Contemporary Archaeologies of the Southwest

8: Ancient Social Boundaries Inscribed on the Landscape of the Lower San Pedro Valley

168 Joe Lally and A. J. Vonarx Prehistoric structural fires in the Gallina region of northern New Mexico are typically assumed to be incendiary because of warfare (Cordell 1979:143). Physical evidence seems to refute this notion in at least one case, however. Scorpion House (LA 57386) is in a heavily wooded area of Rio Arriba County, New Mexico. This Fuel wood burning in corner. Photograph by Kate Harrison. 169 Fire: aCCidental or intentional? is a classic Gallina structure in the heart of Gallina country that experienced fire at some point in its history. Sintered adobe plaster was located on the exterior of the structure. No thermal alterations were observed within the structure, indicating that the fire at Scorpion House was clearly on the exterior. The fuel that caused the thermal alterations was most likely from the heavily wooded area surrounding Scorpion House. Providential wildland fires are a frequent occurrence in the mountains of northern New Mexico, occurring as often as every seven years (Lentz, Gaunt, and Willmer 1996). Flame length can easily exceed 7 meters in a wildland fire (Pyne, Andrews, and Laven 1996:67). A seven-meter flame length would produce 10,265 kW. Such a fire 1 meter from a structure would result in over 245 kW radiant heat flux impinging on that structure—more than the 196 kW that fused the adobe roof fragments in the adobe test fire. It is unknown if this fire occurred during occupation or after abandonment. One can easily imagine a forest growing up around an abandoned structure and eventually falling victim to a lightning-caused wildland fire. We were fortunate to have access to the land, the labor necessary to build the test structures, and the cooperation of our local fire departments to set the structures on fire. Not everyone is so fortunate. A lack of land or labor need not keep anyone from designing and testing fire scenarios, however. The National Institute of Standards and Technology (NIST) has developed several mathematical fire-modeling programs as well as computer animation capabilities that allow any potential compartment fire scenarios to be designed and tested. The growth, duration, HRR, and thermal effects of these computer-modeled fire scenarios can be solved mathematically and visually displayed with the help of computer-generated animation. Best of all, they are available at no cost on the NIST website (www.nist.gov). The most common fire-modeling programs are Consolidated Fire and Smoke Transport (CFAST) and Fire Dynamics Simulator (FDS). CFAST can be run on almost any home computer and is not difficult to use. FDS may be beyond the capabilities of many personal computers and requires the ability to write the computer code defining any given fire scenario. Some FDS calculations may take hours, if not days, to complete. With CFAST, it is as simple as defining your compartment(s)’ length, width, height, ventilation, construction material, fuel load, and location and then clicking “run.” You will soon learn if your suspected fuel load and point of origin are capable of producing the thermal alterations observed. CFAST is easy; FDS is more difficult, but it provides a more complete picture. appliCation to the arChaeologiCal reCord Fire tends to preserve the remains of prehistoric earthen structures; thus the number of burned prehistoric structures may be overrepresented in the archaeo- 170 Joe Lally and A. J. Vonarx logical record. This frequency of burned structures may have caused archaeologists to overemphasize the importance of fire in southwestern prehistory. When one is considering the significance of an ancient burned building, he or she must remember that not all structural fires are intentional incendiary fires. Arguments suggesting a motive for a fire, such as ritual or warfare, should not be made until that fire has been demonstrated to be an intentional fire. Evidence of the incendiary nature of a fire should be demonstrated, not assumed. The size or intensity of a fire alone cannot be used as evidence of intentionality. The high standards of excavation methods used by most archaeologists today are sufficient for discovering evidence of intentional fire. The excavation methods used by modern arson investigators closely resemble those of modern archaeology . Both groups are attempting to build a model of a past event. The quantity and quality of the evidence collected will determine the completeness and accuracy of that model. Any attempt to reconstruct the cause and origin of ancient structural fires must determine: The physical dimensions...