Mile 937: Avenue of Rocks

“On the high, dry Wyoming plains, the Earth sheds her former grassland modesty and bares her rocky skin, wrinkled by time and mountain upheaval. The beveled edges of bent, tilted strata poke up everywhere as irregular fins and ridges. At Avenue of Rocks, about 20 miles west of present Casper, the trail twists between ghoulish hogbacks cut on upended sandstone beds, looking “like the vertebrae of some great sea serpent.” Edwin Bryant described passing “immense piles of rocks, red and black, sometimes in columnar and sometimes in conical and pyramidal shapes, thrown up by volcanic convulsions. These, with deep ravines and chasms, and widespread sterility and desolation, are the distinguishing features of the landscape.”

The trail passes through the eroded cores of anticlines—huge arching folds of rock strata, like a stack of magazines bowed up in the middle—and through synclines, the reverse of anticlines, where the strata bow down in the middle. Lonesome pump jacks bob gracefully on the sagebrush hills. They are parked over anticlines, sucking up the crude oil trapped in the bowed-up layers. Oil underground percolates upward until it runs into rock that stops it. Rising droplets of oil collect against impermeable rock layers within the arching anticlines, forming caches of black gold. More than anything, oil has put Wyoming on the map. And more than any other rock formation, the Mowry Shale has put the oil in Wyoming. You can’t miss the Mowry. It ranges dusk-gray to nightblack from organic residue and smells faintly of decay.”