Washita Battlefield: the ground decided how it went

Before dawn on 27 November 1868, the 7th U.S. Cavalry under Lieutenant
Colonel George A. Custer attacked the Southern Cheyenne village of Peace Chief
Black Kettle. The 326 acres that are now Washita Battlefield National Historic
Site hold the camp, the place Black Kettle and his wife were killed, and the
pony kill site where the cavalry slaughtered an estimated **650 Cheyenne
horses**.

The geology is not background here. It decided how the attack went.

A camp placed for winter, and what that cost

Black Kettle's winter camp was on the floodplain of the Washita, a perennial
meandering river that loops through a series of horseshoe bends. The low
ground was chosen for shelter from the wind that crosses the Great Plains in
winter.

Low ground with a river bending around it is also a trap, and once the
attack began that is what it became.

The cavalry used the same terrain the other way. Topographic highs served as
lookouts — the attack started from one still called Custer's knoll — and the
ridgelines let them encircle the camp. The manoeuvres ran both inside and
outside what is now the park boundary. Black Kettle National Grasslands
surround it.

Washita Battlefield and the Horseshoe Hills. NPS photo.

The rock

Most of the surface is Quaternary: modern Washita floodplain sediment, mostly
fine grained; older terrace deposits carrying pebbles and gravel; and
paleodunes left by wind.

Two Permian bedrock units have been mapped in the uplands, both poorly
exposed:

Cloud Chief Formation — red-brown and greenish-grey shale and siltstone
with gypsum beds near the base, about 190 ft (58 m) thick in central Roger
Mills County. Laid down between roughly 262 and 255 million years ago in
an arid coastal setting: shallow marine, tidal, mud flat including sabkha, and
river.

Doxey Formation — above it, darker, with a gradational contact, deposited
underwater on what was probably a coastal mudflat. It shows in the higher
ground and on the knolls and bluffs around the site — including the ones the
cavalry used.

The basin underneath

Washita lies in the Anadarko Basin, the deepest sedimentary and structural
basin in the interior of the United States: about 50,000 square miles, holding
over 40,000 ft of Cambrian-to-Permian sedimentary rock, reaching from
western Oklahoma across the Texas Panhandle into south-western Kansas and
south-eastern Colorado. It carries one of the country's most significant
natural gas deposits and important helium reserves.

At the surface this is the Western Red-Bed Plains — gently rolling hills on
nearly flat-lying Permian red sandstone and shale. The High Plains begin to
the west, a flat upland of Tertiary and Pleistocene alluvial and windblown
sand, cut by rivers.

Sources

Written from the National Park Service's Geodiversity Atlas entry for Washita
Battlefield National Historic Site, a work of the United States government in
the public domain.