Devil’s Tower, WYO 2012
While driving through eastern Wyoming, I had the opportunity to see and visit the infamous Devil’s Tower at the end of June 2012.
Devils Tower is a butte, possibly laccolithic, composed of igneous rock in the Bear Lodge Ranger District of the Black Hills, near Hulett and Sundance in northeastern Wyoming, above the Belle Fourche River. It rises 386 m above the Belle Fourche River, standing 265 m from summit to base. The summit is 1,559 m above sea level.
Stephen Speilberg chose it for a starring role in “Close Encounters of the Third Kind” in 1977; local rancher W. Rogers conquered it using a ladder in 1893; and in the 1940s a parachutist sat there for six days wen he lost the rope that should have allowed him to climb down.
Devils Tower is composed of phonolite porphyry, a hard crystalline rock, and is one of a series of igneous intrusions of similar composition that intruded the sedimentary rocks of the western flank of the Black Hills 55-33 million years ago. The intrusives domed the sedimentary rocks that they intruded, but there was some withdrawal of magma and subsidence of the center of the dome after the intrusions stopped.
The sedimentary rocks, which overlie Precambrian igneous and metamorphic rocks, range in age from Cambrian to Oligocene. Exposed at the base of Devils Tower is the Sundance Formation of Jurassic age and, in the vicinity of the Tower, are the Spearfish Formation of Triassic age. The Morrison Formation, of Jurassic age, is exposed northwest of the Tower.
The most striking feature of Devils Tower are the columnar joints. Columnar joints are also conspicuous in the other exposed igneous masses. On the basis of the joints, the distribution of the intrusives, the origin of Devils Tower has been consider to be the remnant of an intrusive that did not reach the surface, a remnant of a laccolith and the remnant of a volcanic neck.
Devil’s Tower was formed 50 million years ago when hot, molten lava cooled, contracted, and formed vertical rows of hexagonal columns. Softer rocks that surrounded the intrusion were worn away, leaving the tower exposed above the surrounding land.
Devils Tower is almost homogeneous in composition compared to basalt, which is basic. Most of the columns are 5 sided, but some are 4 or 6 sided. The larger columns measure 2-3 meters in diameter at the their bases and taper gradually up toward to about 1 meter at the tops. The columns are bounded by well-developed smooth joints in the middle part of the Tower, but as the columns taper upward, the joints between them, rather than being smooth, may be wavy and some of the columns may unite. Numerous cross-fractures in the upper part of the Tower divide the columns into many small irregularly shaped blocks.
The local Kowa people believe that Devil’s Tower was conceived when a bear chased a group of girls onto a low rock. The rock rose up carrying the girls out of the bear’s reach, but the bear scratched at the mound with its powerful claws, leaving furrows down the tower’s side. Eventually, the bear died, and the girls lived forever as the seven stars of the Pleiades.