Culture and Ecology of Chaco Canyon and the San Juan Basin

Environment and Natural Resources 47ago as Antevs (1955) suggested. Instead of a gradualchange from one climatic regime to another (e.g., the1,000 years proposed by Antevs [1948]), the changewas much more rapid, perhaps within 200 years.Differences in vegetation at both high and lowelevations did not correspond with today's patterns;the rate of changes in vegetation between the twoelevations were not identical-one could change morethan another during the same period.Hall (1988) later examined the pollen content ofpack rat middens studied by Betancourt and VanDevender (1980) from Atlatl Cave and elsewhere inthe canyon. He re-evaluated these data as anindependent check of pollen and plant macrofossil dataand expanded his interpretations of the vegetation andenvironment of Chaco Canyon and the San Juan Basin(Hall 1988; 1990). He concluded that the alluvialrecord presented earlier (Hall 1977) was correct.Figure 2.6 illustrates the stratigraphy and processes asthey relate to the archaeology, climate, and vegetationof the area. A summary follows.Throughout the Holocene (since 10,600 yearsago), Chaco has been a shrub grassland. Althoughgrasses dominate the alluvial floor, today's vegetationincludes saltbush (Atriplex) and greasewood (Sarcobatusvermiculatum). In the uplands, sagebrush(Artemisia) and Mormon tea (Ephedra) are thepredominant shrubs. Single junipers are present onthe slopes. On Chacra Mesa, pinon is more prevalenton the eastern end and juniper on the western end. Inthe mountains that surround the San Juan Basin arepine forests (P. Ponderosa), oak (Quercus), Douglasfir(Pseudotsuga) , spruce (Picae), and fir (Abies).Because the sediment samples were dominated bychenopods and pine in different amounts over theQuaternary period, Hall thought that the modem andprehistoric environments were much the same; theyjust varied in proportions at different times.During the late Pleistocene, when the sedimentsexhibit a thick cross-bedded Fajada gravel, a ponderosapine forest was probably present on Chacra Mesa;pinons probably grew in lower elevations of the Chacodrainage. Today, these areas contain only grasses andoccasional shrubs. Hall (1988:589) suggested thattemperatures during the Late Pleistocence-EarlyHolocene were slightly lower than today, and thatprecipitation was possibly slightly higher. The exactnature of the climate during the soil formationprocesses represented by the red paleosol is unknown.Prior to 6,700 B.P., erosion and re-entrenchment ofthe canyon floor had occurred.Between approximately 5,800 and 2,400 B.P.(Hall's Middle Holocence period) was the period ofgreatest aridity. At the onset of this arid period, lakesbegan to dry up. Pinon woodlands and ponderosaforests rapidly retreated to higher elevations and werediminished in size. There was deep trenching ofalluvial valleys. Hall (1988:589) admitted that therecord for Chaco is incomplete and needs much morestudy.Around 2,400 B.P., a period of erosion began;it ended by approximately 2,200 B.P. At this timesediments began to accumulate in the canyon andrunoff spread over the flat valley floor. The climatewas warmer and slightly drier than at present. Hall(1990:326) suggested that the Chaco Unit alluvium,deposited between 2,200 and 1,000 B.P., is almostentirely upstream in origin. About 1,000 years ago,there was a change to moister conditions, such as existtoday. Not until ca. A.D. 1100 was there an increasein rainfall and runoff sufficient to erode the post­Bonito channel. Snail remains analyzed by Hall(1980) indicate that there were pools of shallow waterthat supported Stagnicola cockerelli in the main valleyfill from Basketmaker through Pueblo occupations.He proposed that this aquatic snail was eliminatedwhen its habitat was destroyed-when the post-Bonitochannel eroded. Hall (1980:61) suggested that betweenA.D. 600 and 1200, there was no deep arroyochannel like the one present today and that runofffrom storms spread across the valley floor.Sometime between A.D. 1300 and 1400, thepost-Bonito channel began to fill. During this periodthe pinon woodlands again expanded, reaching theirpresent abundance and range. Hall thought the climatewas similar to that of today. He also thought theponderosa forest in the mountains nearby may havereached its present abundance only within the past 100years. The modem arroyo began to erode in 1860;erosion was halted after 1935 when measures weretaken to stabilize the wash to protect the archaeologicalsites (Hall 1977:1617).