NPS Geodiversity Atlas—Bent's Old Fort National Historic Site, Colorado

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.

bent's old fort gri report with wetlands image
In-depth geologic information is contained in the baseline inventory products of the Geologic Resources Inventory, see table below.

Geologic Features and Processes

[Site Under Development]

Bent’s Old Fort National Historic Site is part of the geologic heritage of the Great Plains. The national historic site lies in the Colorado Piedmont section of the Great Plains province. The area ranges in elevation from 3,975 feet (1,212 m) at the point where the Arkansas River enters Bent County to 5,200 feet (1,585 m) north of Delhi, Colorado. Generally speaking, the area around Bent’s Old Fort consists of a series of flat to gently rolling surfaces with steep intervening slopes.

The Colorado Piedmont is situated at the foot of the Rockies. After leaving the mountains, the South Platte River, to the north, and the Arkansas River, to the south, have excavated the Tertiary (65.5- to 1.81- million years old) sedimentary rock layers removing great volumes of sediment. These Tertiary rock units were originally deposited by shifting stream channels, floodplains, swamps, and occasional volcanic ash (Kiver and Harris, 1999).

Bent’s Old Fort lies on the northern edge of the Arkansas River floodplain. Three well- formed terrace levels flank the river’s floodplain, and remnants of a number of well- formed higher land surfaces are preserved between the river and the mountains. Around the national historic site these Quaternary terraces rest unconformably upon Cretaceous shales and limestones.

In the Colorado Piedmont, the erosional effects of streams are the most conspicuous features of the landscape. These fluvial features are enhanced by the steep tilting of layered rocks along the western margin of the Colorado Piedmont and modified by wind action, which has softened the landscape with a widespread cover of windblown sand and silt (Trimble, 1980).

River Systems and Fluvial Landforms

The Arkansas River is a broadly meandering stream with a braided stream channel in the vicinity of Bent’s Old Fort National Historic Site. It flows approximately 6 miles (10 km) to cover a linear distance of 3 miles (6 km). The river’s meanders are developed on a floodplain that, in general, is less than 0.75 mile (1.2 km) wide. The floodplain is entirely developed in unconsolidated materials, underlain by 30 to 40 feet (9 to 12 km) of fill. The river gradient is about 5 feet per mile (1.5 m/1.6 km), and the sediment load carried is moderately heavy, consisting largely of silt and sand in this reach of the river.

During the period of stream gauging recorded at La Junta, the flow of the Arkansas River ranged from no flow to 200,000 cubic feet per second (5,664 m3 /s) (Swenson, 1970). Large diversions of water are made for irrigation, which accounts in part for the wide range in discharge of the stream. Discharge is also affected by rapid snowmelt in the mountains and cloudbursts within the basin, which can cause heavy flooding in the area. The maximum flood recorded occurred on June 4, 1921; it resulted from a cloudburst during a period of heavy snowmelt (Swenson, 1970).

The area surrounding the fort includes several wetlands (Woods and MacDonald, 2002). The 52- acre (21- ha) Arch wetland lies on the Arkansas River floodplain immediately north of the fort. National Park Service personnel report that the Arch wetland has gradually increased in extent over the last 15–20 years. The 1- acre (0.4- ha) Casebolt wetland lies approximately 1,476 feet (450 m) southwest of the fort in a meander bend of the Arkansas River, and the 0.5- acre (0.2- ha) Day Pond is located adjacent to the Arkansas River about 984 feet (300 m) southwest of the fort (Woods and MacDonald, 2002).

Although analysis has not been conducted, wetlands at Bent’s Old Fort National Historic Site have the potential to serve a significant ecological purpose by acting as natural filters of pollutants (National Park Service, 1993). Fertilizers, organic products, and other chemicals used on the agricultural lands surrounding the national historic site may be reducing the quality of the subsurface waters, and the wetlands could be sequestering and filtering these pollutants. Wetlands also have the potential to serve as flood buffers (Berger and Iams, 1996).

The original fort was built of adobe on the edge of a low terrace projecting southeastward into the floodplain of the Arkansas River (Swenson, 1970). Figures showing the river course and floodplain in Swenson (1970) plot Bent’s Old Fort as lying at the northern edge of—but not actually in—the floodplain. However, in 1921 the “Great Pueblo Flood” crested above the level of the fort grounds causing most of the remaining original adobe walls to collapse (National Park Service, 1975a). The site also was threatened by flooding in 1965; however, floodwaters did not reach the ruins or cause any property damage (National Park Service, 1975a).

Aeolian (Dunes) Landforms and Processes

Windblown sand and loess (windblown dust) blanket and smooth terrain features across large parts of eastern Colorado, including the south side of the Arkansas River near La Junta eastward to Kansas. Northwesterly winds, which frequently blow with near- hurricane velocities, have whipped fine material from the floodplains of the streams and spread it eastward and southeastward over much of the Colorado Piedmont.

Blowout dunes are the most common dune type in the vicinity of Bent’s Old Fort (Madole, 1995). Blowout dunes are large accumulations of sand derived from a “blowout”—a saucer- or trough- shaped hollow formed by wind erosion in a preexisting sand deposit.

Within the Great Plains, dry climatic episodes during the past 1,000 years have generated desert conditions with windblown sand and dust. Presently, most dune sand in the vicinity of Bents Old Fort is stable and covered with vegetation. However, the surounding semiarid region is close to the threshold where sediment could again become mobile and convert thousands of square miles into active dune fields (Muhs and others, 1993).

Paleontological Resources

Potential for fossils within the boundaries of Bent’s Old Fort exists in the Bridge Creek Member of the Greenhorn Limestone (Upper Cretaceous). This formation is known to contain Durania cornupastoris, an extinct group of bivalved (pelecypods) mollusks that are a rare find in the Western Interior. Additionally, the Pleistocene- age Louviers Alluvium has produced a mammoth (Mammothus columbi) tusk and a horse (Equus sp.) tooth in the vicinity of the national historic site. This stream- deposited sediment hosts fossils of other vertebrates, including camels and great bison, and fossil mollusks in other areas of eastern Colorado. Hence, these paleontological resources are also likely to occur within the boundaries of Bent’s Old Fort National Historic Site.

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).

Geology Field Notes

Students and teachers of college-level (or AP) introductory geology or earth science teaching courses will find that each park's Geologic Resource Inventory report includes the Geologic History, Geologic Setting, and Geologic Features & Processes for the park which provides a useful summary of their overall geologic story. See Maps and Reports, below.

Regional Geology

Bent's Old Fort is a part of the Great Plains Physiographic Province and shares its geologic history and some characteristic geologic formations with a region that extends well beyond park boundaries.

Maps and Reports

Geologic Resources Inventory

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 2715. To search for additional information, visit the Data Store.

Soil Resources inventory

NPS Soil Resources Inventory project has been completed for Bent's Old Fort National Historic Site and can be found on the NPS Data Store.

Source: Data Store Saved Search 2782. To search for additional information, visit the Data Store.

Related Articles

Bent's Old Fort National Historic Site

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.

For more information on the NPS Geodiversity Atlas, contact us.

Last updated: October 24, 2019