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. . regions; however, a similar claim for increased xerophytic adaptation can be made for the Colorado Plateau, where the terrain has been dominated by vast exposures of bedrock during much of the Cenozoic era. . The developing topographic diversity of the Colorado Plateau during Cenozoic time also influenced vertebrate evolution, for many of the same reasons cited above for plants. For example, the boreal group of the genus Bufo, including B. woodhousei, B. cognatus, and B. punctatus, are xeric adapted and have been evolutionarily isolated from other Bufo for a considerable time (Blair 1972). Quaternary Events Pleistocene volcanism in the Grand Canyon. Significant geologic events occurred in the Grand Canyon region during the latter half of the Pleistocene. Volcanism in the western Grand Canyon beginning 1.2 m.y. ago repeatedly dammed the Colorado River between river miles 178 and 189 (Hamblin 1989). The largest of these dams inundated much of the Grand Canyon and creating at least one lake more than 300 km long and 700 m deep. Evidence of these lakes include several anomalous geological features: (1) Pleistocene gravel and travertine terraces in Nankoweap, lava- Chuar, and Havasu canyons; (2) massive slope failure events at river kilometers (RK) 83 and 215, and other locations where the soft, Cambrian Bright Angel Shale was exposed at river level (Rogers and Pyles 1980); (3) fine, water-laid silt deposits more than 350 m above the present river channel in lower Marble Canyon; and (4) river "anticlines" between RK 225 and 260 which consist of folding in the Cambrian Muav limestone about an axis parallel to the sinuous Colorado River, superimposed kink bands and discontinuous thrust faults, and attributed to gravity-driven slippage (axial loading) on saturated Cambrian shale (Collier 1978). Massive travertine deposits at RK's 91-97 and 185-188 may be attributable to higher ambient moisture levels 250,000 to 600,000 years ago or possibly to saturation of the aquifer (Devonian and Mississippian limestones) by lake waters. Aquatic and riparian life along the Colorado River at the time of this volcanic activity would have been eliminated. Virtually all Canyon life would have been destroyed in the lava and ash flows produced by volcanic eruptions, and the river undoubtedly boiled off as lava poured into it. Impoundment would have eliminated all riparian vegetation above the dam sites for several to many centuries, and the river's flow below the lava dams would have been greatly reduced or eliminated for up to several decades after the larger events. Aquatic and terrestrial . . . . . . . 11 .