All the layers of sedimentary rocks preserved in the Monument above the
dark Proterozoic ones were deposited by wind and water during the
Mesozoic Era. This long era has been called the age of reptiles, for
reptiles, including dinosaurs (meaning terrible lizards), were then the
dominant land animals. The Mesozoic Era has been divided into three
parts—the Triassic, Jurassic, and Cretaceous Periods. Rocks of each of
these periods crop out in the Monument.