Introduction to Physical Geology
Geologic Structures
Understanding the formation of geologic structures in a region is important in reconstructing its geologic history. Generally, the greater the number of structures, the more complex the geologic history. Structures often offset, rearrange, or bury blocks of bedrock, making geologic interpretation more difficult. Understanding geologic structures is important not only to those in academic fields, but to those in industrial and engineering fields as well. Knowing the structural history of an area is important for finding petroleum and metallic ore bodies and for determining rock stability (for example, in order to locate dams and nuclear reactors on structurally stable bedrock).
Structural events are often inferred from how the bedrock has moved. For example, the law of original horizontality suggests that sedimentary rocks were deposited as flat-lying layers on a relatively horizontal surface. If these rocks are now exposed at the surface and are still flat-lying, it can be concluded that they were uplifted by an evenly applied tectonic force. If they are tilted in one direction, it can be concluded that the uplifting stress was greater at one end and pushed the layers into an inclined position. Occasionally, although the bedrock is horizontally layered, the sedimentary structures and age relationships shown by fossils indicate the top layer is the oldest—this arrangement indicates that somehow the entire sequence has been overturned by tectonic forces and what was the oldest layer on the bottom is now on top.
