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.
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 .
The two intrusions are labeled as X and Z; the surrounding rock (called the "country rock") is labeled as D.
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.
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.
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.
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.
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. You can open a larger version of this diagram by clicking on it.