For approximately 6,000 years, between about 8,000 and 2,000 years ago, the Archaic period in the Great Plains was a time of human adjustment to changing ecological conditions. Paleo-Indian bison hunting decreased markedly after about 9,000 years ago, due to a steady deterioration of ecological conditions. Subsequently there were several late Paleo- Indian groups, such as Lusk, Angostura, Frederick, and James Allen, which were beginning to shift toward the use of small animal and plant resources. By about 8,000 years ago, both bison and human populations in the Great Plains had decreased significantly. Some groups may have moved into the foothills and mountains to the west and others into the prairies on the east. The Plains was not abandoned during the Archaic period, but ecological conditions made it a much less desirable place.

Much of what is known about the Plains Archaic period comes from archeological sites on the edges of, and just outside, the Plains. A widespread change in projectile point styles from lanceolate to notched forms is arbitrarily used to mark the beginning of the Plains Archaic period. Some argue that the change was the result of new groups moving into the area, while others believe it was simply a technological modification accepted by the existing residents. The best evidence for these changes is found in areas adjacent to the Plains and in areas of topographic relief within the Plains, such as the Black Hills of western South Dakota and northeast Wyoming and the Pryor Mountains in southern Montana.

In the Northern Plains, the Early Plains Archaic, also known as the Altithermal, is dated from about 8,000 to 5,500 years ago; the Middle Plains Archaic lasted from about 5,500 to 3,000 years ago; and the Late Plains Archaic lasted from about 3,000 years ago until between 2,000 and 1,500 years ago. A slightly different chronology is used in the Central and Southern Plains. The Early Archaic period there is 8,500 to 6,500 years ago; the Middle Archaic is 6,500 to 4,500 years ago; and the Late Archaic is 4,500 to 2,500 years ago.

Evidence from the Early Plains Archaic suggests that the Black Hills may have been a kind of oasis where bison were able to maintain their numbers. The Hawken site in northeast Wyoming, a 6,500-year-old bison kill site, contained animals that were intermediate in size (probably Bison occidentalis), smaller than the ones found in earlier Paleo-Indian kills but larger than the modern bison found in kill sites after about 5,500 years ago. The Itasca site, close to the Plains in Minnesota and dating from between 7,000 and 8,000 years ago, contained similar bison. The Cherokee site in Iowa has Late Paleo-Indian and Early Archaic bison evidence. The Rustad and Smilden-Rostburg sites in western North Dakota are Early Archaic sites with bison remains. The lowest level at the Oxbow Dam site in southern Saskatchewan is Early Archaic. The Logan Creek complex in eastern Nebraska and Iowa and the Sutter site in eastern Kansas are also Early Archaic. Early Archaic sites in the Southern Plains include the Gore Pit site in south-central Oklahoma and the Wilson Leonard site on the Edwards Plateau, which has a stratified sequence of Early through Late Archaic levels.

Pronghorn remains are found in Great Plains sites of all ages. The Trappers Point site at a seasonal migration route along the Green River, just beyond the Plains in western Wyoming, contains communal pronghorn kills dating from about 8,000 to 5,000 years ago and is a strong indicator of similar pronghorn procurement throughout much of the Plains.

The Medicine House and Split Rock pit house sites along the North Platte River in southern Wyoming contain evidence of the use of grinding stones, plants, and small animals. Rock shelters and open sites in the Big Horn Mountains of northern Wyoming and in the Pryor Mountains of southern Montana also hold Early Archaic evidence.

Improved climatic conditions and a resurgence of bison hunting appeared during the Middle Plains Archaic, around 5,500 years ago on the Northern Plains. The Head-Smashed- In buffalo jump near Lethbridge, Alberta, is one of the more spectacular of these bison procurement features, showing continuous use from the end of the Early Plains Archaic into historic times. Oxbow Dam, Mortlach, and Long Creek are deep, stratified sites in southern Saskatchewan with both Middle and Late Archaic levels.

McKean sites, named after the McKean site in the Wyoming Black Hills, are widespread over the Northern Plains. The Scoggin site, for example, a bone bed inside an artificial bison corral, is located in western Wyoming close to the North Platte River. The Laidlaw site in southern Alberta is Middle Archaic, with drive lines and a pit used to trap pronghorns. Flat stone grinding slabs, manos (handheld grinding stones), and stone-filled fire pits are numerous in the southern part of the McKean occupation area, indicating a trend toward broad-spectrum hunting and gathering.

The Middle Archaic of the Edwards Plateau of Texas is well represented at the Wilson Leonard site by a wide variety of diagnostic traits. Middle Archaic levels are also present at the Magic Mountain site on the Front Range west of Denver, Colorado.

Late Plains Archaic sites are widely distributed throughout the Great Plains. First thought to be late Paleo-Indian because of large lanceolate projectile points, the Late Archaic Nebo Hill complex, widespread along the Kansas-Missouri border, contains a wide variety of artifact material, features, and faunal and plant food resources. The Late Archaic Pelican Lake complex, named from sites in southern Saskatchewan, is found over most of the Northern Plains. Pelican Lake levels are found at Head-Smashed-In buffalo jump in Alberta and in jump and arroyo bison kills in Montana. South of the Montana border, Pelican Lake demonstrates less dietary emphasis on bison and more on smaller animals, including pronghorn and deer. Grinding stones and stone-filled food-preparation pits are also common. Along the Powder River in northern Wyoming and Montana are arroyo bison kill sites known as Yonkee from a site in southern Montana, with dates of about 2,500 years ago. Stone circles (tipi rings) are widespread features from this period, most of which are believed to have held down the edges of hide coverings of conical lodges.

Besant cultural groups, another Late Archaic group named from sites in southern Saskatchewan, may have been the most sophisticated pedestrian bison hunters to appear in the Plains. The Ruby site in eastern Wyoming is a large bison corral alongside a religious structure. Farther west is the Muddy Creek site, a large Besant bison corral located in a depression, with a wooden ramp built to stampede the animals into the enclosure. Large stone circle concentrations represent associated living areas, and a large boulder pile on a high point overlooking the site is a religious structure.

In the Central and Southern Plains, the Late Archaic subsistence strategy was broadspectrum hunting and gathering. There is a 2,600-year-old bison jump at Bonfire Shelter at the mouth of the Pecos River in Texas. Late Archaic groups on the eastern margins of the Great Plains may have been encouraging the propagation of native plants, but there is no evidence of corn, beans, or squash.

Following the Archaic in the Northern Plains was the Late Prehistoric period, between about 2,000 and 1,500 years ago, which witnessed the appearance of the bow and arrow and intensified bison hunting. Avonlea, one of the earliest hunting groups to use the bow and arrow, was contemporaneous with some late Besant groups. On the eastern edge of the Plains, agricultural villagers with ceramics and Central American cultigens appeared and persisted into historic times.

See also PHYSICAL ENVIRONMENT: Bison.