for the last nine posts. Yesterday we made our way south from Glacier National Park over a corner of the Great Plains. Today (that is, June 23rd) we would be headed someplace different: down to the deepest part of the Earth's crust, and into the depths of geologic time. We were going to have a look at some of the oldest rocks on the planet.
How does one get to the base of the Earth's crust, or even into the mantle? Given that the base of the crust is 15 or 20 miles beneath us, and the deepest tunnel ever dug is 2 1/2 miles, one cannot walk or ride there. What we have to do instead is find a place where the crust has been brought up to us. Such a place is the Beartooth Mountains on the Montana/Wyoming border near Yellowstone.
Glacier National Park), is called the Laramide Orogeny. The rocks of the Beartooth Mountains were pushed up and over Cretaceous sedimentary rocks. Way, way up. The rocks originated in the deepest part of the continental crust, and these rocks are old. Very, very old.
The Beartooth Mountains take their name from the "fang" seen in the picture below, beyond the head of the circular valley called a cirque. These bowl-shaped valleys in the highest reaches of the mountains were the origin point for the glaciers (snow would blow off the highest summits and ridges, so glaciers couldn't form on them, but in the shaded cirques instead). Sharp knife-edged ridges between glacial valleys are called aretes (not pictured).