You learned about the laws of superposition and cross-cutting relationships in class and figured out how to reconstruct the geological history of a region based on rock outcrops. The last two homework assignments (1, 2)gave you a good idea of how things are supposed to work in an ideal world. Now its time to head out into the field and apply what we have learned in class. Before we head out on our (virtual) field trip, you should download a copy of the fieldtrip worksheet.

To access the worksheet you'll need Adobe Acrobat reader, which you can download free of charge from here.

 

To give you a little bit of background information: We are in Northern Arizona, just outside Flagstaff, south of the Grand Canyon, where we spent the last couple of days studying the sedimentary and volcanic rocks of the Colorado Plateau. For this exercise we are heading east into the Navajo Indian Reservation to Grand Falls.

By now the van is packed and we are ready to go!
For almost an hour we have been driving past dozens of cinder cones like the one seen on the right. When we finally arrive at our destination: a small picnic area above a rather impressive though dry waterfall on the Little Colorado River. Even though its dry now there are plenty of signs of recent water. Large plunge pools at the base of the falls are still filled with muddy, yellow puddles, and some of the mud in the holes of the dry river bed is still moist.

The Little Colorado river approaches the falls area from the Southeast, loops briefly to the North, loops back South, plunges into a rather deep canyon and continues its course towards the Northwest. The following link gets you to a topographic map showing the the road we came in, our present location (the picnic area) and the course of the Little Colorado River.

Hint: the default setting at Topozone brings up a very small map. You might want to change the map size to large and/or change the scale to see the entire region (don't forget to click the "Update Map" button). You can also scroll around to get a feel for the region.

Assignment 1:
Have a look at the topographic map and draw a sketch map of the area on your worksheet. The sketch should include:

  • the course of the Little Colorado River
  • the location of the waterfall
  • the canyon of the Little Colorado River
  • the location of the picnic area
  • an approximate scale bar
  • an arrow showing the North direction

Below is a picture of Grand Falls and the canyon, as well as a view of the picnic area, including our van.
We notice that the rocks around our picnic area are all black in color and form a rather irregular surface. This is in stark contrast to the reddish brown rocks we had seen on our way in. The two rock types are clearly visible in the picture above. The left (southwest) side of the canyon is made up of dark black rock, the right side (northeast) side of the canyon is composed of reddish brown, layered rocks. Below are a few close-up images of the two rock types.

For you guys who want to get really close:

black rock: close-up, even closer
brown rock: close-up
Assignment 2:

Identify the two rocks shown in the images below. What information did you use to arrive at your answer?

Note: Some experts take a look at the rock and know the answer right away, but sometimes it pays off to look around, check out the landscape (I hope you didn't sleep in the van on the way to Grand Falls), and pay attention to the large scale features in the area.

 
While our professors have a little snack or go off bird watching we start exploring the area around the falls. Of course we take lots of photos. Below is a sketch map from one of our field notebooks indicating the location of our images. To see the image click on the number on the map. The arrow on the map indicates the direction of view.