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. . the lower portion of the Moenkopi redbed formation (Baars 1987b). The fossil-rich Kaibab limestone forms the caprock of the Grand Canyon and is a prominent erosional surface throughout northern Arizona. It was deposited in a sea lying west and south of the highlands and embayments accumulating terrestrial sediments. Uranium-rich deposits have been recently discovered in breccia pipes in the Kaibab limestone in the Grand Canyon region. The Mesozoic era. Mesozoic age rocks are poorly represented in the Grand Canyon area, with only a few exposures of Moenkopi redbeds remaining along the rim; however, Mesozoic strata are well preserved in the Canyonlands area, the San Juan Basin and lake Powell (Fig. 7). There, Mesozoic strata reach 1,000 to 1,500 m in thickness and are represented by the following strata in ascending order (Molenar 1987): . (1) the Moenkopi formation, an Early to Middle Triassic silty sandstone; (2) the freshwater fluvial/lacustrine Chinle formation, with the mid-section Petrified Forest member; (3) the Glen Canyon group, with the thick eolian Wingate formation, the fluvial Kayenta formation, and the eolian, Triassic-Jurassic Navajo sandstone which was deposited in a desert environment far from the coastline; the San Rafael group, with the Page sandstone, Carmel formation, Entrada formation, and Curtis-Summerville formations; (4) . . . . . (5) the Morrison formation, well known for its dinosaur fossils; (6) the early Cretaceous Cedar Mountain and Burro Canyon formations; . (7) the late Cretaceous, coastal plain Dakota formation; (8) the Mancos shale was deposited in a shallow marine environment and reaches nearly 1,100 m thickness under the Book Cliffs; and (9) the sandstone, shale and coal-bearing Mesaverde group, which reached more than 700 m in thickness. As the break-up of Pangaea proceeded durin~ Triassic times, the Colorado Plateau became a broad lowland reglon. Streams and adjoining floodplains deposited and dissected redbeds of mud and . . 5 .