The cliff was formed around 500,000 years ago during one of the basaltic floods of the Yellowstone Caldera. As the lava cooled it contracted and formed a set of joints perpendicular to the cooling surface. Today much of the basalt has tumbled down from the cliff due to weather erosion.
This loose rock is perfect habitat for the yellow-bellied marmot. These large members of the rodent family make their burrows under rock piles to help hide them from predators such as wolves, coyotes, and fox.
Unfortunately this marmot seemed to be habituated to humans and was not afraid of people at all. He came right up to both Michelle and I, as well as other people who were there, while we were shooting pics.
There were also plenty of chipmunk hanging out around Sheepeater. I am guessing that they also live in burrows under the rocks.
Sheepeater Cliff is just one example of columnar basalt cliff in Yellowstone. There are many other basalt cliffs in the Yellowstone area but most, like the ones near Bunson Peak are pretty in accessible. Fortunately Sheepeater Cliff is located adjacent to the Northern Grand Loop Road just south of Mammoth.