Last updated: June 17, 2024
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NPS Geodiversity Atlas—Bighorn Canyon National Recreation Area, Montana and Wyoming
Geodiversity refers to the full variety of natural geologic (rocks, minerals, sediments, fossils, landforms, and physical processes) and soil resources and processes that occur in the park. A product of the Geologic Resources Inventory, the NPS Geodiversity Atlas delivers information in support of education, Geoconservation, and integrated management of living (biotic) and non-living (abiotic) components of the ecosystem.
Introduction
There are three primary structural features in the vicinity of Bighorn Canyon National Recreation Area are the Bighorn Mountains, Pryor Mountains, and a structural basin between the two mountain ranges.
The northern end of the Bighorn Mountains is a northward-plunging anticline with steep limbs and a broad top (Richards 1955). A segment of this anticline is exposed in Bighorn Canyon. For 24 km (15 mi) upstream (south) from the Yellowtail Dam, Bighorn Lake bisects this massive anticline, displaying rocks that span more than 250 million years.
East of the recreation area, the sheer cliff face of the Pryor Mountains is the first in a series of major faults that were part of the uplift of the mountain range. The sedimentary rocks of the mountains were broken along nearly vertical planes. The North Pryor fault abruptly terminates the north end of the Pryor Mountains (Mapel et al. 1975). Maximum displacement on this fault is 610 m (2,000 ft). Upthrown on the south, the fault brings the Cretaceous Cloverly Formation (KJcm) on the north against the much older Madison Group (Mm) on the south (Mapel et al. 1975).
Both the Bighorn and Pryor mountains can be characterized as “fault-propagation folds,” after the model proposed by McConnell (1994). The differences between the two ranges are that in the Pryors, the fault makes it to the surface and cuts the steeply dipping beds along the mountain front. In the Bighorns, the faults do not make it to the surface, and there is a back thrust on the west side that produced the fold in the monocline area (David Lopez, geologist/independent consultant, written communication, January 31, 2011).
Between the Bighorn Mountains and Pryor Mountains is a structural basin, referred to as the Dry Head–Garvin Basin, in which mostly Triassic and lowermost Jurassic rocks are exposed (Mapel et al. 1975).
The combination of originally horizontal rock layers and later deformation has resulted in scenic displays of faulted and folded strata. Structural displays include a near-vertical fault at least 10 km (6 mi) long that strikes eastward across Bighorn Canyon (Richards 1955). This fault is easy to see: the north side of the fault, which includes a pillar of rock called “the Sentinel,” is about 60 m (200 ft) higher than the south side of the fault (National Park Service 2010a).
River Systems and Fluvial Landforms
Fluvial features at Bighorn Canyon National Recreation Area are related to segments of the Bighorn and Shoshone rivers, and the extreme lower reaches of numerous small streams that flow into the east and west sides of Bighorn Lake (Jacobs et al. 1996). Bighorn Canyon is geomorphically distinctive for at least three reasons: first, it formed via superimposition; second, it hosts entrenched meanders; and third, it contains ancient abandoned meanders.
Superimposition
Superimposition is the process by which a stream or drainage system—which originally developed on a cover of easily eroded rocks that has since eroded away— becomes established on a new surface and maintains its course, despite different rock types and structures encountered as it erodes downward. The present-day canyon of the Bighorn River cuts completely through resistant rocks such as the cliff-forming Madison Group (Mm), and structures such as folds of the Bighorn Mountains, without being diverted. Thus, in the process of creating the present-day Bighorn Canyon, the river’s course was “superimposed” on the rocks and the structurally uplifted features it encountered (Lopez 2007). Erosion itself occurred as a combination of chemical weathering (dissolution of limestone) and mechanical weathering (breakdown of rock along the riverbed by fluvially transported cobbles, pebbles, and sand grains) (Lopez 2007).
Entrenched Meanders
The ancient Bighorn River meandered across unconsolidated floodplain sediments, which were underlain by rocks that have long since been removed by erosion. As erosion continued, these meanders became entrenched (established via downward erosion) when the river was superimposed on the hard underlying Madison Group. Such a deepened meander, which preserves its original pattern with little modification, suggests rejuvenation of a meandering stream under conditions of rapid vertical uplift or lowering of base level (Neuendorf et al. 2005). This was the case for Bighorn Canyon during the Laramide Orogeny—a period of mountain building and uplift starting about 70 million years ago.
Abandoned Meanders
In a meandering river system, the highest rate of erosion is on the outside bends of the meanders, called the “cutbanks.” At the same time, deposition occurs on the opposite/inside bends of the river, the “point bars.” During an earlier stage of entrenchment, the Bighorn River’s course was through the Natural Corrals. However, erosion on the two cutbanks of the meandering river cut through a wall separating them, allowing the river to take a new shorter route and in the process abandoning the previous meander. Continued downcutting left the Natural Corrals high and dry (Lopez 2007). This outstanding example of an abandoned meander is across from Devil Canyon Overlook.
Terraces
For decades, the Bighorn Basin has been a classic area for the study of fluvial geomorphology and Quaternary stratigraphy (the study of rock strata) (Reheis 1992). The well-preserved fluvial terraces along major rivers such as the Bighorn, Shoshone, Greybull, Clarks Fork, and Yellowstone have provided evidence of the dynamic nature of fluvial processes and tectonic activity over the past 2.02 million years (Reheis 1984a, 1992).
Generally speaking, the paired nature of terraces on opposite sides of a stream channel is one of the most distinctive characteristic of terrace morphology. The pairs represent the same former floodplain, with higher pairs being older than lower pairs, chronicling the river’s downward erosion to the modern floodplain level. Unpaired terraces indicate the removal by erosion or burial by deposition of the terrace on one side of the valley or the other. Tectonics may also disturb terrace forms, shifting terrace elevations along faults.
A distinctive feature of the terraces at Bighorn Canyon National Recreation Area is the sediment that makes up these deposits. Investigators have deduced the histories of the drainages in the Bighorn Canyon area by analyzing and dating these sediments.
Aeolian (Dunes) Landforms
Strong winds generally can be expected at Bighorn Canyon National Recreation Area during any season of the year (National Park Service 1971). Gusts of 121 km/h (75 mph) have been recorded at Fort Smith, Montana (National Park Service 1971). At present, prevailing winds in the area are westerly.
At the scale of geologic mapping, no active eolian deposits are shown on the digital geologic map for the national recreation area. However, eolian sediments from the Holocene Epoch (the last 11,700 years) are present north and east of Crooked Creek. These deposits form sand sheets, wedges, and dunes approximately 3 m (10 ft) to 5 m (16 ft) thick, and have evidence of significant prehistoric human occupations (Judson Finley, assistant professor, University of Memphis, written communication, April 18, 2011). Ancient “paleodunes” preserved in the Tensleep (PNt) and Morrison (Jm) formations are from the Pennsylvanian (318 million to 299 million years ago) and Jurassic (200 million to 146 million years ago) periods respectively.
Eolian deposits have been indentified as consisting primarily of volcanic ash (from the Yellowstone caldera) that was transported and deposited against the mountain front east of the national recreation area (National Park Service 2005). Some of these deposits are as thick as 30 m (100 ft), and in the past have been mined for sand used in glassmaking (see “Mineral Resources and Mining”).
Reheis (1987) noted that eolian processes add silicates, calcium carbonate, and gypsum to the soils of Big Horn County, Wyoming. Windblown silt- and sand-sized particles make up a large proportion of the upper parts of the soils that have formed on the Kane alluvial fans, near Lovell, Wyoming. These soils have accumulated gypsum over time, which is added chiefly as eolian dust from both local and distant sources. Sources include the clay and gypsum dunes in the Bighorn Basin, as well as exposed gypsum in the Sheep Mountain anticline to the southwest, on the slopes of the Pryor Mountains to the northwest, and in the Cody area on the west side of the Bighorn Basin (Reheis 1987).
Although eolian deposits are often underrepresented on geologic maps, they are significant indicators of past climatic conditions (Madole 1995; Muhs et al. 1999). According to Reheis (1987), the arid but cool climate has permitted gypsum to accumulate continuously for the past 600,000 years, indicating that the effective moisture in the area has not increased substantially during this time.
Paleontological Resources
Fossils at Bighorn Canyon National Recreation Area are primarily marine invertebrates, which lived in the marine ecosystems that characterized the region for hundreds of millions of years. The Madison Limestone is highly fossiliferous and contains a diversity of marine invertebrates including horn coals, bryozoans, brachiopods, molluscs, snails, gastropods, lithopods, and crinoids. The fossils range in age from the Upper Ordovician Period (approximately 450 million years ago) to the Cretaceous Period (approximately 65 million years ago).
In addition, much younger fossils occur within unconsolidated deposits at the national recreation area. Musk ox vertebra with an estimated age between 175,000 and 130,000 years old and corresponding to the peak of the Illinoisan glaciation have been documented in the park.
All NPS fossil resources are protected under the Paleontological Resources Preservation Act of 2009 (Public Law 111-11, Title VI, Subtitle D; 16 U.S.C. §§ 470aaa - 470aaa-11).
Cave and Karst
There are caves found within Bighorn Canyon that are located on the cliffs well above water line along the Bighorn River channel (Some caves were inundated by the lake). The Madison limestone (Mississippian in age) and the Bighorn Dolomite (Ordovician in age) host many of the caves in the area.
The Bighorn Cavern is within the NRA but is on lands managed by the Crow Reservation. However, through agreement with the Crow Reservation, this cave is managed by the NPS. Bighorn Cavern is part of a larger cave system that is 14 miles long. This cave extends beyond Bighorn Canyon boundaries onto BLM lands in Wyoming with another entrance named Horsethief Cave.
The NRA also has karst springs associated with limestone and dolomite layers that underlie much of the park and are exposed in cliff faces along the river channel. There are 33 known springs in Bighorn Canyon and numerous seeps that emerge from various geologic units.
All NPS cave resources are protected under the the Federal Cave Resources Protection Act of 1988 (FCRPA)(16 U.S.C. § 4301 et seq.).
Regional Geology
Bighorn Canyon National Recreation Area is a part of the Rocky Mountain System Physiographic Province and shares its geologic history and some characteristic geologic formations with a region that extends well beyond park boundaries.
The Geologic Resources Inventory produces digital geologic maps and reports for more than 270 natural resource parks. The products listed below are currently available for this park, check back often for updates as many maps, reports, and posters are still in progress.
- Scoping summaries are records of scoping meetings where NPS staff and local geologists determined the park’s geologic mapping plan and what content should be included in the report.
- Digital geologic maps include files for viewing in GIS software, a guide to using the data, and a document with ancillary map information. Newer products also include data viewable in Google Earth and online map services.
- Reports use the maps to discuss the park’s setting and significance, notable geologic features and processes, geologic resource management issues, and geologic history.
- Posters are a static view of the GIS data in PDF format. Newer posters include aerial imagery or shaded relief and other park information. They are also included with the reports.
- Projects list basic information about the program and all products available for a park.
Source: Data Store Saved Search 2722. To search for additional information, visit the Data Store.
A NPS Soil Resources Inventory project has been completed for Bighorn Canyon National Recreation Area and can be found on the NPS Data Store.
Source: Data Store Saved Search 2784. To search for additional information, visit the Data Store.
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National Park Service Geodiversity Atlas
The servicewide Geodiversity Atlas provides information on geoheritage and geodiversity resources and values within the National Park System. This information supports science-based geoconservation and interpretation in the NPS, as well as STEM education in schools, museums, and field camps. The NPS Geologic Resources Division and many parks work with National and International geoconservation communities to ensure that NPS abiotic resources are managed using the highest standards and best practices available.