ASBOG Practice Exam

What indicates a plunging fold on a geological map?

1. A circular outcrop pattern

2. A U-shaped outcrop pattern

3. A linear outcrop pattern

4. A random pattern without shape

The correct answer is: A U-shaped outcrop pattern

A plunging fold on a geological map is characterized by a U-shaped outcrop pattern. This type of fold occurs when the axis of the fold is not horizontal but tilted or "plunging" at an angle into the Earth. As a result, the layers of rock exposed at the surface reflect this geometry, often creating a U-shaped appearance in their outcrop. This is indicative of a syncline where the rocks bow downward in the middle, or an anticline where rocks arch upward, but the plunging nature of the fold leads to the U-shaped configuration as it descends below the surface. In contrast, a circular outcrop pattern typically represents a dome or basin, where the layers wrap around a central point. A linear outcrop pattern may suggest more straightforward geological features like faults or vertical layers, while a random pattern without shape does not provide the orderly structure needed to indicate a fold. The U-shaped pattern is thus the most informative about a plunging fold's orientation and structure in geological mapping.