ESS 211
CONSTRUCTION OF GEOLOGIC CROSS-SECTIONS
GOAL: To learn the techniques of constructing geologic cross sections using an actual
geologic map. You will also learn about the geometry of reverse faults and how their
outcrop patterns relate to their disposition at depth, below the earth’s surface.
Your assignment is to draw a geologically reasonable and plausible cross section.
You can choose one of three laminated U S Geological Survey [USGS] geologic maps:
 Section line A—A' on USGS GQ-1549, the Pine Creek quadrangle,
Wyoming
 Either section line B—B' or C—C' on USGS GQ-1092, the Clause Peak
quadrangle, WY
 Section line A—A' on USGS GQ-711, the Castle Reef quadrangle, Montana.
Your cross section should extend from A to A. For the eastern half, where
Cretaceous strata are exposed, you only need to show the geology above a
depth of 4500. However, you are welcome to extend the section to a greater
depth.
The explanations for all the map symbols and units are not laminated and will be
posted or left on the tables in the lab room.
PLEASE DO NOT WRITE OR MARK ON THE LAMINATED MAPS OR THE
EXPLANATIONS, EVEN IN PENCIL.
We suggest that you look at a cross section on one of the posted USGS maps, like the
ones that you used in the map-interpretation exercise, to appreciate the general
appearance of professionally drafted cross sections. Note that each unit in the section is
labeled so that the units can be identified using a simple accompanying legend. Also
examine the maps and cross sections from the Canadian Rockies, published by the
Geological Survey of Canada. Note especially that thrust faults that dip steeply at the
earth’s surface may dip progressively more gently at depth. Faults of this character are
called listric.
BEFORE YOU BEGIN: An excellent habit to develop is to make a rough sketch on a
separate piece of scratch paper of what you think the final, professionally drawn cross
section will look like. For this sketch, you do not have to worry about an accurate
topographic profile, nor does the sketch have to be at the same scale as the map. Don’t
exaggerate the vertical scale, however! Sketches like these will improve your ability to
examine any geologic map and envision the essential attributes a cross section drawn
along any randomly chosen line.
STEP 1. SKETCH A TOPOGRAPHIC PROFILE. On your piece of Clearprint
drafting film, construct a section box with a vertical scale equal to the horizontal scale of
the map. Trace a topographic profile from those provided in lab.
STEP 2. ADD STRUCTURAL AND STRATIGRAPHIC DATA. Just like you did in
the earlier, simple cross-section exercises, mark contacts and faults on the topographic
profile. These surface data are the most important controls for projecting the contacts,
units, and structures to depth. The locations of the contacts in particular cannot be
fudged. To estimate the dips of the units and the contacts, use the strike-and-dip symbols
closest to the section line. Don’t worry if the section line is not exactly perpendicular to
the strikes. You can correct the dip measured in the field to an apparent dip using the
nomogram from an earlier lab exercise. You will probably have to choose an average
strike and dip to account for the natural and actual variability of attitudes in the field.
LOOK ON EITHER SIDE OF THE SECTION LINE, along the strike of the units, to
see if there are units or structures present that will appear on the cross-section below the
surface. We say that these project into the line of section. For example, a unit that does
not crop out along your section line may crop out along strike. This common situation
will give you confidence that the unit is present at depth along your section.
HOW DEEP AND HOW HIGH? You have to decide how much of the blank area
below the earth's surface you want to fill in with geology. The general guideline we use:
include those units and structures that you are fairly confident are present. Try to fill in a
strip that extends approximately 2000-3000 feet below the topographic profile [note the
instructions above about Castle Reef section A—A']. If you show something in a cross
section, you should be able to explain to a skeptic the reasoning behind your action. You
might say, for example, “This unit crops out one km north of the section line and there it
dips to the south.” If folds or faults are present in your section, you can dash them into
the air above the surface to indicate their general form. After all, these features and the
rocks that they affect used to occupy this space before they were eroded!
You may choose to leave part of the section box blank, because you are not confident
enough about the rocks and structures actually present.
KEEP IN MIND that these rocks were all deformed in the upper crust at relatively low
temperatures and pressures. Therefore, each unit will retain a relatively constant
thickness in the cross section. You will probably find that you will have to slightly adjust
the dips plotted at the surface to ensure that this attribute is respected.
CHECK FOOTWALL AND HANGING-WALL CUTOFFS. If your section portrays
a hanging-wall cutoff and a footwall cutoff that you infer were matched prior to slip
along a reverse fault, the template constraint requires that all of the beds that are
truncated by the fault in either cutoff are matched by those in the other.
STEP 3. LABEL AND COLOR THE UNITS & LABEL THE STRUCTURES. Use
the abbreviations on the maps (e.g. Kgu). Use normal line-widths for contacts, and
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heavier lines for faults. Be sure to include a horizontal scale, and plot elevations along
both sides of the section. Your colors do not have to exactly match those on the map, but
it’s a good idea to take them from the same part of the spectrum.
ENSURE THAT YOUR SECTION HAS A PROFESSIONAL APPEARANCE. You
do not need to print labels using a computer, but try to make your lettering, line work,
and coloring as neat as possible.
A FEW THINGS TO KEEP IN MIND:
STRAIGHT EDGES MAKE STRAIGHT LINES. They are great for drawing the
boundaries of the section box and underlining words. Don't use a straight edge for
drawing folds and faults in your sections, especially where you can depict faults as listric.
GRADING. Your score will be based on criteria including location and orientation of
folds, faults, and bedding; legend, labels; geologic plausibility; and neatness.
NO separate write-up or discussion is necessary.
CHECK LIST OF ITEMS THAT SHOULD BE PRESENT ON YOUR CROSS
SECTION:
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

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
Title (Cross section A-A´, Pine Creek…]
Scale bar; V = H
Your name
Colored boxes for each geologic unit shown on the section
For each box, name of the unit and its age
Names of major faults and folds identified on the geologic map
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