AGE (millions of years) GEOLOGIC AGE NAME OF ROCK FORMATION KIND OF ROCK AND HOW IT IS SCULPTURED BY EROSION THICKNESS (feet) NAMED FOR OCCURRENCE AT OR NEAR 80 Late Cretaceous Mancos Shale Gray and black shale, and thin beds of sandstone and limestone. Contains sea shells. Eroded from Monument, but underlies Grand Valley and forms lower part of Book Cliffs. 3,800 Mancos, Colo. Dakota Sandstone Coaly shale, sandstone, conglomerate, and lignite coal. Contains plant remains. Forms benches and slopes. Caps highest hill in Monument. 150 Dakota, Nebr. 115 Early Cretaceous Burro Canyon Formation Green siltstone and shale, and sandstone and conglomerate. Forms benches and slopes. Crops out on highest hill in Monument. 60 Burro Canyon San Miguel Co., Colo. EROSIONAL UNCONFORMITY 150 Late Jurassic Morrison Formation Brightly colored siltstone and mudstone, and sandstone and limestone. Contains dinosaur bones and fresh-water shells. Forms slopes and badlands. Lower third with sandstone lenses is Salt Wash Member, upper two thirds is Brushy Basin Member. 600 Morrison, Colo. 170 Middle Jurassic Summerville Formation Brightly colored siltstone and mudstone, and thin sandstones. Forms slopes. 54 Summerville Point San Rafael Swell, Utah Entrada Sandstone White and salmon-red sandstone. Upper level-bedded Moab Member forms stair steps, lower mostly cross-bedded Slick Rock Member forms cliffs. 150 Entrada Point Moab, Utah Slick Rock, Colo. 195 Jurassic and Triassic(?) (missing) EROSIONAL UNCONFORMITY 210 Late Triassic(?) Kayenta Formation Red and purple siltstone and shale, and sandstone and conglomerate. Forms bench between two cliffs and mesas between canyons. 45-80 Kayenta, Ariz. Late Triassic Wingate Sandstone Buff and light red sandstone. Cross-bedded and level-bedded. Forms highest cliffs and most of named rock features in Monument. 350 Fort Wingate, New Mex. Chinle Formation Red siltstone and shale, and some limestone conglomerate. Forms steep slopes at foot of cliffs. 80-100 Chinle Valley N.E. Ariz. GREAT UNCONFORMITY 240-1000 Triassic, Paleozoic, Younger Proterozoic (missing) Unnamed Schist, gneiss, granite, and pegmatite dikes. Floors main canyons and forms high bluff above The Redlands. Unknown 1500 Older Proterozoic