The Geology of Bent's Old Fort National Historic Site, Colorado

Bent's Old Fort was a trading post on the mountain branch of the Santa Fe Trail, where traders, trappers, travellers, and the Cheyenne, Arapaho and other tribes came together peacefully to trade. It was in use from 1833 until 1849, when disease and disasters led to its abandonment. In 1976 the adobe fort was rebuilt on its original foundation, and today, set in its landscape of native plants and animals, it lets visitors step back in time. Its geology, as the National Park Service's Geodiversity Atlas tells it, is part of the story of the Great Plains.

The sweeping views of the Great Plains from the fort are an important part of the park's cultural landscape. Credit: National Park Service.

On the trail

The fort stands on the floodplain of the Arkansas River near La Junta, Colorado. Until the end of the Mexican-American War the Arkansas was the boundary between the United States and Mexico. The Santa Fe Trail's mountain route followed the river for much of its length; it was longer and harder than the Cimarron route but thought safer, with plenty of water. A few miles west of the fort the trail left the river and turned southwest towards Raton Pass, its most difficult stretch — so the fort was the place to stock up, make repairs and eat before the hard road ahead.

A seaway's rocks, cut by rivers

The site lies in the Colorado Piedmont, a deeply eroded section of the Great Plains between the High Plains to the east and the Rocky Mountains to the west. Its most striking features are the work of streams. The Arkansas and South Platte Rivers and their tributaries have stripped away the thick Paleogene sediments that washed off the Rockies after they rose — sediments that still lie beneath the High Plains — and exposed older Cretaceous rock wherever younger deposits do not cover it. To the south, the Raton section of the Great Plains, with mesas capped by lava flows, rises gradually from the river.

The Cretaceous rocks were laid down in the Western Interior Seaway, which covered much of central North America from approximately 100 to 70 million years ago and at its largest reached from the Arctic to the Gulf of Mexico. Near the fort, from oldest to youngest:

Because the fort sits beside the river, only Quaternary deposits lie at or just below its surface. The country around is typical of eastern Colorado: flat to gently rolling surfaces with steep slopes between them, the uplands largely blanketed by windblown silt.

The Arkansas River in the historic site. Credit: National Park Service.

The Arkansas River

The Arkansas is the sixth-longest river in the United States, flowing 1,469 miles from the Rocky Mountains to the Mississippi. Near the fort it is a sand-bottomed river meandering across its own sediments, winding about 6 miles within the park boundary. Its floodplain is generally more than a mile wide, bordered by low bluffs and underlain by 30 to 40 feet of loose fill over Cretaceous bedrock. The river falls about 6.7 feet per mile and carries a fairly heavy load, mostly silt and sand.

In the 19th century the river ran only part of the time, in a wide, shallow, braided channel. Dams — especially Pueblo Dam, upstream of Pueblo — and irrigation have since changed its flow, and invasive tamarisk changed its character too, although tamarisk has now been removed from the stretch inside the park.

Its floods vary enormously. Near La Junta the peak discharge expected every 1.5 years is 2,700 cubic feet per second; the 10-year peak is 14,200 and the 500-year peak 64,800. The highest flow on record at La Junta, 200,000 cubic feet per second, came in the Pueblo Flood of June 1921, when an intense cloudburst struck during heavy snowmelt; it brought down most of the fort's remaining original adobe walls. Other floods hit the La Junta area in 1935, 1942, 1955 and 1965.

The fort was built in 1833 on the edge of a low terrace on the northern side of the floodplain, approximately 300 yards from the river. The floodplain holds alluvium, fine flood-laid sediment and wetlands. The largest wetland, the 52-acre Arch wetland immediately north of the fort, has grown since the 1980s, probably because leakage from the Fort Lyon Canal — which runs roughly parallel to the river about 1.5 miles north of the fort — and runoff from excess irrigation have raised the water table. The wetlands store floodwater, recharge groundwater, provide wildlife habitat and may help filter pollutants.

The Arch wetland, with Bent's Old Fort behind it. Credit: National Park Service.

Older river terraces of Pleistocene age line the valley. The oldest, the deeply weathered Verdos alluvium with its well-developed ancient soils, dates from the middle of the early Pleistocene and is mapped on the uplands west of the fort. The Slocum alluvium is middle Pleistocene. The Broadway alluvium, gravel, sand and silt in terraces above the river, is mapped inside the site.

Wind at work

Windblown sand and loess (windblown dust) smooth much of the Colorado Piedmont. Northwesterly winds have carried fine material off stream floodplains and spread it southeastward. North of the river, loess covers the uplands; south of it, sand blown mainly from the Arkansas River's own alluvium forms a belt of dunes, most of them blowout dunes — large heaps of sand that come from a "blowout," a saucer- or trough-shaped hollow scoured by wind in older sand. None of these deposits lie inside the park. Most are now stable and covered in plants, but in droughts such as the Dust Bowl this semi-desert country's sand moves again.

Fossils

The site's best-known fossil is the tusk of a Columbian mammoth, found in a borrow pit in 1964. A few other pieces of mammoth tusk or teeth have turned up, along with pollen and phytoliths — microscopic plant structures made of silica — and the Quaternary deposits may yet yield more. The Bridge Creek Limestone holds bivalves, microfossils and nannofossils from marine phytoplankton; the Greenhorn, Carlile and Niobrara units, though not exposed in the site, hold many other fossils, mostly marine invertebrates. All fossils in national parks are protected by the Paleontological Resources Preservation Act of 2009.

Hazards

The fort lies within the 100-year floodplain. The Arkansas floods in two ways: longer spring floods from snowmelt, and summer floods from storm runoff with higher peaks but smaller volumes.

Eastern Colorado's earthquake hazard is fairly low. The USGS 2014 Seismic Hazard Map gives the area a 2% chance of shaking with a peak ground acceleration between 8 and 10 %g — roughly V to VI on the Modified Mercalli Intensity Scale — being exceeded in 50 years, and the expected number of damaging earthquake shakings in eastern Colorado in 10,000 years is between 2 and 4. A magnitude 3.4 earthquake struck approximately 14 miles south of the fort in 1998.

Part of the geologic map of the park. Image from the National Park Service's Geologic Resources Inventory.

Maps and more

The Park Service's Geologic Resources Inventory, which produces digital geologic maps and reports for more than 270 natural resource parks, and its Soil Resources Inventory have both covered Bent's Old Fort; their products are on the NPS DataStore.

Inside Bent's Old Fort. Credit: National Park Service.

Sources

Based on "NPS Geodiversity Atlas—Bent's Old Fort National Historic Site, Colorado," National Park Service Geologic Resources Inventory; rewritten in hubnx's own words.

• The atlas gives the river's straight-line distance through the park as "3 miles (6 km)" and the fill beneath the floodplain as "30 to 40 feet (9 to 12 km)"; the conversions do not match, so the straight-line distance is left out and only the feet are given.
• The atlas's hazards section names "Sand Creek Massacre NHS," evidently copied from another park's entry; this page applies the section to Bent's Old Fort, the park the rest of the entry describes.
• "South Platt" in the atlas is corrected to South Platte.
• The Geologic Resources Inventory report cover shown on the atlas page is not reproduced.