Laboratory Manual for Introductory Geology 4e

EXERCISE 10.3
Determining Strike and Dip (continued)
Name:
Course:
Section:
Date:
N
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Strike: 0°
Angle of dip: —————————––
Direction of dip: ————————––
Strike: 290°
Angle of dip: —————————––
Direction of dip: ————————––
10.3 Working with Block Diagrams
We start our consideration of how to depict geologic features on a sheet of paper
by considering block diagrams, which represent a three-dimensional chunk of
the Earth’s crust using the artist’s concept of perspective (FIG. 10.4a). Typically,
geologists draw blocks so that the top surface and two side surfaces are visible.
The top surface is called the map view, and the side surfaces are cross-section
views. In the real world, the map view would display the topography of the land
surface, but for the sake of simplification, our drawings portray the top surface
as a flat plane. In this section, we introduce a variety of structures as they appear
on block diagrams.
10.3.1 Block Diagrams of Flat-Lying and Dipping Strata
The magic of a block diagram is that it allows you to visualize rock units underground
as well as at the surface. For example, FIGURE 10.4b shows three horizontal
layers of strata. If the surface of the block is smooth and parallel to the layers, you
can see only the top layer in the map view; the layers underground are visible only in
the cross-section views. But if a canyon erodes into the strata, you can see the layers
on the walls of the canyon, too (FIG. 10.4c).
Now, imagine what happens if the layers are tilted during deformation so that they
have a dip. FIGURE 10.4d shows the result if the layers dip to the east. In the front
cross-section face, we can see the dip. Because of the dip, the layers intersect the mapview
surface, so the contacts between layers now appear as lines (the traces of the contacts)
on the map-view surface. Note that, in this example, the beds strike due north,
so their traces on the map surface trend due north. Also note that, in the case of tilted
strata, the true dip angle appears in a cross-section face only if the face is oriented
perpendicular to the strike. On the right-side face in Figure 10.4d, the beds look horizontal
because the face is parallel to the strike. (On a randomly oriented cross-section
face, the beds have a tilt somewhere between 0° and the true dip.) Practice drawing
tilted strata in Exercise 10.4.
246 CHAPTER 10 INTERPRETING GEOLOGIC STRUCTURES