The beginning of the Cenozoic Era 65 million years ago—give or take a
few million years—marked the beginning of a long span of geologic time
during which mammals became the ruling land animals. Remains of some
small primitive mammals have been found in Mesozoic rocks (p. 50), but
these tiny newcomers did not have a chance to flourish until the
formidable dinosaurs died out.
The Cenozoic Era is divided into the long Tertiary Period—The Age of
Mammals—and the short (about 2 million years) Quaternary Period—The Age
of Man. The Tertiary in turn is divided into five epochs—the Paleocene,
Eocene, Oligocene, Miocene, and Pliocene (fig. 61). Events during parts
of the Tertiary Period had an important bearing upon the Monument even
though no rocks of this period now occur in the area.
[Illustration: COMMON TYPES OF ROCK FOLDS. Top, anticline, or
upfold; closed anticlines are called domes. Middle, syncline or
downfold; closed synclines are called structural basins. Bottom,
monocline, a common type on the Plateau in which the dip of the beds
changes in amount but not in direction; axes may be mapped along
trends of upper fold, middle flexure, or lower fold. Top and middle
diagrams from Hansen (1969, p. 31, 108). (Fig. 27)]