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


RELATIVE DATING EXERCISE

@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. Relative Dating of Geologic Cross-Sections:

Cliffs, road cuts, and non-vegetated landscapes allow us glimpses into geology which is often hidden from view.   Cliffs and road cuts are "side views" or "geologic cross-sections" of the topography which show the relative positions of various rock layers and structures at a given spot.  

Applying the principles of relative dating to these rock exposures (also called "outcrops"), we can reconstruct the sequence of events that created the geologic features which we see.  Events can be the deposition of a sedimentary layer, the eruption of a lava flow, the intrusion of magma to form a batholith, a fault (break) in the rock that shifts one side relative to the other side (and causes an earthquake), a fold that bends and distorts rock layers, or any number of other geologic processes.  Just as Sherlock Holmes used his power of observation to decipher the clues to a suspect's past actions, we will let the blemishes and behaviors of the rocks tell us their past story.  (Am I getting too carried away here?)  

Remember that relative dating involves determining "which came first" rather than "exactly when did this happen."  The first step to untangling the geologic history of an area is often to figure out what happened first, second and third, etc. without knowing the absolute ages at which the rocks themselves formed.

To review our principles of relative dating as applied to such geologic cross-sections, 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.  Practice each several times till you get them correct.  

Now let's practice on a couple more imagined cross-sections:

Question 1:  What is the sequence of events that can be inferred from the above cross-section?  What principle(s) of relative dating did you use in order to arrive at your interpretation of the relative timing of each event?  The various sedimentary layers are labeled as B, E, K and W.  The timing of the fault (break) in the rocks (labeled as Q) must be included in the sequence of events.

 

Question 2:  What is the sequence of events that can be inferred from the above cross-section?  What principle(s) of relative dating did you use in order to arrive at your interpretation of the relative timing of each event?  The two intrusions are labeled as X and Z; the surrounding rock (called the "country rock") is labeled as D.  

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II. Reconstructing a Regional Geologic Story:

We have seen that a cliff or a road cut is a local "geologic cross-section" -- a side view of the geology at one location.  As geologists piece together the information at various outcrops, they can begin to assemble a "geologic map" (like a road map) of an entire region (consisting of many square miles).  This map displays the large-scale (also called "regional") geologic features they have inferred are present beneath the landscape.

Along with these geologic maps, we can reconstruct a regional geologic cross-section which would be like a great "geologic slice" through the landscape.  In the next lab, we will learn how to use local geologic information from outcrops to begin to build such regional geologic maps and geologic cross-sections, but for now we just want to practice how to read them.

Remember when we drew a topographic profile for lab manual exercise #1 (page 18) on Topographic Maps?  We could draw such a profile across several miles of landscape so we would see a side-view of the land's surface over which we might be hiking.  For example, we could use a ruler to draw a straight line (a "transect") from the northwest corner to the southeast corner of the topographic map in our lab kit; then we could draw in the topographic profile along this transect by using the contour line information on the map (as done on page 18).  

In the same way, such a transect could also show the inferred profile of the geology underfoot -- the expected rock layers and structures beneath the land from the northwest corner to the southeast corner of the map. 

Take a look at the geologic cross-section below.  You can open a larger version of this diagram by clicking on it.  Notice that the various sedimentary layers have been labeled with letters.  Also an igneous intrusion is present (labeled T) and a fault is present (labeled A). 

Question 3:  What is the sequence of events that can be inferred from the above cross-section?  What principle(s) of relative dating did you use in order to arrive at your interpretation of the relative timing of each event?  All 13 lettered events need to be included in your sequence.  

Question 4:  Using the relative dating method you have now practiced, derive the history of the hypothetical landscape on page 173 (also shown below).  Include all the events which can be inferred from the drawing.  List which relative dating principles apply to the order of each event.  

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III. The Geology of the Grand Canyon:

Shown below is a real transect across the entire Grand Canyon (in two parts).  This profile is composed of both the surface topography and the inferred geology underfoot.  Notice that the rock layers are each labeled with several letters.  Each rock layer was deposited at a different geologic time.  We will learn more about this method of identifying rock units and geologic time periods in next week's lab.

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Question 5:  On the South Half, approximately how high is Hopi Point on the Coconino Plateau?  What is the approximate elevation of the Colorado River (shown by the arrow pointing downwards)?  What is the elevation difference?

Question 6:  On the South Half, which rock unit is likely to be the youngest (most recent)?  Which rock unit is likely to be the oldest?  What relative dating principle tells us this?  Use the letter labels to refer to the distinct rock layers.

Question 7: On the South Half, we can also notice that many faults have occurred in the area of the Cheops Pyramid.  These are the dark lines that slice through the rock formations.  When did all this faulting take place (that is, between the times of which two sedimentary layers did the faulting occur)?   What relative dating principle tells us this?

Notice the "Great Angular Unconformity" shown on the North Half of the profile.   This is not labeled as such -- but see how the rocks at the bottom of the profile have been tilted while the younger rocks on top are horizontal.  The earlier sequence of rock formations had to have been tilted before the more recent sequence of rock formations was deposited on top of it.

Question 8:  Pinpoint the time at which this tilting took place.  Find the youngest layer which has been tilted and the oldest layer which has NOT been tilted.  The tilting must have occurred between these two times.  Between which two rock formations (use the letter labels) did this tilting occur?

I hope this first virtual trip to the Grand Canyon was a bit of fun along along with the very hard work.  We will visit it again (but only virtually!) next week.

 

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