The broad inland basins that were formed late in the Cretaceous Period
received sand, silt, and mud brought in by streams from the uplifted or
folded areas. These materials became compacted into the Wasatch
Formation—the red or pink rock from which Bryce Canyon National Park was
sculptured. One such basin lay just northeast of the Monument. The
Monument probably was covered by some of these stream deposits after the
main basin was partly filled.
The mammals that roamed the area during the Paleocene Epoch were
primitive, but more advanced forms appeared later, in Eocene time. Some
of their fossil remains have been found in the Wasatch Formation in
Plateau Creek Valley north of Grand Mesa and near Rifle, about 60 miles
northeast of Grand Junction. The entire 5,000 feet of the Wasatch may be
seen along U.S. Highway I-70 between the towns of DeBeque and Grand
Valley, and much of it helps support towering Grand and Battlement
Mesas.