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. . silt. The streambed deposits were rich in organic debris and wood, conditions which subject groundwater to a reduction environment. Petrified wood with local replacement by economically important uranium minerals has resulted in these Triassic beds. The late Triassic landscape was covered by extensive eolian sand deposits as the climate became arid and hot. These sandstones comprise the Glen Canyon Group and form spectacular cliffs throughout the southern half of the Colorado Plateau. Jurassic age rocks of the plateau were derived from terrestrial sand deposits to the east and shallow marine environments to the west. One of the best known Jurassic strata is the Morrison Formation, whose stream and lake environments preserved a great diversity of dinosaurs at Dinosaur National Monument. Inland seas and coastal environments dominated the region during the Cretaceous period. Transgressions left the widespread Dakota sandstone, overlain on the Colorado Plateau and to the north by thick deposits of organic-rich shales derived from a seaway that divided the continent. Locally interbedded sandstones indicate brief regressions and contain economically important oil deposits. The close of Cretaceous time was marked by the extinction of dinosaurs and numerous forms of marine life. The Cenozoic era. Tertiary time was marked by the Laramide orogeny which re-deformed the Rocky Mountains east of the Colorado Plateau, and reactivated older fault and clinal systems, such as the Comb Ridge and East Kaibab monoclines and the Monument upwarp. Extensive volcanism and intrusion of igneous rocks occurred abundantly around the margin of the Plateau and locally within it, with limited deposition of economically important ores. Significant volcanic fields in the southern Colorado Plateau include the Lava Falls and San Francisco Peaks fields. During the late Tertiary period regional uplift accompanied by local subsidence and rifting created the Basin and Range Province west and south of the plateau. As Cenozoic groundwaters dissolved away the upper portions of subsurface Paradox basin salt deposits, subsidence or, in some cases, uplift of local areas occurred, causing the replacement of near-surface halite by gypsum and increasing salinity of surface water in the Canyonlands area. Thus, the stage was set for integration of the Colorado River's course, as well as erosion and weathering of the Colorado Plateau landform, during the late Tertiary and Quaternary periods. Evolution and Integration of the Colorado River Despite its orderly sequence of strata, the Colorado Plateau encompasses a complex landscape that shaped and was shaped by the Colorado River system during post-Laramide times. The Colorado River system traverses two contrasting terrains on the Colorado . . . . . . . . 6 .