Geology of Sand Creek Massacre National Historic Site, Colorado

Part of the NPS Geodiversity Atlas, which describes each park's significant geologic features and processes. Geodiversity is the full variety of a park's natural geologic resources — rocks, minerals, sediments, fossils, landforms and physical processes — and its soils. The atlas comes from the NPS Geologic Resources Inventory.

A monument at Sand Creek Massacre National Historic Site. Image from the NPS's page.

The site

Sand Creek Massacre National Historic Site was dedicated and joined the national park system in 2007. It protects the ground along Sand Creek where, in 1864, a peaceful village of Cheyenne and Arapaho people led by Chief Black Kettle was attacked by about 700 men of the 1st and 3rd Regiments, Colorado (U.S.) Volunteer Cavalry, under Colonel John M. Chivington. About 230 Cheyenne and Arapaho were killed, most of them women, children, elders and the infirm. The site is deeply significant to the victims' descendants and tribes, a place to remember ancestors, and a reminder of the tragedies of the Indian Wars.

Big Sandy Creek, an intermittent tributary of the Arkansas River, runs diagonally across the site for about 3.25 miles (5.23 km). The village was camped along it, and during the attack people dug holes — sand pits — into the creek bank in a desperate effort to escape. Trees that were seedlings at the time of the massacre still grow along the creek.

The ground

The site lies on the Colorado Piedmont, part of the Great Plains in eastern Colorado — rolling hills and valleys between the High Plains and the Rocky Mountain foothills. Erosion has stripped away the Paleogene rocks that underlie the High Plains, leaving upper Cretaceous rock at the surface wherever younger deposits don't cover it. Exposures are poor, but three materials matter:

• Niobrara Formation — deposited in the Western Interior Seaway 82 to 81 million years ago, with the Fort Hays Limestone below and the Smoky Hill Shale above. The shale is mapped in the site's north, usually under a thin skin of silty clay, and weathers quickly to a yellowish gray visible in the soil. Its loamy soils carry shortgrass prairie.
• Windblown (eolian) sand and loess — widespread at the surface, especially in the south, forming sand hills west and south of the creek with sand sagebrush. These sandy soils are easily disturbed.
• Quaternary alluvium — along the creek.

Windblown sediments exposed in the bluff above Big Sandy Creek. NPS photo.

The most prominent landform is a bluff about 20 ft (6 m) high southeast of the creek, made mostly of windblown sand. All but its top 2.5 feet is too old to hold cultural material from the time of the massacre; floodplain alluvium from that time lies along the creek.

The bluff above Sand Creek; the small shrubs are sand sage. NPS photo.

Big Sandy Creek

• Drainage basin: about 2,631 square miles (6,814 km²) of eastern Colorado; the creek flows mostly southeast and joins the Arkansas River about 31 miles (50 km) south of the site.
• An alluvial stream: its bed and banks are made of sediment it carries — clay to gravel, mostly alluvium with some windblown material on top, filling the valley 23 to 30 feet (7 to 9 m) deep.
• Rarely flowing: it is a meandering channel too small for its broad valley, but because it flows only every 5 to 10 years, little meandering or channel-building happens now. In places there is no defined channel at all — just a broad grassy swale marked by cottonwoods. Three stream terraces sit above the modern floodplain.

Big Sandy Creek in flood. NPS photo.

Cottonwoods in the Big Sandy Creek riparian zone. NPS photo.

More: a riparian condition assessment of the creek.

Fossils and hazards

• Fossils: none found in the site. The Niobrara elsewhere holds many marine vertebrates and invertebrates, and the windblown and creek deposits could hold pollen. All park fossils are protected by the Paleontological Resources Preservation Act.
• Floods: rare, usually from intense summer thunderstorms.
• Earthquakes: low hazard. The USGS 2014 map gives a 2% chance in 50 years of shaking above 4–8% of gravity (about V on the Modified Mercalli scale), and 2 to 4 damaging shakings expected in 10,000 years. A magnitude 3.9 quake struck east of Eads in 2009, about 11 miles southwest of the site.

Maps and reports

The site lies in the Great Plains province. A Soil Resources Inventory project has been completed. More: the site's geophysical and hydrological research.

The Sand Creek landscape. Image from the NPS's page.

Sources

Based on "NPS Geodiversity Atlas—Sand Creek Massacre National Historic Site, Colorado," National Park Service; a work of the United States government in the public domain. The atlas spells the tribe both "Arapaho" and "Arapahoe"; Arapaho is used here.