But deciphering earth history is not as simple as it might appear. In many areas the [rock] layers are not always found in the sequence in which they were originally deposited. In places, great structural disturbances have caused some of the rocky “pages” to become shuffled and out of place; others may be missing completely. Many rocks have been destroyed by [weathering] and erosion or greatly altered by [metamorphism]. As a result, the story recorded in these particular rocks is lost forever. These missing “pages” make the ancient story even more difficult to interpret so the geologist must then depend on other evidence that will permit him to “fill in the blanks.”
The record revealed in the [rocks] indicates that our planet is at least 4½ billion years old and that life has been present for more than 3 billion years. During this vast span of time the earth and its inhabitants have undergone many changes.
THE GEOLOGIC COLUMN AND [GEOLOGIC TIME] SCALE
The geologic column refers to the total succession of [rocks], from the oldest to the most recent, that are found in the entire earth or in a given area. For example, the geologic column of Texas includes all rock divisions known to be present in the State. By the same token, the geologic column of Palo Duro Canyon consists of the [geologic formations] exposed there. Thus, by referring to the geologic column previously determined for a specific area, the geologist can determine what type of rock he might expect to find in that particular region.
The [geologic time scale] ([fig. 6]) is composed of named intervals of [geologic time] during which were deposited the [rocks] of the geologic column. These time intervals bear the same names that are used to distinguish the various units of the geologic column. For example, one can speak of [Permian] time (referring to the geologic time scale) or of Permian rocks (referring to rock units of Permian age in the geologic column).
Both the geologic column and the [geologic time scale] are based upon the principle of [superposition]. This basic geologic concept states that unless a series of [sedimentary rock] has been overturned, a given [rock] layer is older than the [strata] above it, and younger than all of the layers below it. Thus, the field relationship of the rocks plus the type of [fossils] (if present) give the geologist some indication of the relative age of the rocks. Relative age does not imply age in years; rather, it fixes age in relation to other events that are recorded in the rocks.
Within recent years, however, it has become possible to assign ages in years to certain [rock] units. This is accomplished by a system of rock dating based on very precise measurements of amounts of radioactive elements (such as uranium). When present in the rocks, radioactive [minerals] change or decay at a known rate so that they are natural “clocks.” This method of dating has made it possible to devise a time scale in years which gives some idea of the tremendous amount of time that has passed since the oldest known rocks were formed. It has also been used to verify the previously determined relative ages of the various rock units.
The largest unit of [geologic time] is an [era], and each era is divided into smaller time units called [periods]. A period of geologic time is divided into epochs, which, in turn, may be subdivided into still smaller units. The [geologic time scale] might be roughly compared to the calendar in which the year is divided into months, months into weeks, and weeks into days. Unlike years, however, geologic time units are arbitrary and of unequal duration, and the geologist cannot be positive about the exact length of time involved in each unit. The time scale does, however, provide a standard by which he can discuss the age of [fossils] and their surrounding [rocks]. By referring to the time scale it may be possible, for instance, to state that a certain event occurred during the [Paleozoic] Era in the same sense that one might say that something happened during the American Revolution.
There are five eras of [geologic time], and each has been given a name that is descriptive of the degree of life development that characterizes that [era]. Hence, [Paleozoic] means “ancient-life” and the era was so named because of the relatively simple and ancient stage of life development.
The eras, a guide to their pronunciation, and the literal translation of each name is shown below.