In order to interpret geologic history, the earth scientist must attempt to gather evidence of the great changes in climate, geography, and life that took place in the geologic past. The record of these changes can be found in the rocks, and here is found the story of the various events in earth history.
GEOLOGIC COLUMN AND TIME SCALE
In order to discuss fossils and the age of the rocks containing them, it is necessary to become familiar with the geologic column and the geologic time scale ([Pl. 1]).
The geologic column refers to the total succession of rocks, from the oldest to most recent, that are found either locally or in the entire earth. Thus, the geologic column of Texas includes all [rock] divisions known to be present in this State. By referring to the geologic column previously worked out for any given area, the geologist can determine what type of rocks he might expect to find in that particular region.
The geologic time scale is composed of units which represent intervals of geologic time, during which were deposited the rocks represented in the geologic column. These time units are used by the geologist to date the events that have taken place in the geologic past.
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.
[Cenozoic] (SEE-no-zo-ic)—“recent-life” [Mesozoic] (MES-o-zo-ic)—“middle-life” [Paleozoic] (PAY-lee-o-zo-ic)—“ancient-life” [Proterozoic] (PRO-ter-o-zo-ic)—“primitive-life” [Archeozoic] (AR-kee-o-zo-ic)—“beginning-life”
[Archeozoic] and [Proterozoic] rocks are commonly grouped together and referred to as [Precambrian] in age. The Precambrian rocks have been greatly contorted and metamorphosed, and the record of this portion of earth history is most difficult to interpret. Precambrian time represents that portion of geologic time from the beginning of earth history until the deposition of the earliest [fossiliferous] [Cambrian] strata. If the earth is as old as is believed, Precambrian time may represent as much as 85 percent of all geologic time.