[Illustration: DARK ANGEL, a shaft of the Slick Rock Member that is an erosional remnant of a once high, narrow fin. About one-half mile northwest of Double O Arch. Photograph by National Park Service. (Fig. 57)] [Illustration: “INDIAN-HEAD ARCH,” in upper Devils Garden. Arch and most of head are in Slick Rock Member, top of head is basal part of Moab Member. Opening is 4 feet wide and 4½ feet high. Photograph by Professor Dale J. Stevens, Brigham Young University. (Fig. 58)] [Illustration: GEOLOGIC TIME SPIRAL, showing the sequence, names, and ages of the geologic eras, periods, and epochs, and the evolution of plant and animal life on land and in the sea. The primitive animals that evolved in the sea during the vast Precambrian Era left few traces in the rocks because they had not developed hard parts, such as shells, but hard shell or skeletal parts became abundant during and after the Paleozoic Era. (Fig. 59)] GEOLOGIC TIME The Age of the Earth The Earth is very old—4.5 billion years or more according to recent estimates. Most of the evidence for an ancient Earth is contained in the rocks that form the Earth’s crust. The rock layers themselves—like pages in a long and complicated history—record the surface-shaping events of the past, and buried within them are traces of life—the plants and animals that evolved from organic structures that existed perhaps 3 billion years ago. Also contained in rocks once molten are radioactive elements whose isotopes provide Earth scientists with an atomic clock. Within these rocks, “parent” isotopes decay at a predictable rate to form “daughter” isotopes. By determining the relative amounts of parent and daughter isotopes, the age of these rocks can be calculated. Thus, the results of studies of rock layers (stratigraphy), and of fossils (paleontology), coupled with the ages of certain rocks as measured by atomic clocks (geochronology), attest to a very old Earth! Professor Stevens found 14 arches in what he called upper Devils Garden, northwest of Double O Arch, and two arches in the northwesternmost extension of the park known as Eagle Park (fig. 1). One of the unnamed arches in upper Devils Garden is shown in figure 58. I am tentatively calling it “Indian-Head Arch,” because of the rather obvious resemblance. This ends our journey through Arches National Park, but there remains for consideration a summary of the principal geologic events leading to the formation of this beautiful part of the Colorado Plateau and a brief comparison with the geology of other national parks and monuments on the Plateau. Summary of Geologic History