The park's Geologic Resources Inventory report. Image from the National Park Service's page
Bighorn Canyon National Recreation Area, on the Montana–Wyoming line, sits among three great structures: the Bighorn Mountains, the Pryor Mountains, and the Dry Head–Garvin Basin between them, where mostly Triassic and earliest Jurassic rocks are exposed.
Folds and faults
- The Bighorn anticline. The northern end of the Bighorn Mountains is a northward-plunging anticline with steep limbs and a broad top. For 24 km (15 miles) upstream of Yellowtail Dam, Bighorn Lake cuts straight through it, exposing rocks that span more than 250 million years.
- The Pryor front. East of the park, the sheer cliff of the Pryor Mountains marks the first of several major faults that lifted the range. The North Pryor fault ends the range abruptly, with up to 610 m (2,000 ft) of displacement, setting the Cretaceous Cloverly Formation against the far older Madison Group.
- Two kinds of fold. Both ranges are fault-propagation folds. In the Pryors, the fault breaks the surface, cutting the steep beds of the mountain front; in the Bighorns, the faults stay buried.
- The Sentinel. A near-vertical fault at least 10 km (6 miles) long crosses the canyon; its north side, with a rock pillar called the Sentinel, stands about 60 m (200 ft) higher than its south side.
Part of the park's geologic map. Image from the National Park Service's page
How the river carved the canyon
| Feature | What happened |
|---|---|
| Superimposition | the river first flowed across soft rocks that have since eroded away, then held its course as it cut down into hard Madison Group limestone and through the folds of the Bighorn Mountains — dissolving limestone and grinding the bed with cobbles, pebbles and sand |
| Entrenched meanders | the river's old meandering path, set in soft floodplain sediment, was locked into the bedrock as it cut down — a sign of rapid uplift or falling base level, during the Laramide Orogeny, the mountain building that began about 70 million years ago |
| An abandoned meander | erosion on two cutbanks broke through the rock wall between them, letting the river take a shortcut and leaving its old loop, the Natural Corrals, high and dry — visible across from Devil Canyon Overlook |
| Terraces | paired terraces along the Bighorn, Shoshone, Greybull, Clarks Fork and Yellowstone rivers record 2.02 million years of downcutting and tectonic movement, making the Bighorn Basin a classic place to study rivers and recent geologic time |
Wind
Strong winds blow in every season; gusts of 121 km/h (75 mph) have been recorded at Fort Smith, Montana. North and east of Crooked Creek, Holocene windblown sand forms sheets, wedges and dunes 3–5 m (10–16 ft) thick, with evidence of significant prehistoric human occupation. Ancient "paleodunes" are preserved in the Tensleep (Pennsylvanian) and Morrison (Jurassic) formations. Volcanic ash from the Yellowstone caldera, piled up to 30 m (100 ft) thick against the mountain front east of the park, was once mined for glassmaking sand.
Wind also adds gypsum to local soils. Gypsum has built up continuously for about 600,000 years, a sign the region's climate has stayed dry all that time.
Fossils
Most fossils are marine invertebrates from the seas that covered the region for hundreds of millions of years, dating from the Late Ordovician (about 450 million years ago) to the Cretaceous (about 65 million). The Madison Limestone is especially rich: horn corals, bryozoans, brachiopods, snails and crinoids. Much younger, a musk ox vertebra 130,000–175,000 years old dates from the Illinoisan glaciation. Park fossils are protected under the Paleontological Resources Preservation Act of 2009.
Caves and springs
Caves dot the cliffs well above the river — some were drowned by the lake — mostly in the Madison limestone and the Bighorn Dolomite. Bighorn Cavern, on Crow Reservation land within the park, is managed by the NPS by agreement with the Crow. It is part of a 14-mile cave system that continues onto BLM land in Wyoming, where it has another entrance, Horsethief Cave. The park also has 33 known springs and many seeps. Caves are protected under the Federal Cave Resources Protection Act of 1988.
Maps and reports
The NPS Geologic Resources Inventory products for the park, and a completed Soil Resources Inventory, are in the NPS DataStore. More: the park's pages on geologic activity, fossils and caves.
Sources
Based on "NPS Geodiversity Atlas—Bighorn Canyon National Recreation Area, Montana and Wyoming," National Park Service, citing Richards (1955), Mapel and others (1975), Reheis (1984–1992), Lopez (2007) and others; a work of the United States government in the public domain. The article lists the Madison Limestone's fossils as including "horn coals"; they are horn corals.
In these publicationsBighorn Canyon National Recreation Area
Licence: CC0 1.0 (public domain) · Adapted from www.nps.gov
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