Name__________________
GLS 100: Physical Geology
Structural Geology Lab
INTRODUCTION
Structural geology is the branch of geological sciences that examines the deformation of rock
units and attempts to reconstruct the causes for the observed deformation.
When a rock is placed under stress, it can behave in either a ductile or brittle manner depending
on the local conditions (heat, pressure, nature of stress) at the time of deformation. If ductile
deformation occurs, then the rocks are folded, and the resulting structure is a curved fold. If
brittle deformation occurs, then the rock fractures, and if movement occurs along the fracture,
the resulting structure is a fault.
Figure 1. Anticlines and synclines in sedimentary rocks.
Stress can come in three different varieties: compressional (pushed together), tensional (pulled
apart), or shear (sliding past). In the cases of compressional and shear stresses, either folds or
faults can result. In the case of tensional stress, only faults will be formed.
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In this lab you will conduct a scaled-down geologic field investigation of a field area (lab room)
and produce a geologic map of the area. You will then use your observations to determine the
geologic structure present. This information will allow you to determine the type of stress that
the region experienced, and will allow you to hypothesize as to the plate boundary type that your
area represents.
Strike and Dip
In any discipline, it is necessary to adopt tools to use to be able to explain your observations.
For structural geologists, it is important to be able to describe how a given rock layer is oriented
with respect to compass direction and angle of tilt. We use two measurements, strike and dip,
to describe this orientation.
The figure below illustrates both strike and dip of an inclined rock bed. If you imagine a
horizontal surface (in this case we represent it with water level), the strike is simply the direction
in which you would walk if you followed the shoreline. Note that this can be one of two compass
directions (If you were walking northeast along the shoreline and turned around to walk the
other way, you would be walking southwest). The dip angle is the angle that the rock layer
makes with the horizontal plane. The dip angle can by 0 (horizontal), 90 (vertical) or
somewhere in between (example shows dip of 30). Note that there is also a dip direction, in this
case to the east.
Structural geologists plot the rock type and strike and dip information on geologic maps, and
then interpret this information to determine what structural features are present in the region.
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Figure 2. Determining strike and dip.
Folded Rocks
Hard, brittle rock may fold and not break if the stress is applied slowly and continuously over
a very long period of time. Folds form purely from compressional stresses. Let's look at the
anatomy of a fold...
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Figure 3. Components of a fold.
Fold Types:
Syncline: Limbs dip towards the axial plane (shaped like a Smile - side view) Anticline:
Limbs dip away from the axial plane (shaped like an 'A' - side view)
Figure 4. Idealized cross-sections of a syncline and anticline.
Anticlines and Synclines may be:
Symmetrical (the above pictures; axial plane is vertical; limbs dip at the same angle)
Asymmetrical (picture below (side view; Asymmetrical Anticline); note how axial plane
is tilted; limbs dip at different angles)
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Figure 5. An asymmetrical Anticline
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Broken Rocks
Joint: A break in a rock (crack) in which there is no relative movement of either side across
the break.
Fault: A break in the Earth in which the rocks on either side of the break have been
displaced (vertically and/or horizontally) relative to each other (see pictures below).
Strike-Slip fault: Movement of the Foot Wall and the Hanging Wall blocks is parallel to the
strike of the fault plane. These faults form from shear stresses.
How do you name the strike-slip fault?
Just imagine you are standing on one side of the fault. How did the other side move
relative to the side you're standing on? If it appeared to move left, it is a left-lateral strikeslip fault. If it appeared tomove right, it is a right-lateral strike-slip fault.
Dip-Slip fault: Movement of the Footwall and the Hanging Wall blocks is parallel to the
dip direction of the fault plane. These faults form from tensional stresses or
compressional stresses.
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The picture on the previous page displays the "anatomy" of a Dip-Slip fault.
Let's go over it in more detail... Notice the terms 'Footwall Block' and 'Hanging
wall Block'.
These are old mining terms for the two sides of the fault (where the rocks have moved
relative to each other):
 The side a miner could walk down (put his feet on) was called the 'Footwall Block'
 The side a miner could hang his lantern on was called the 'Hanging Wall Block'
 The Footwall and Hanging Wall Blocks are separated by the Fault Plane
The Fault Plane can be thought of as the plane in which the rocks slide past one another in
opposite directions Movement arrows are placed around the fault plane in order to indicate
relative movement of the fault blocks. There are 2 kinds of dip-slip faults: Normal and
Reverse Dip-Slip Faults...

In Normal Faults, the Footwall Block moves UP relative to the Hanging Wall Block
An easy acronym is F.U.N. -Footwall Up = Normal
These form from tensional ("pulling apart") stress

In Reverse Faults, the Footwall Block moves DOWN relative to the Hanging Wall
Block
An easy acronym is F.D.R. -Footwall Down = Reverse These form from
compressional ("pushing together") stress
Another important observation regarding Dip-Slip faults is that they move older rocks
next to younger rocks.
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Map Symbols:
Strike and Dip
Horizontal Beds
Vertical Beds
Normal Fault: (Note that the arrow points in the direction the fault plane is dipping; it also
(therefore) always points to the hanging wall block!)
Reverse Fault (Thrust): (Note that the 'teeth' point in the direction the fault plane is dipping; it
also (therefore) points to the hanging wall block!)
Synclines:
(Dark line marks fold axis; arrows denote direction limbs are dipping)
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Anticlines:
(Dark line marks fold axis; arrows denote direction limbs are dipping)
LABORATORY EXERCISE
Part 1: Data Collection and Map Generation
1. The first step to any geologic study is to orient yourself on your map. Examine the map
of our lab room and determine where you are sitting and where the sample locations are.
2. For each sample (there are 9) first find it on the map. Then use the following table to
determine the rock name, strike direction, and dip direction:
Sample #
Rock Name
Strike Direction
Dip Direction
1
2
3
4
5
6
7
8
9
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3. Identify the rock at each station. Use the following color scheme to choose the colors that
are appropriate for your map. (red=arkose, orange=conglomerate, blue=limestone,
brown=shale, gray= black mudstone, yellow=halite, and black =coal)
4. On your map, color each sample location with the appropriate color and draw the
appropriate strike and dip symbol (based on your strike direction and dip direction from
#2).
5. Look for patterns in your colors and strike directions. Draw contacts on your map to
separate different rock units. Your contacts should be drawn with single lines, and they
should be parallel to the strike directions.
6. Complete coloring your map by extrapolating between the contacts. At this point your
whole map should be colored and you should have nine strike and dip symbols plotted.
Part 2: Creating a geologic cross section and stratigraphic column.
1. The line labeled A – A’ shows the location for which you will construct a cross section of
your field area.
2. Start by taking a piece of scrap paper and laying it out along line A – A’. Make a mark
on the paper at each contact between rock type.
3. Take your scrap paper and transfer the contact information to the blank cross section on
the bottom of your map.
4. Now use your dip directions to extrapolate the rock contacts below the surface of the
ground.
5. Color your rock types the appropriate color.
6.
Using your cross-section determine the relative ages of the rock units and fill in the
stratigraphic column below. Color and the number of each rock in the box and label
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the rock type in the blanks on the right.
Part 3. Identifying the structure and probable tectonic setting
1.
Using the information presented at the beginning of lab, what structural feature have
you found in your field area? _____________________________
2.
What type of stress occurred in this region to produce the structure that you found?
_________________
3.
Propose a plate boundary type at which this stress could have occurred.
__________________________
4.
Finally, place the appropriate map symbol (see page 6) on your map to show the
structure that you found.
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