North Seattle Community College's
PHYSICAL GEOLOGY 101
Instructor:  Tom Braziunas

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Glacier National Park, Montana
Photo by Paul Carrara
USGS Public Domain Photograph

  

GEOLOGIC STRUCTURES

EXERCISE

(Summer Quarter)

@2002 -- The information contained in this document is copyrighted.
No reproduction may be made without prior approval from the author (Dr. Tom Braziunas).

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I. An Introduction

A local rock outcrop (an exposure such as an erosional cliff or a roadcut) reveals the layering (strata) and structures in rock sequences.  For example, look at the layers exposed in the mountains of Glacier National Park shown in the photograph above.  We see some relatively horizontal sedimentary layers as well as some folded sections as well.   Trace these layers with your eye and finger.  Note that these layers and folds do not follow the slopes and peaks of these mountains.  The surface topography is due to uplift and erosion a long time after these rock strata were deposited and lithified.

This week we learned some principles of relative dating that we can apply to the rocks exposed at such an outcrop to determine the sequence of events that took place to produce the geologic arrangement which we see.   In Chapter 8 we learned about the principles of superposition, horizontality, cross-cutting relations, and inclusions.  In Chapter 9 we learned some of the terminology for the varieties of geologic structures that complicate the "rock record".   Geologic structures include folds (bends), faults (breaks) and unconformities (gaps) in rock strata.

In this lab exercise, we will become more familiar with geologic features, how to measure and interpret them, and how to represent them through symbols on geologic maps. We have 11 questions to answer along the way, each worth two or three points.  Use the "Week 6 -- Lab Homework Part 1" form to submit your answers.  TOTAL POINTS POSSIBLE = 25.

 

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II. Principles of Relative Dating:

To review our principles of relative dating, we will make use of a neat learning tool available on the Internet.  "Athro Limited" is a private company which provides education modules on the Internet.  Click here to access the activities related to the interpretation of geologic sequences as exposed at rock outcrops.

Question 1 (3 points). Find the list of hypothetical geologic examples and click on "fault."  We are asked to determine the correct sequence of geologic events shown by the cross-section.    In order to do this, we need to apply the principles of relative dating which we have learned.   As your answer to this question, complete the sequence correctly and explain the logic and principle behind your choice for each event.  Your explanations are as important as the correct sequence in earning the points for this exercise.

Question 2 (3 points). Return to the list of hypothetical geologic examples and click on "folds and an intrusion."   We are again asked to determine the correct sequence of geologic events shown by the cross-section.  Again, complete the sequence correctly and explain the logic and principle behind your choice for each event.

Question 3 (3 points).  Finally, return to the list of hypothetical geologic examples and click on "two intrusions."  Again, complete the sequence correctly and explain the logic and principle behind your choice for each event.  This is a much more difficult exercise than the previous two because we will find several possibilities for the sequence of geologic events .  You do not need to complete the second half of this particular exercise (about resolving these ambiguities in the relative dating).

 

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III. Strikes and Dips:

The strike is the direction of a line formed by the intersection of a rock layer and a horizontal plane, in other words, the direction you would need to walk (if you were standing on a flat surface) in order to stay in contact with a particular rock layer. The dip is the angle between the inclined rock stratum and a horizontal plane, in other words, the direction that a ball would roll down a particular rock layer if it could. Strikes and dips also apply to other features such as faults or unconformities.

Click on this website to view an excellent diagram of the relation of strike and dip to rock strata:  http://courses.unt.edu/hwilliams/images/strike.htm

IMPORTANT NOTE:  The figure above is a "block diagram,"  one of three ways to view geologic or topographic information.  This view imitates a three-dimensional look at the landscape.  In Lab Manual Lesson 2 (Structural Geology), Figure 2.6 is a block diagram.  The other two standard ways to illustrate geologic information is through a "map view" (like a roadmap) and a "geologic cross-section" (like a topographic profile).

VERY IMPORTANT NOTE!   When you read and work on Lab Manual Lesson 2 (Structural Geology),  be sure to know whether you are looking at a geologic diagram "from above" (map view) or "from the side" (geologic cross-section).  This is critical!  For example, map views are shown in Figures 2.3 and 2.7.  But geologic cross-sections (side views) are given in Figures 2.2, 2.4 and 2.5.  Also, the illustration on page 37 (which you need to answer one of the multiple-choice questions) is a geologic cross-section, NOT a map view.

Strikes and dips do NOT necessarily relate to the slopes and contours of the surface topography.  Look at the mountain cliffs behind and to the left of St. Mary Lake in Montana as shown in the photograph below (at the red arrow).  Do you see the layering?  It is impossible to determine the dip without determining the strike of these layers.  Without this complication, let's pretend we can determine the dip from the view we have.  Let's also say that we are looking due North.

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St. Mary Lake, Montana
Photo by Paul Carrara
USGS Public Domain Photograph


It appears that the strata dip slightly to the west.  Do you see this?  As explained in your lab manual, dip is measured relative to a horizontal plane.  The strata in the photograph dip about 10o to the west.  We would state that the dip is 10o W (10 degrees West).

Now let's visit the Lower Ugab valley in Namibia.  If you can't fly there in person (!), then open your textbook to Figure 9-1 on page 152.  What a beautiful spot to park our land rover and explore.  We can see such folding and faulting -- and it doesn't take a geologist to appreciate the earth stresses involved!  Of course, our friends and/or family (along for the ride) expect us to provide some interpretation of exactly what we are seeing.

Question 4 (2 points). What type of fold is shown in this photo?  Look at the types of folds shown in Figure 9-8 of our textbook.   Which one(s) of these might best describe this fold? _________________

Question 5 (2 points). The folding is so extreme that it is difficult to decide which rock layers are the younger ones and which are older. What sedimentary structures could we look for to help us decide which layers are younger and which are older? (See page 139 of your textbook). _____________________

Question 6 (2 points).   If the rock layers get younger as you go from right to left across the outcrop (so the most recently formed layers are to the left), is the biggest fold an anticline or syncline?  Why? (Remember about the distinction based on the relative age of the innermost rocks.) _________________

Question 7 (2 points).  Walk directly across the road from your rover (trace your path with your finger on the photo!) .  Right where you are standing next to the outcrop, measure the dip of that very thick light-color sedimentary layer near the bottom of the cliff.   Use your protractor on the photo.  What is the dip direction and angle? _____________________   NOTE:  For this exercise, assume that we are looking North in the photograph (in other words, the cliff is north of the land rover).

Question 8 (2 points).Follow this same layer to where it curves and reaches the top of the outcrop.  What is its dip direction and angle now? ______________________

Question 9 (2 points).  Notice the fault that runs through the structure just behind the jeep (from bottom left to top right).  The layers on one side of the fault are offset from the layers on the other side of the fault.  It is pretty obvious where it cuts through the thickest light-color unit about one-third the way up the outcrop.  What type of fault is this (normal, reverse, thrust)? Why?  _____________________

Question 10 (2 points).  One centimeter on this photograph in your textbook is equivalent to about 2 real meters (6 feet) on the real outcrop.  For example, the land rover is 2 cm long in the photo.  This is equivalent to 4 meters (12 feet) for the actual vehicle.  So now we can use our ruler to determine how much of a shift has occurred along this fault.  Measure the distance (in meters) on the observed offset for that lower thick light-color sedimentary layer.  How much offset is there? __________

Question 11 (2 points).  Carefully follow the fault to the top of the outcrop.  You can follow this fault all the way to the top of the outcrop (use a ruler and follow the straight line of this fault) from bottom to top.  Look for the patterns in the layers on each side of the fault to see which ones match up.  What is the approximate offset (in meters) along this fault line at the top of the outcrop?  What layers did you base this estimate on? _____________________

 

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