The Pierre Shale Formation
Pierre Shale - A view skyward, from the bottom - The ocean layers on the right were laid down earlier, and become progressively younger, moving to the left. The youngest layers are still some 70 million years old.
Pierre Shale - A view skyward, from the bottom - The ocean layers on the right were laid down earlier, and become progressively younger, moving to the left. The youngest layers are still some 70 million years old.