T90 Topics in Geoarchaeology: Reconstructions of Ancient Landscapes and Paleoenvironments

Monday, 10 October 2011
8:00 AM - 12:00 PM
Room 101FG

EXPERIMENTS IN THE HEAT-TREATMENT OF OGALLALA FORMATION SILICIFIED SEDIMENT NODULES

Paper number: T90-10
Program: Technical Sessions
Time: 10:10 AM – 10:30 AM

Authors

Abstract

Heat-treatment of siliceous stone is a well-documented technique used globally by past hunter-gatherer groups. Heat-treatment decreases fracture toughness for increased ease and control of the fracture process during stone tool manufacture. In heat-treatment, a stone is often reduced to a thinner biface or flake preform to decrease fracture damage, and to ensure the entire piece was effectively altered. However, at a Late Archaic (4,500-2,000 B.P.) campsite (41GR862), located on the eastern escarpment edge of the Southern High Plains of Texas, an analysis of stone tool reduction sequences indicate hunter-gatherer groups were heat-treating large Potter member (Ogallala Formation silicified sediment) nodules instead of using flake blanks or prepared preforms. The site rests on an eroded bench overlooking an unnamed gully. Erosion has occurred since the site was occupied, and no intact features were encountered. A surface assemblage of 2,221 artifacts was mapped, collected, and analyzed. Over 71% of the artifacts consist of Potter member debitage and tools; ~57% of these artifacts exhibit evidence of heat-treatment.

A series of experiments document the methods Late Archaic groups may have used and the advantages of heat-treating large unaltered nodules. Results indicate minimum temperatures between 246-260 °C were necessary for over 10 hours for effective heat-treatment. To obtain these temperatures in experimental heat-treating pits, the unaltered nodules were placed directly on coals. To prevent thermal shock from rapid temperature increases in the nodules, the Potter member nodules were pretreated slowly increasing their temperature by placing them around the fire pit before directly placing them on top of the coals. Results of the experiments suggest the heat-treatment of Potter member nodules involved the careful pretreatment of nodules before heat-treatment, and that the nodules also may have functioned as hearthstones for the conservation of wood.