rock types, relationships, textures, features (such as cross‐bedding), and structures (such as folds and faults) as well as cross‐cutting relationships of intrusive rocks, rock mineral contents, and fossils. Detailed directional measurements along structures, when plotted on a map, can reveal a bigger picture of how the rocks have been folded and faulted.

One of the most useful measurements is the strike and dip of a tilted rock unit (Figure 1). The strike of the unit is the direction (compass bearing) of the line formed by the intersection of the tilted bedding plane with the horizontal plane. The dip angle is the angle between the horizontal plane and the tilted bedding plane. Compasses equipped with a device called an inclinometer can determine the dip angle. The direction of dip is always perpendicular to the strike direction. For example, in Figure the rock strikes north‐south and dips 25 degrees to the east. A rock that is perfectly flat‐lying has no strike direction and no dip. A rock unit that has been tilted into a vertical position has a maximum dip of 90 degrees.

Figure 1

  Strike and Dip

A plan (two‐dimensional) geologic map shows the locations and shapes of the outcrops at an appropriate scale and indicates, through a variety of geologic symbols, features such as folds, faults, contacts between different rock units, and strike and dip. A geologic cross section, a vertical slice across the map area, can be constructed from the structural information on a geologic map. It depicts the spatial relationships of the rock units and structures beneath the surface (Figure ). A cross section supplies a third dimension to the plan geologic map. A good geologic map is critical to understanding and interpreting structures, when they formed, and how they fit into the overall geologic picture.

Figure 2

A Vertical Cross Section