The Geology of Devils Tower National Monument, Wyoming

Devils Tower National Monument in northeastern Wyoming was the nation's first national monument, established by Theodore Roosevelt in 1906. The tower is a steep-sided igneous monolith famous worldwide for its columnar jointing, and it is the best-known landmark of the northern Great Plains.

The National Park Service's Geodiversity Atlas summarizes the park's geology, drawing on the Geologic Resources Inventory.

The tower

• Summit elevation: 1,560 m (5,117 ft) above sea level, on a fairly flat top.
• Size of the top: about 55 m (180 ft) east to west and 91 m (300 ft) north to south.
• Height: about 386 m (1,267 ft) above the Belle Fourche River, which flows north. The river dug the buried igneous body out of the surrounding rock and is still cutting into its south side.

The tower sits on the northwest flank of the Black Hills, a north–south elongated dome (a doubly plunging anticline) raised during the Laramide Orogeny, a mountain-building episode of the Cretaceous–Tertiary about 45–65 million years ago. The forested range covers about 2 million acres of southwestern South Dakota and northeastern Wyoming. Erosion has stripped Paleozoic and Mesozoic layers off the dome's center, exposing its Precambrian core, and the younger sedimentary rocks circle that core in rings.

Devils Tower. National Park Service photo by Deanna Greco.

The rock

The tower is made of phonolite porphyry. Phonolite is a fine-grained igneous rock composed mainly of feldspar; a porphyry has coarse crystals set in a finer groundmass. In fresh samples the groundmass is light to dark gray or greenish gray, studded with white feldspar crystals about 0.6–1.3 cm (0.25–0.50 in) across and smaller, very dark green crystals of pyroxene, a group of dark silicate minerals. Weathered surfaces look light gray or brownish gray, and lichens can tint the rock green, yellowish green or brown.

The columns

The columns formed when the shallow body of magma cooled quickly: solid rock takes up less space than molten rock, and uneven shrinking as it cooled cracked it apart. The fractures meet at about 120° and make four- and six-sided columns.

• The larger columns are 1.8–2.4 m (6–8 ft) across at the base and taper to about 1.2 m (4 ft) at the top.
• In the middle of the tower they are bounded by smooth, well-formed joints; higher up the joints can turn wavy, some columns merge, and cross-fractures break the rock into many small, irregular blocks.
• In the central and upper tower the columns stand nearly vertical, but at the bench about 30 m (100 ft) above the base they flare outward, and on the southwest side they lie almost horizontal. Several can join into a larger, less distinct column that merges with the solid base.

The talus apron

Around the tower lies an apron of talus — fallen blocks at the foot of the cliff — together with landslide material and the remains of a large mass of alloclastic breccia, a coarse rock formed by volcanic processes. The talus and landslide debris come from the tower and from the Hulett Sandstone Member of the Jurassic Sundance Formation. The apron spreads about 335 m (1,100 ft) out from the tower. It is about 46 m (150 ft) thick near the base but thins quickly with distance. Pieces range from a few centimeters to whole column sections up to 2.4 m (8 ft) across and 7.6 m (25 ft) long. Three outcrops of phonolite within the talus may be radiating dikes or part of the original, uneroded igneous mass.

How did it form?

Geologists have proposed four explanations:

1. It is the eroded remnant of a volcanic neck, from a volcano that vented to the surface through more than a hundred meters (several hundred feet) of sediment.
2. It is the remnant of a laccolith or sill.
3. It is an intrusive body such as a small pluton.
4. It sits in the remains of a crater blasted out when rising magma met groundwater, and is the eroded remnant of the lava and hot pyroclastic material that filled it.

The vertical columns in the center and the flaring ones around the base could fit a volcanic neck, a shallow intrusion or a sloping basin floor. Every hypothesis still has discrepancies.

The surrounding hills

The hills near the tower are red, yellow, green or gray beds of sandstone, siltstone, shale and gypsum, about 122 m (400 ft) thick. The oldest is the Triassic/Permian Spearfish Formation, overlain by Jurassic layers. No Cretaceous or Tertiary sedimentary rocks are exposed in the monument, but Cretaceous layers and the Oligocene White River Formation occur at the Missouri Buttes, smaller columnar-jointed igneous bodies northwest of the tower. The Permian, Triassic and Jurassic red beds are poorly cemented and weather easily into rolling hills and gentle slopes. Recent erosion by the Belle Fourche River and its tributaries gave the tower its present shape, and Quaternary alluvium, glacial material and other deposits fill the main drainages.

Fossils

The Redwater Shale Member of the Sundance Formation holds marine fossils including clams, oysters and belemnites, but none have been found in the monument's Spearfish or Gypsum Spring formations. All fossils in national parks are protected by the Paleontological Resources Preservation Act of 2009.

Maps and reports

The monument's digital geologic map. Image from the National Park Service's page.

The monument is part of the Great Plains Physiographic Province. The Geologic Resources Inventory's maps, report and poster for it are on the NPS DataStore, as is its soil resources inventory.

Sources

• National Park Service: "NPS Geodiversity Atlas—Devils Tower National Monument, Wyoming," citing Robinson (1956), Karner and Halvorson (1987, 1989), Kiver and Harris (1999) and others.
• The source gives the Black Hills dome as "105 by 200 km (125 by 65 mi)," with the conversions in the opposite order, so neither measurement is given here. Its image of the inventory report's cover is not reproduced.