Culture and Ecology of Chaco Canyon and the San Juan Basin

~~~--~~~~ --30 Chaco Project SynthesisTable 2.1. Five alluvial units identified by Hall. aAlluvial UnitFajada UnitComment:Comment:Gallo UnitChaco UnitDescription- Late Pleistocene.- Found in two main locations: a) gravel pit approximately 0.8 km southwest of visitor center and b) as a terracealong the base of the sandstone cliff 3 to 5 m above alluvial valley of Chaco Wash northeast of its junction with theEscavada Wash.- Composed mainly of cross-bedded sand and gravel, plus shale, quartz, quartzite, chert, jasper, agate petrifiedwood, and dark reddish-brown sandstone.Red paleosol on top of Fajada gravels probably Early Holocene in age.Early Holocene alluvial deposits not documented (Hall 1990:325). Gap in pollen sample record from 9,700 to7,OOOB.P.- 7,000 to 2,400 P.B. (End date estimated)- Up to 4 m thick. Found in Gallo Wash above Fajada Unit and below Post-Bonito alluvium. Similar materialidentified in Mockingbird Canyon Arroyo, south of Chetro Ketl, also 500 m upstream from eastern boundary ofpark.- Composed of un bedded, blocky, yellowish-brown silt; occasional clay beds and lenses of sandy gravel.Approximately 4 m thick. (Some material from small tributaries and reentrants are local alluvium. In axis ofcanyon gray-brown, silty, fine alluvium of upstream origin. )- Haynes (1968:612) noted an absence of radiocarbon-dated deposits between 5,800 to 7,100 B.P. in alluvium ofthe southwest. Hall's data fill in this gap.- 2,200 (estimated) to 1,000 or 850 B. P. (A.D. 1100 or 860, respectively). Both beginning and end datesestimated.- Principal valley fill; in central part of valley extends to unknown depth.- Composed of pale brown clayey silt, largely without regular bedding; upstream origin. Pueblo pottery describedby Bryan (1954:30-32,37) found in two distinct zones represents late and early pottery types.Post-Bonito UnitHistoric Unit- 600 B.P. to A.D. 1860. Dating of the channel-cutting is uncertain. Last building construction at Pueblo Bonitodated ca. A.D. 1100-1130; therefore, erosion thought to have begun around A.D. 1100. Filling of channel is lesscertain.- In channel cut in older sediments, ca I m thick in central part of canyon, especially near Pueblo del Arroyo- Composed of clayey silt and sand.Abraded pottery fragments similar to types recovered at Pueblo Bonito.- Bryan (1941 :231, Table 1) correlated fill in this unit with similar fill found in northeastern Arizona and northcentralNew Mexico prior to A.D. 1400. There is no good evidence with which to date the horizons in this unit.- 1935 to present.- In Chaco Wash arroyo, about 2 m.- Clay, silt, and sand, probably resulting from planting of willow, tamarisk, wild plum and cottonwood for erosioncontrol of Chaco Wash. Institution of checkdams, cable-willow-rock jetties during 1949 and 1950s .• Taken from Hall (1977, 1988, 1990) and supplemented by (D. Love 1980).that arroyo-cutting occurs with a transition from aridto less arid climate (or increased precipitation), thatarroyo-cutting does not occur during a transition to amore arid environment, and that erosion is notcontinuous during a period of aridity.In summary, the intermediate-scale adjustmentsalso can encompass the small -scale changes to an innerchannel that is affected by minor changes inprecipitation. Understanding more recent changes instream development, however, is complicated byconservation projects initiated during the 1930s.Several investigators considered these issues. WhenDeAngelis (1972) rechecked Bryan's 1920s measurementsof several sections of the arroyo, it was aboutthe same depth. In 1924-1925, the flat-floor, braidedgully existed; in 1972, the active channel wasentrenched as much as 3 m (10 ft) below the arroyofloor. Using 1934 photographs, DeAngelis determinedthat the arroyo had widened considerably by