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).