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U B about 1. 8 mill ion years ago. Recent convergence of deep-sea core data and terrestrial evidence suggests that there were some 18 glacial cycles duri ng thi s peri od and tha t each full cycle 1 as ted about 100,000 years (Kukla 1977). However, the onset of glaciation has not been well dated. According to Fairbridge's model (1972), a full glacial cycle begins with a relatively wann interglacial, progresses towards an increasingly cold anag1 ac i a 1 phase, cu1 mi na tes in a eo1 d, ari d p1 en; gl aei a1 phase and becomes somewhat milder and moister in the brief kataglacial phase that precedes the next cycle. Paleobotanical evidence and other paleoclimatic indicators suggest that mean annual temperatures in the continental U.S. were at least 80 C (Brakenridge 1978), and probably more like 10 to 130 C (Gates 1976; t1ears 1981 ) colder than present during 1 ate Wi scons i n glaciation at about 18,000 to 20,000 years ago. While there were no glaciers in the Grand Canyon itself, the Rocky t~ountains were extensively glaciated and runoff and sediMent 1 bad from these gl ac i ers must have si gni ficantly increased the competence and capaci ty of the Colorado River. The San Franci sco Peaks to the south were gl aci atedat 1 east three times duri ng the last 210,000 years, and glaciers extended down to at least 2,257 ,m (3,400 feet). Inactive rock glaciers on Kendrick Peak, also in the San Francisco volcanic field, occur at 2,490 m (8,169 feet) (Barsch and Updike 1971) indicati n9 that the peri gl aci a1 zone extended down at 1 east that far and therefore must have incl uded parts of the North Rim of the Grand Canyon. Brackenridge (1978) estimates that the regional snowline was depressed about 1,000 m (3,280 feet) juring the late Wisconsin. Assuming a modern snowline of about 3,901 m (12,800 feet) on the San Francisco Peaks, the Wisconsin snowline would have been near 2,901 m (9,518 feet). Leopo1 d (1951). however, .sth.ates that the regional snowline in northern New~1exico was lowered to about 2,377 m (7800 feet). Vegetation zones ill the Southwest were al so depressed by an estimated 900 to 1,200 m (2,952 to 3,937 feet) during the late Wisconsin glaciation (Martin and Mehringer 1965). Lowe (1964) estimates that tundra extended down to about 2,137 m (7,010 feet) and the spruce- fi r communi ty extended to 1,579 m (5,181 feet) . On the basi s of analysi s of packrat middens in the Grand Canyon, Col e (1982) ;'-~ J ,~ I k...-....~.' Q lJ ( '~ ; J ;1 \~,,iJ 17