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The remains of Archaic cultures, especially those dating 8,000 to 4,000 B.P., are not well documented in the archaeological record of the Central Plains of the United States. Some researchers have suggested that people abandoned the region during this period because of increased aridity and associated displacement of game. However, there is strong evidence that geomorphic processes, particularly erosion and deposition, have greatly affected the preservation and detection of Archaic and younger cultural deposits.

Chronostratigraphic evidence gleaned from more than 65 localities reveals spatial and temporal patterns of Holocene erosion and deposition in river basins of the Central Plains. Both of these geomorphic processes, as well as periods of net transport and storage of alluvium, were diachronous within individual drainage basins, but were roughly synchronous in similar-sized streams of the drainage networks. Radiocarbon ages from alluvial fills in valleys of large streams (≥ fourth order) span the Holocene, whereas nearly all of those from alluvial fills associated with small streams (≤ third order) are less than 4,000 years old. The early through mid-Holocene gap in the alluvial chronology of small streams is largely filled when alluvial/colluvial fans are considered. From 9,000 to 4,000 B.P., fans typically developed where first and second-order valleys join large valleys.

Although valley erosion and deposition may have several causes, major bioclimatic changes generally explain the pattern of Holocene fluvial activity detected in the alluvial stratigraphic records of the Central Plains. Reduced vegetative cover, combined with infrequent but intense rainfalls during the warm, dry Altithermal (8,000 to 5,000 B.P.), favored erosion and net transport of sediment within small valleys. As mean annual precipitation increased during the late Holocene, vegetation recovered and erosion rates decreased, promoting sediment storage in small valleys.

The time-space distribution of alluvial deposits in the Central Plains explains the paucity of documented Archaic sites in the region. Within large valleys, it is likely that most of the Archaic record is deeply buried in early through late Holocene valley fills and alluvial fan deposits. Within small valleys, erosion during the early and middle Holocene probably removed most of the Early and Middle Archaic cultural deposits, and aggradation during the late Holocene favored deep burial of Late Archaic and younger cultural remains. Also, surfaces of landforms that dominate valley bottoms throughout the drainage systems are geologically quite young, often postdating 2,000 B.P. Hence, apparent gaps in the archaeological record may be a result of (1) deep burial of cultural deposits, (2) removal of alluvial deposits that contain cultural materials, and (3) young surfaces dominating valley landscapes.

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