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.
The oldest era is at the bottom of the list because this part of geologic time transpired first and was then followed by the successively younger eras which are placed above it. Therefore, the geologic time scale is always read from the bottom of the chart upward. This is, of course, the order in which the various portions of geologic time occurred and during which the corresponding rocks were formed.
As mentioned above, each of the eras has been divided into periods, and most of these periods derive their names from the regions in which the rocks of each were first studied. For example, the [Pennsylvanian] rocks of North America were first studied in the State of Pennsylvania.
The [Paleozoic] era has been divided into seven periods of geologic time. With the oldest at the bottom of the list, these periods and the source of their names are:
[Permian] (PUR-me-un)—from the Province of Perm in Russia [Pennsylvanian] (pen-sil-VAIN-yun)—from the State of Pennsylvania [Mississippian] (miss-i-SIP-i-un)—from the Upper Mississippi Valley [Devonian] (de-VO-ni-un)—from Devonshire, England [Silurian] (si-LOO-ri-un)—for the Silures, an ancient tribe of Britain [Ordovician] (or-doe-VISH-un)—for the Ordovices, an ancient tribe of Britain [Cambrian] (KAM-bri-un)—from the Latin word Cambria, meaning Wales
The Carboniferous [period] in Europe includes the [Mississippian] and [Pennsylvanian] periods of North America. Although this classification is no longer used in the United States, the term Carboniferous will be found in many of the earlier geological publications and on many of the earlier geologic maps.
The periods of the [Mesozoic] era and the source of their names are:
[Cretaceous] (cre-TAY-shus)—from the Latin word creta, meaning chalky [Jurassic] (joo-RAS-ik)—from the Jura Mountains of Europe [Triassic] (try-ASS-ik)—from the Latin word triad, meaning three
[In] Texas, the [Cretaceous] has two divisions, known as either Lower Cretaceous and Upper Cretaceous or as Comanche [series] and Gulf series, respectively. These designations are for rocks of nearly equivalent age, and both sets of terms have been used by geologists and in publications. In this handbook, both sets of terms are used interchangeably, that is, Lower Cretaceous and/or Comanche series and Upper Cretaceous and/or Gulf series.
The [Cenozoic] periods derived their names from an old outdated [system] of classification which divided all of the earth’s rocks into four groups. The two divisions listed below are the only names of this system which are still in use:
[Quaternary] (kwah-TUR-nuh-ri) [Tertiary] (TUR-shi-ri)
While the units discussed above are the major divisions of geologic time, the geologist usually works with smaller units of rocks called formations. A geologic [formation] is identified and established on the basis of definite physical and chemical characteristics of the rocks. Formations are usually given geographic names which are combined with the type of [rock] that makes up the bulk of the formation. For example, the Beaumont clay was named from clay deposits that are found in and around Beaumont, Texas.