Wednesday, June 12, 2013

Research Rocks! Literally :-)

Craig working on some XRF analysis of basalt in the PGM lab.
The prehistoric Hohokam who inhabited Pueblo Grande village were sedentary irrigation agriculturalists.  The primary crops of these farmers included maize (i.e., corn), beans, and squash. Dependence on these domestic cultigens required the use of stone grinding tools in order to prepare the plants for consumption.  The two most common groundstone tools found in archaeological contexts include milling stones (called manos) and grinding slabs (called metates).  These implements were used together, with grains being ground between the stationary metate and the handheld mano.

The Hohokam preferred to manufacture groundstone tools from vesicular basalt.  This stone is a volcanic rock that has cavities throughout and in many cases large-grain mineral inclusions.  This irregular texture provides an ideal surface for shearing large-grained seeds, such as corn.  Additionally, the stone maintains an effective grinding surface after repeated use, whereas other material types, such as granite, must be regularly roughened to maintain grinding efficiency.  These favorable performance characteristics were well-known to the Hohokam, as evidence by the abundance of vesicular basalt groundstone tools at their settlements.
Mano & Metate grinding exhibit at PGM
Though preferred by the Hohokam, vesicular basalt is not readily available within the vicinity of Pueblo Grande.  Instead, the material can only be found at a handful of outcrops in the region.  Sources of the material closest to the site are located in relatively small portions of the Phoenix, Santan, and McDowell Mountains.  Thus, the Hohokam had to import this material from the natural source areas to their sites.

My research seeks to determine the method or methods by which Hohokam villagers acquired vesicular basalt for groundstone tool production.  To accomplish this goal, I first collected hundreds of rock samples from nearly two dozen vesicular basalt outcrops in the Hohokam region.  These source materials were then analyzed using Energy-Dispersive X-ray Fluorescence Spectroscopy (EDXRF), a nondestructive analytical technique that characterizes the chemical composition of the sample.  The analysis found that the majority of vesicular basalt outcrops used by the Hohokam are chemically distinct.  This observation is important because it allows us to analyze vesicular basalt groundstone artifacts from archaeological contexts and match them to the source from which they were derived.
Example of an XRF analysis graph
By comparing vesicular basalt use at different archaeological sites it is possible to assess raw material acquisition practices.  For example, artifacts from Pueblo Grande were selected for this study because it features publically constructed architecture (including a ballcourt and platform mound), suggesting that the site was an important ceremonial location.  Therefore, when the Pueblo Grande data are compared to other sites, a picture will be begin to emerge on whether ceremonial activities played a role in the transport of vesicular basalt.  Other possible explanations that are also being evaluated include direct-procurement, where people simply walked to the source themselves to get the stone; mutualistic exchange, where the Hohokam bartered among themselves for the stone; and more complicated market exchange practices in which the stone was acquired through less personalized transactions that were organized in a more formal and organized system.


Posted by Craig Fertelmes, Gila River Indian Community - Cultural Resource Management Program Archaeologist & Arizona State University Doctoral Candidate in Archaeology/Anthropology

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