LATE-PLEISTOCENE AND HOLOCENE GEOMORPHOLOGY IN THE SAN LUIS VALLEY, COLORADO: IMPLICATIONS FOR ARCHAEOLOGICAL RESEARCH

The San Luis Valley (SLV) of Colorado is home to the headwaters of the Rio Grande, two mountain ranges with peaks over 4,250 meters, and a large semi-arid alpine valley floor. The San Juan and Sangre de Cristo mountains were both glaciated in the late Pleistocene. Four mammoth digs are active; all are near paleo-water sources and three are in alluvial fans. Fluvial geomorphology provides data on watershed-scale climatic and environmental change. A high water table in the SLV has constructed paleo and modern wetlands, preserving climatic data and attracting prehistoric cultural activity. Spatial and temporal patterns of erosion, deposition, and stability in the glacial and fluvial landforms, wetland peats, and alluvial fans relate to the archaeological story of the SLV. Almost a decade of radiocarbon and OSL data help to construct a map of likely locations for archaeological preservation. In-situ Paleoindian materials may be found in the stratified peats of paleowetland sites, aeolian sediments on the SLV floor, high terrace fills along the mainstem of the Rio Grande and the South Fork of the Rio Grande (larger basins), and deeply buried in younger fans deposited during Holocene fires and mass wasting events. Archaic materials may be found on older landform surfaces or shallowly buried in aeolian sediments on the SLV floor, shallowly buried in younger fan sediments, deeply buried in terrace fills of tributary basins, and in the middle and low terrace fills of larger basins. Prehistoric/Ceramic and Protohistoric age sediments are widely stored in terrace fills of tributary basins and are common in low terrace fills of larger basins, and are shallowly buried in fans and aeolian SLV floor sediments. For those who seek the elusive pre-Clovis cultures, accessible and stratified low-energy fluvial sediments are buried below peats in paleowetlands on the SLV floor where late-Pleistocene faunal remains are also common. Late-Pleistocene alluvium and glacial outwash also is stored below extensive high terraces in larger basins, and deeply buried soils are found in the massive set of coalescing Pinedale fans along the mountain front.