Laboratory Manual for Introductory Geology 4e

FIGURE 10.11 Constructing a cross section.
N
A
X
30
B
C
D
X
v
Contact position
30º
Tick indicating dip angle
v v v
X'
v v v v
Scrap of
paper
30
30
30
E
X'
Step 2: Identify contact positions. Add dip marks at correct angles.
X
X'
v v v v
E
A
B C D
N
Map
Step 1: Mark data locations on the cross-section paper.
(a) Example with gently dipping beds.
Y
A
X
40
30
W
20
20
10
Scrap of
paper
(b) Example with folded beds (syncline).
X
Y
30
35
v v v v v
Q
40
Step 1: Mark data locations on the cross-section paper.
A'
Map
Step 3: Draw contacts so they obey location and dip data.
Contact position Tick indicating dip angle
A v v v v 35º v A'
Step 2: Identify contact positions. Add dip marks at correct angles.
A v v
W
v v v A'
X
Y
Q
Interpreted contact
Step 3: Draw contacts so they obey location and dip data.
The cross section is a vertical plane inserted into the ground along the line of section.
Take a strip of paper and align it with your line of section. Mark the points where contacts
cross the strip of paper. Transfer these points to the cross-section face on the right.
Using a protractor, make a little tick mark indicating the dip of the contact; use the
strike and dip symbol closest to each contact to provide this angle. Next, in the subsurface,
sketch in lines that conform to the positions of the contacts and the dip angles
(in Fig. 10.11a, the contacts dip gently to the southeast). In Figure 10.11b, the contacts
curve underground to define a syncline. Unless there is a reason to think otherwise, the
layers should have constant thickness. Note that, because of this constraint, Layer Q in
Figure 10.11b appears in the lower left corner of the cross section; it would come to the
surface to the west of the map area. Construct your own cross sections in Exercise 10.12.
10.4.3 Basic Geologic Map Patterns
Geologic maps can get pretty complex, especially where structures are complicated or
where topography is rugged. But, by applying what you have learned so far about block
diagrams, you can start to interpret them. To make things simple, we begin with some
very easy maps of areas that have no topography (i.e., the ground surface is flat), as in
the block diagrams that you’ve worked with. Exercise 10.13 challenges you to look at a
map and imagine the three-dimensional structure it represents. Keep in mind that sedimentary
and extrusive igneous rocks are commonly deposited in horizontal layers, with
the youngest layer at the top of the pile and the oldest at the bottom.
260 CHAPTER 10 INTERPRETING GEOLOGIC STRUCTURES