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I . preserved at Petrified Forest National Park. Rushforth (1971) described the flora of the Cenomian (Cretaceous) Dakota Formation from the Westwater Canyon area in the Colorado River. He found a diverse flora consisting of 19 species of ferns and angiosperms, including five species of Platanaceae and one Salix species. He concluded that the climate supporting this coastal assemblage was tropical or subtropical, with gymnosperms in nearby highlands. Extensive temperate conifer-deciduous forests of Arcto- Tertiary affinities occurred in the north during late Mesozoic times, while a Neotropical Geoflora occurred to the south on broad plains. Precipitation may have reached 1,000 mm/yr, but increasing decertification during the Cenozoic era favored development of a sub-humid Madro-Tertiary Geoflora. These taxa were closely related to those of the present-day. Miocene rolling terrain (1,000 m elevation) on the Colorado Plateau supported Quercus, chaparral and grasslands, with an overall gradient of forests to the north and shrublands to the south. Aridity of the southwestern United States increased during the Tertiary period, especially with increasing rain shadow effects generated by the Basin and Range orogeny (Axelrod and Raven 1985). Decertification increased as the Sierra Nevada rose nearly 3,000 m above the 1,000 m Miocene floor (West 1980), reaching maximum severity during mid-Pliocene time (Axelrod 1948). Evolution of the Sonoran Desert flora probably began on dry sites during the late Cretaceous, and proceeded gradually (Axelrod 1979). Riparian plant species became increasin~ly restricted to wetland habitats, and some desert riparian taxa (e.g., the Salicaceae) are considered relictual; however, the thesis that riparian taxa are older species than those of surrounding non-riparian habitats has yet to be examined in detail. Evolutionary adaptation of plant taxa to xeric conditions may result from several environmental attributes of rain shadow deserts (Stebbins 1972): (1) topographic, soil, and microclimatic gradients diversify the environment, in effect creating more niches; (2) population size and genetic interchange are reduced; and (3) increased abiotic selection pressure strongly favors morph010gical and physiological adaptations to drought resistance, including architectural, pollination and reproduction strategies, as well as optimization of water retention during photosynthesis (i.e., C4 and CAM photosynthetic pathways). Axelrod (1972) su~gested that tectonism by itself, without rain shadow (topographlc) effects, may drive a flora towards these same xeric adaptations. Tectonic exposure of large shields of bedrock effectively reduces moisture availability to plant life by increasing erosion, preventin~ soil formation, and generally increasing substrate instabillty. Axelrod's (1972) thesis focused on unstable crystalline bedrock in tropical and subtropical . . . . . . . . 10 .