Laboratory Manual for Introductory Geology 4e

of structures on the ground (the map surface) as well as on one or two vertical slices
into the ground (cross-sectional surfaces). A geologic map represents the Earth’s
surface as it would appear looking straight down from above, showing the boundaries
between rock units and where structures intersect the Earth’s surface. A cross
section represents the configuration of structures as seen in a vertical slice through
the Earth. FIGURE 10.2 shows how these different representations depict Sheep
Mountain in Wyoming.
EXERCISE 10.1
Picturing Stress
Name:
Course:
For each of the phenomena described below, name the stress state involved.
Section:
Date:
(a) You spread frosting on a cake with a knife. The frosting starts out as a thick wad, then smears into a
thin sheet.
(b) You step on a filled balloon until the balloon flattens into a disk shape.
(c) You pull a big rubber band between your fingers so that it becomes twice its original length.
(d) A diver takes an empty plastic milk jug, with the lid screwed on tightly, down to the bottom of a lake. The jug
collapses inward from all sides.
The purpose of this chapter is to help you understand the various geometries of
geologic structures and develop the ability to visualize those structures and other
geologic features by examining block diagrams, geologic maps, and cross sections.
In addition, this chapter will help you to see how the distribution of rock units,
as controlled by geologic structures, influences topography, as depicted on topographic
maps and digital elevation maps (DEMs). Geologic structures can be very
complex, and in this chapter we can work with only the simplest examples. Again,
our main goal here is to help you develop the skill of visualizing geologic features
in three dimensions.
10.2 Beginning with the Basics:
Contacts and Attitude
10.2.1 Geologic Contacts and Geologic Formations
When you look at Figure 10.2d, you see patterns of lines. What do these lines represent?
Each line is the trace of a contact, the boundary between two geologic units.
In this context, a trace is simply the line representing the intersection of a planar
feature with the plane of a map or cross section; a unit may be either a stratigraphic
10.2 BEGINNING WITH THE BASICS: CONTACTS AND ATTITUDE
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