3.3. Regional patterns and major sources of obsidian

Sources of tool-quality obsidian occur in discrete locations along the Western Cordillera in the south-central Andes. The spatial pattern of obsidian sources in southern Peru, spaced at an interval of approximately 100-300 km along an arc trending north-west to south-east, is evident from maps of these source distributions. It is volcanic processes at a continental scale that have resulted in emplacement of obsidian at 3000 and 5000 meters above sea level. As will be discussed in more detail in Chapter 4, obsidian sources appear to be regularly spaced paralleling the spine of the Western Cordillera, however geologists believe that "the complex tectonic and lithospheric variables at continental subduction zones preclude the development of any regular pattern of volcano spacing" (Clapperton 1993: 77-80). Nevertheless, volcanoes do occur at major crustal fractures and at fracture intersections.

Figure 3-5. Known south-central Andean obsidian sources used in prehistory with least cost paths (Tobler's function on SRTM DEM data) from the three major sources to sites in the region.

In the prehispanic central and south-central Andes obsidian from three sources dominated the archaeological use of this material, followed by obsidian from a number of minor, local sources that were only used for tool production in the area of geological origin (Figure 3-5). Finally, there were countless sources of geological obsidian that were used negligibly, or not at all, by humans apparently due to the undesirable attributes of the glass. Outside of the study region, additional major Andean obsidian sources have been studied to the south in northern Argentina and surrounding terrain (Yacobaccio, et al. 2004;Yacobaccio, et al. 2002) and to the north in Ecuador (Burger 2003;Burger, et al. 1994).

The three most widely-circulated obsidian sources in Peru and north-west Bolivia are the Quispisisa, Alca, and Chivay types, and these three types will be investigated here in more detail. Burger et al. (2000: 348) argue that it is the large nodule size and the homogeneity of the glass that is the primary determinant of how widely obsidian was circulated in prehistory and by these criteria these three sources are exceptional in the Andes.

In the course of this research a relational GIS database has been assembled using published data on obsidian sourcing in the south-central Andes (Burger, et al. 2000;Craig 2005: 908-916;Frye, et al. 1998;Giesso 2000) and using these data a few summaries have been generated for the purposes of this study. The trends in these data show broad patterns in the use of obsidian by chemical type, but given the early stages of this kind of research, these values also reflect a sampling bias based in the history of research and the collections available for sampling. With further development of sourcing technologies, including the more widespread use of portable XRF units, obsidian from a wider variety of sites will be characterized in coming years.

Type	Samples	Sites	Ave. Distance (Km) *	Ave. Time (Hrs) **	Max. Distance (Km) *	Max. Time (Hrs )**
Chivay	531	48	171.0	40.0	553.6	120.8
Alca	305	50	249.6	66.0	988.7	260.7
Quispisisa	525	63	260.3	66.7	846.7	208.5

Table 3-2. Three major Peruvian obsidian sources showing average and maximum distances and times. * based on straight-line distance measure, ** Least-cost path walking time based on Tobler's(1993) hiking function.

Figure 3-6. Cumulative frequency graph showing three major Peruvian obsidian sources by consumption site altitude.

As will be discussed below, the dominant pattern that has emerged for both the Chivay and the Alca obsidian types is one of highland interaction. Throughout the prehispanic period, artifacts of Chivay obsidian have never been found below an altitude of 1280 masl, and that was in the context of a Tiwanaku colony, at the site of Omo in Moquegua (Burger, et al. 2000: 338;Goldstein 1989). Alca type obsidian artifacts are similarly found mostly at high altitude; the coastal Quebrada Jaguay paleoindian finds are anomalous for Alca distributions. The next lowest altitude context for Alca is also at Middle Horizon Omo. Quispisisa, on the other hand, is commonly found at coastal sites from the Nasca area and northward, beginning with a strong representation at the cotton preceramic site of San Nicolas (Burger and Asaro 1978;Vaughn 2005).

One possible explanation for the lack of Chivay and Alca obsidian in low elevation contexts is that camelid caravans were less common in the low elevation coastal areas of southern Peru and northern Chile. While there is strong evidence of camelid corrals in northern coastal Peru (Shimada 1982), the examples of pastoral sites in the coastal areas of the south-central Andes are rare. Stanish (1992: 57) states that there no sites with corrals below 1,500 masl in the Moquegua drainage, and he presents other evidence for the scarcity of pastoral sites in southern coastal Peru. However, Wheeler et al. (1995) examine the remains of 26 camelids, both llamas and alpacas, at the Chiribaya site of El Yaral at 1000 masl in Moquegua. Moving northward in the Pacific coastal drainages, both Christina Conlee (2000) and Kevin Vaughn (2005) report evidence of domesticated camelids at low altitude in the Nasca area. Vaughn (2005: 92) describes a corral-like structure at the site of Marcaya at 1000 masl. As mentioned, the Nasca zone is rich in Quispisisa obsidian. A variety of factors may have contributed to the pattern of Quispisisa obsidian consumption on the coast and Alca and Chivay consumption in the highlands including the greater breadth of the altiplano in the Titicaca Area, the position of geological source relative to the coast, and the severe aridity in the Atacama area of southern Peru and northern Chile.