Laboratory Manual for Introductory Geology 4e

EXERCISE 10.14
Name:
Course:
Horizontal and Vertical Contacts on a Geologic Map (continued)
Section:
Date:
(b) The gray stripe is a vertical basalt dike. Remember that when you are looking at a geologic map, you are looking
straight down from the sky. With this in mind, explain why the dike appears as a straight line on the map.
Block diagram.
Dike
Contour
Contact
Stream
Geologic map.
10.5 Structures Revealed in Landscapes
Unless you live in the Great Plains or along the Gulf Coast of the United States,
you know that landscapes tend not to be as flat as the tops of the idealized block
diagrams that we’ve worked with so far. In many cases, the distribution of rock
units controls the details of the landscape, so erosion may cause structures
to stand out in the landscape, especially in drier climates. For example, resistant
rocks form cliffs, whereas nonresistant rocks form gentler slopes. Thus, a
cliff exposing alternating resistant and nonresistant rocks develops a stair-step
profile. Where strata are tilted, resistant rocks form topographic ridges, whereas
nonresistant rocks tend to form valleys (FIG. 10.13a). Generally, the ridges are
asymmetric—a dip slope parallel to the bedding forms on one side, and a scarp
cutting across the bedding forms on the other. In the Appalachian Mountains
of Pennsylvania, ridges trace out the shape of plunging folds (FIG. 10.13b). A
region in which the structure of bedrock strongly influences topography is called
a structurally controlled landscape, and you will see how this is manifested in
Exercise 10.15.
264 CHAPTER 10 INTERPRETING GEOLOGIC STRUCTURES