The Lower Colorado River Valley: A Pleistocene Desert

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Abstract
A chronological sequence of plant macrofossil assemblages from twenty-five pack rat middens provides a record of desert scrub vegetation for most of the last 13,380 yr B.P. from a hyperarid portion of the lower Colorado River Valley. At the end of the late Wisconsin, and probably during much of the Quaternary, the Picacho Peak area, Imperial County, California, supported a typical Mohave Desert association ofLarrea divaricata(creosote bush),Coleogyne ramosissima(blackbrush),Yucca brevifolia(Joshua tree), andY. whipplei(Whipple yucca). Recent arrivals of Sonoran Desert plants such asOlneya tesota(ironwood) andFouquieria splendens(ocotillo) suggest that the area supported relatively modern Sonoran desert scrub species for relatively short periods during interglaciations.