Culture and Ecology of Chaco Canyon and the San Juan Basin

46 Chaco Project Synthesisnel present at 3,700 B.P. and one at 2,400 B.P. Loverecognized that pit structures are present 2 to 4 mbelow the present surface and suggested that thechannel was entrenched at the time of their constructionand use. Judd (1964) documented severalold channels south of Pueblo Bonito between A.D.900 and 1127. Love (1977b:215) cited 29SJ550, a. Pueblo II field house with cored masonry betweenWijiji and Shabik'eshchee at a depth of 2 to 3 m, aspossibly having been constructed on the banks of aburied channel. South of Pueblo del Arroyo is an oldchannel containing walls with late masonry styles thatdate post-A.D. 1050. These data suggest the possibilityof more than one channel. Laminated gray claylayers, which Nichols (1975) dated at A.D. 1250,were found in the stratigraphy near Chetro Ked.Similar layers were found in Senter's (1937) post­A.D. 1050 strata. They suggest flooding of the canyonfloor; there probably was no channel at that time.Thus, the record for the past 1,000 years suggestsseveral cut and fill episodes, not all of which were 12m deep, but they do indicate alternate periods ofentrenchment and filling of the Chaco arroyo, probablyto different heights during different times (Figure2.3).Although the presence of trees on the valleyfloor during the early Pueblo occupation has beendebated, D. Love (1977b:232-234) found no indicationof large trees. Each tree species has specificrequirements. For example, Love (l977b) reportedthat ponderosa pine requires 355.6 cm (14 in) ofrain,but precipitation only averages about 222.5 cm inChaco. Judd (1964:18) documented a small stand ofpines at the head of Chaco Canyon and four dead pinesin Werito's Rincon in the 1920s, plus the remains ofone rotted pine in the West Court of Pueblo Bonito.Windes et al. (2000) recorded several individualponderosas on Chacra Mesa near the Chaco Eastcommunity. Judd (1954:10) cited Jackson (1878),who indicated that willows and cottonwoods werenumerous in 1877; by the 1920s there were stillseveral on the south side of the canyon. These speciesgrow shallow roots, and a decrease in the water tablecould cause their disappearance. Shattuck (personalcommunication, 2003) indicates some seep populusremain even during the recent five-year droughtperiod. The presence of a prehistoric forest isdoubted; but the presence of some species duringtimes when a high water table was present is likely.Palynological ReconstructionHall (1975,1977) accepted Bryan's (1954) geomorphologicalresults, in which the latter indicatedthat two periods of erosion and sedimentation in theChaco arroyo served as a baseline. Hall's initialanalysis of 18 radiocarbon dates (Hall 1977:Table 1)and alluvial pollen samples from four locations in theChaco Wash, two in the Gallo Wash, and sevensurface pollen stations (Hall 1977:Figure 1) provideddata that allowed him to describe five alluvial unitsand their relative dates, as well as shifts in thedominant arboreal vegetation over a period of 7,000years (Hall 1975, 1977, 1983).Hall assumed that the major source of pollen insurface material is upstream. "If the conclusion isaccepted that pollen in alluvium comes from sheeterosion of surface materials and that the surface pollenis a mixture of many years of accumulation, thenfluctuations in climate and in plant communities ofless than many years duration will likely beundetected" (Hall 1975:39-40). Theperiodsmeasuredwould fall within 100 years, at best. D. Love (1980)realized that Hall did not recognize the differentsources of alluvial fill (headwater and local) of thedifferent facies within the fill at the time of his earlypublication. Because it is necessary to understandconditions that affect the formation of each unit(where the material came from and how long it took toform the unit), Love suggested that Hall's sequencewas the result of alternating different kinds ofdeposits, and that it therefore may not be climaticallyaccurate. Later work by Hall recognized this problem(Hall 1990:325).Hall (1977:Figure 13) divided his data into threepollen zones and seven sub zones. These subzonesmay be more specific to Chaco than they are to theregion (Hall 1977:1609). The climatic history in theChaco area fit the broad regional pattern of "coolerand more moist conditions during glacial times, amoderately warm and dry early postglacial period, amid-postglacial interval of greater dryness andwarmth, and a late postglacial reverse in climate tomoderately warm and dry conditions" (Hall 1977:1613). The details for Chaco, however, differed; Hallnoted an abrupt onset of arid conditions around 5,800years ago. The period of greatest aridity fell between5,600 to 2,400 B.P. rather than 7,500 to 4,000 years