Recent estimates suggest that the earth was formed at least 4.5 billion years ago. To visualize the length of geologic time and the relations between the stratigraphic and absolute time scales, let us imagine a yardstick as representing the length of time from the origin of the earth to the present (fig. 19). On one side of the yardstick we plot time in years; on the other, we plot the divisions of the stratigraphic time scale according to the most reliable absolute age determinations. [Illustration: Table 1. The stratigraphic time scale.] Era System or period Series or epoch Cenozoic Quaternary Recent Pleistocene Tertiary Pliocene Miocene Oligocene Eocene Paleocene Mesozoic Cretaceous Jurassic Triassic Paleozoic Permian Pennsylvanian Mississippian Devonian Silurian[1] Ordovician Cambrian — Precambrian [1]_The Silurian is the only major subdivision of the stratigraphic time scale not represented in Grand Teton National Park._ We are immediately struck by the fact that all of the subdivisions of the stratigraphic time scale since the beginning of the Paleozoic are compressed into the last 5 inches of our yardstick! All of the other 31 inches represent Precambrian time. We also see that subdivisions of the stratigraphic time scale do not represent equal numbers of years. We use smaller and smaller subdivisions as we approach the present. (Notice the subdivisions of the Tertiary and Quaternary in table 1 that are too small to show even in the enlarged part of figure 19). This is because the record of earth history is more vague and incomplete the farther back in time we go. In effect, we are very nearsighted in our view of time. This “geological myopia” becomes increasingly evident throughout the remainder of this booklet. [Illustration: Figure 19. _The geologic time scale—our yardstick in time._] ABSOLUTE TIME (Years ago) INCHES STRATIGRAPHIC TIME SCALE