In Eocene time the northern part of the Colorado Plateau sagged downward
and gradually filled with water until it became a huge lake, now known
as Lake Uinta. The waters in it teemed with plants and animals,
particularly micro-organisms such as algae, whose remains, coated with
calcium carbonate, settled to the bottom along with the sand, silt, and
mud washed into the lake by streams. These sediments compacted into the
remarkable Green River Formation which contains, among many rock types,
large deposits of rich oil shale.
The light-colored Green River Formation, which is about 3,800 feet
thick, may be seen from U.S. Highway I-70 in the upper part of the
towering Roan Cliffs on the northwest side of the Colorado River between
DeBeque and Rifle. It also underlies the volcanic caprock of Grand and
Battlement Mesas. John R. Donnell, of the U.S. Geological Survey,
estimated that the oil shale in the Piceance Creek Basin, northwest of
the Colorado River alone, contains more than one trillion barrels of
oil. The Monument was at or near the south shore of this lake, and may
have been covered with a few hundred feet of the Green River Formation.