Culture and Ecology of Chaco Canyon and the San Juan Basin

Chapter SixThe Classic Adaptation Within Chaco CanyonWhat may have been the purposes of many of their marked peculiarities; what the numbers andcharacteristics of their buildeis; what the relationship, if R..1J.y, bet\veen their in.habit~'lts a..nd the otherfamilies of the great race of early community dwellers, are queries which may be answered in part whenthe investigator shall go back with pick and shovel to uncover the buried rooms, and lay bare that whichhas remained concealed since the death of departure of the ancients. (Bickford 1890:896-897)The Chaco Project staff focused on anexamination of data pertaining to the Classic period inChaco Canyon, including the effects of environmenton culture. Determining the parameters within whichthe people lived and worked was especially relevant tounderstanding this period with the largest population,the great houses and their communities, and the mostspectacular artifacts. This chapter outlines the environmentalparameters within which the Chacoanslived, evaluates the population and its health, andpresents aspects of material culture and practices thatprovide clues to social organization. Other colleaguesstudied astronomy and cosmology to flesh out themodels discussed in Chapter 9.Environmental Parameters AffectingAgricultureBased on research synthesized by Gillespie(1983, 1984b, 1985), the climate of Chaco Canyonduring the Bonito period is assumed to be similar tothat of today. Although average rainfall is approximately22 cm (8 in), it varies considerably from yearto year. Extended periods of above- or below-averagerainfall would have affected crop production; but evenduring wetter periods, dry farming on mesa topswould have been limited. Vegetation would have beensimilar to that of today. Pinon and juniper wereprobably present within the area. Some ponderosapine would have been imported from long distances;and spruce, Douglas-fir, and subalpine fir, whichrequire cooler and wetter climates, would have beenfound only on the perimeters of the San Juan Basin.Because of the low annual rainfall, water wasconsidered the most limiting factor for the agriculturalistswho farmed within Chaco Canyon and itsimmediate surroundings. Regional summer precipitationvalues reconstructed by Robinson and Rose(1979) provided the initial data against whichcomparisons of cultural events could be made. Basedon the Palmer Drought Severity Index (PDSI) calculatedby Rose et al. (1982), the dry interval betweenA.D. 1030 and 1060 and the protracted and severedrought between A.D. 1130 and 1180 are thought tohave affected agricultural production (Dean 1992).Windes (1987[1]:30-37, Figure 2.2) interpretedthe July PDSI values as an indicator that overall theA. D. 900s represent a century of above-average moisture,followed by interspersed dry periods betweenA.D. 1006 and 1029. This period correspondsarchaeologically with the predominance of Red MesaBlack-on-white pottery in the ceramic assemblage.Mild drought conditions from A.D. 1031 to 1050,thought to coincide with the beginning of the Gallupceramic assemblage, were worst in the eleventhcentury. Increased moisture in the A.D. 1060s and1070s was followed by a low from A.D. 1081 to1099. The early twelfth century (A.D. 1100 to 1129,the Late Mix ceramic assemblage) was a wet period,followed by moderate drought conditions from A.D.1130 to 1180-the most severe drought that occurredduring the 250-year Bonito phase (Figure 6.1).When Windes (1993:Figure 2.6) constructed athree-years-running mean for the San Juan Basin, there