The Mancos Formation - Cretaceous
95-80 million years ago...
The Mancos Shale formation started forming approximately 95 million years ago and ceased formation about 80 million years ago. It is named after a town in western Colorado where it was first discovered. It is fine grain in texture, often with a grayish tinge to it. Formation occurred because of an intercontinental sea. As the levels rose and fell it left behind fine grain silt. This continued for many millions of years until the sea began to evaporate. The formation of the Mancos ceased and all that was left was fine grain silt that, under serious pressure, eventually turned into shale.