Geologic dating exercise
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.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?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. ) 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.
The two intrusions are labeled as X and Z; the surrounding rock (called the "country rock") is labeled as D.
Click 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?
We will learn more about this method of identifying rock units and geologic time periods in next week's lab.
Question 5: On the South Half, approximately how high is Hopi Point on the Coconino Plateau? Use the letter labels to refer to the distinct rock layers.