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.