Rock Layers of the Monument

ROCK LAYERS OF THE MONUMENT The rocks of Colorado National Monument record a fascinating story of mountain building, enormous amounts of erosion, and changing climates, as the continent of North America gradually moved northward toward its present position. PRECAMBRIAN The dark-colored rock at the bottom of the with the overlying red sedimentary rocks. canyons is Precambrian in age, dated at The record of about 1.5 billion years of 1. 7 billion years old. These rocks were earth's history is missing! We know from originally sedimentary rocks, but were surrounding areas that this region was changed into metamorphic rocks and partly uplifted into a major mountain range which, melted into igneous rocks when the area that after hundreds of millions of years, was is now Colorado collided with ancient finally eroded low enough that sediments North America and became part of the could be deposited where the mountains continent. There is a huge gap in the once stood. geologic record at the contact of these rocks The lowest and oldest layer of sedimentary deposits, the Chinle Formation records a TRIASSIC rock is the Chinle Formation. Comprised time when this area was close to the equator. chiefly of red stream and floodplain JURASSIC As the continent slowly drifted northward, After Entrada time, a succession of lake and the climate changed and desert conditions stream deposits formed, beginning with the prevailed. The towering cliffs of the wind Wanakah Formation and followed by the deposited (eolian) Wingate Sandstone Morrison Formation. preserve the remnants of sand dunes Here at Colorado National Monument, the formed in that desert. lower part of the Morrison, called the After the Wingate was deposited, rainfall Tidwell Member, was formed as a delta became more abundant and shallow streams built out into a shallow lake. As the delta flowed across the area, depositing the extended further and further into the lake, Kayenta Formation. The irregular, wavy the main stream channels, represented by contact between the Kayenta and the the Salt Wash Member, were able to extend overlying Entrada Sandstone represents across the area. another gap in the geologic record and is Stream and floodplain deposits and layers of all that we have to tell us of a time when volcanic ash that spewed out of volcanoes thousands of feet of wind-blown sand and west of here comprise the uppermost part of other sediments were being deposited west the Morrison, the Brushy Basin Member. of here, in Utah. Dinosaurs were abundant in the area while The Entrada Sandstone was also deposited the Morrison sediments were being by the wind but the climate was not as arid deposited and their bones have been as before. It preserves sand dunes that found at several locations just outside of migrated inland from the shores of an inland the monument. Undoubtedly they were sea located in central Utah at that time. present here as well. CREATACEOUS The youngest rock unit that occurs in the Muds eroding from mountains to the west monument, the Burro Canyon Formation, accumulated on the sea floor, forming the is found only on Black Ridge. It too consists massive deposit that we call the Mancos of stream and floodplain deposits and can Shale. The Mancos Shale is over 4,000 feet be identified by the green shale that occurs (1219 m) thick in this area. It extends across within it. Petrified wood and dinosaur bones the Grand Valley from the Colorado River to are found in this group of rocks also. the Book Cliffs. Our story would be incomplete if we failed Those thousands of feet of Mancos Shale, to mention the next two rock layers. They plus even more rocks that are on top of the do not now occur within the monument, Mancos, once covered the area of Colorado although they certainly did in the past. National Monument-but another episode The Dakota Formation occurs on the very of mountain building elevated this area once top of Black Ridge and along the south again and started a new cycle of erosion. The bank of the Colorado River. It preserves relentless forces of erosion have stripped off sediments deposited on a coastal plain, in those thousands of feet of sediment and lagoons, and on beaches as a great inland have carved our magnificent canyons, sea, extending from the Gulf of Mexico to exposing for us this wonderful story of the Arctic Ocean, invaded the interior of earth's history. North America. Mancoi: Weak, gray, calcareous marine shale. ome Vl (D sandy zones. Underlies most of the :J Q S!hale Grand Valley (4500') 0 Q.._~~~~~-'c=c'"""''"""'°'='~;;;_~~~~~~~~~...;...;'-----'~~~~~--t 5 ::::> Dakota Sandstone, conglomerate & carbonaceo us O shale with occasional thin coals (l 03') ill Sandstone, conglomerate & green u~ Butto Canyon mudstone (100') Red, green, purple and gray mudstone and bentonites with occasional sandstone lenses. Prone to landslides (305') Thick channel sandstones with minor red to green floodplain mudstone (93') Wingate Eolian sandstone with S!:mdJ:tone large, sweeping cross beds (329') Chin le conglomerate and thin limestones (88') .~ £ E Black Canyon Gp. uD s N Colorado National Monument ~~~~~-------------------- ,n~ . B~ Car~!.£'!!:..._______________ --.... , - --, " ....... '- ....... " "' \ ------- ~- --------- - \ \ \ \ \ \ \ \ \ \ I \ I I I I I i I I Grand Valley I I GENERALIZED CROSS - SECTION · • • • . • Enlrada Sandstone '*fP~ f(ayenl~ Fm . f)~: :: " EXPERIENCE YOUR AMERICA .

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