Tuesday, October 23, 2012

homogeneity of rock units

Over the years I have noticed that students have a tendency to assume that rock units are very homogeneous in both shape and texture. They expect the Navajo sandstone to be made of white sand grains in distinctive sand dune cross bedding everywhere they look. When you point out "different" structures in the middle of a rock unit they get confused ( just for a short time). Most of my students also seem to expect sedimentary rock units to be of equal thickness throughout their extent. I guess I can't blame them since I am just as guilty of using these stratigraphic columns that show nice rectangles for rock units.   The White Rim sandstone is a great example of a rock unit that noticeably gets thinner as you ride east. 

A few weeks ago I had the awesome pleasure of riding the White Rim Trail in Canyonlands National Park, an 80 mile jeep track created by the atomic energy commission in the 1950's which has made more money as a tourist destination than all the uranium ore pulled out.

   The White Rim is a resistant rim of Permian aeolian beach sand that delineates the inner canyons of the Green and Colorado Rivers.

 Early in the trip you can see the thickness of the White Rim sandstone.  There is a bicycle on top of the cliff in the distance for scale...It is a big cliff.
 Again, there are dots across the gorge showing the scale of the White Rim cliff.
 Halfway through the trip you can see that the formation thickness is much less than in the first two images.
Here, near the end of the trip we can see the thin white line, all that is left of the White Rim just pinches out in the Cutler formation. The beach sand ends and the debris running off the Ancestral Rockies takes over.