As the land gradually subsided nearer to sea level, swamps which were formed along the coast supported considerable vegetation. As the trees and plants died and were covered by silt and mud, they gradually changed to peat which finally became compacted into coal and brown or black coaly shale containing plant remains. You can dig out some of this coal and perhaps find some plant remains near the top of the west canyon wall just below the highest sandstone bed in [figure 24], which is outside the Monument.
For awhile the coast alternately sank slightly below and rose slightly above sea level. Beach sand covered the swamp deposits, then more swamp deposits covered the sand. Some of the sand contains seashells, such as oysters and clams.
Except on Black Ridge, the Dakota has been entirely eroded from the Monument, but it crops out with the underlying Burro Canyon in a series of low hills south of the Colorado River. The Dakota Sandstone is about 200 feet thick.
The Sea Covers the Plateau
Still later in the Cretaceous Period the whole region sank beneath the sea and stayed there a long time. Silt and limy mud were piled layer upon layer on the sea floor and hardened into the gray and black Mancos Shale. Thin layers of sand were cemented into sandstone, and layers of calcium-carbonate mud became chalk or limestone. Seashells and bones of sharks and seagoing reptiles have been found in the Mancos in many places.
The Mancos and all younger rocks have been stripped off the Monument, but they may be seen one after the other as you travel northeastward. Thin remnants of the Mancos cap low hills just south of the Colorado River, and the entire 3,800 feet of the Mancos underlies the Grand Valley and Book Cliffs. The upper part is clearly exposed in the towering, barren Book Cliffs, where the soft shale is protected by a caprock of hard sandstone—the lowermost unit of the overlying Late Cretaceous Mesaverde Group ([fig. 25]).
The Sea’s Final Retreat
Slow uplift of the Plateau, including the Monument region, caused the gradual retreat of the Mancos sea. Deposition of mud on the sea bottom gave way to deposition of beach sand, coal swamps, and then more beach sand and coal swamps. Finally, in Late Cretaceous time, the sea withdrew entirely, never again to return to the Colorado Plateau region.