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.
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)—“earlier-life”
Archeozoic (AR-kee-o-zo-ic)—“beginning-life”
Archeozoic and Proterozoic rocks are commonly grouped together and
referred to as Precambrian in age. In most places 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. Precambrian time probably represents as much as 85 percent of
all geologic time.
The _oldest_ era is at the _bottom_ of the time scale because this part
of geologic time transpired first and was then followed by the
successively younger eras which are placed above it. 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.
[Illustration: Fig. 7. Generalized geologic map of Palo Duro Canyon
State Park.]
EXPLANATION
Q & T Pleistocene and Pliocene undifferentiated
Rdo Dockum Group
P Permian undifferentiated
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 is 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
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)
Although the units named above are the major divisions of geologic time
and of the geologic column, the geologist generally works with smaller
units of the column called _geologic formations_. A geologic formation
is a unit of rock that is recognized by certain physical and chemical
characteristics. A formation is generally given a double name which
indicates both where it is exposed and the type of rock that makes up
the bulk of the formation. For example, the Beaumont Clay is a formation
consisting of clay deposits that are found in and around Beaumont,
Texas. For convenience in study, two or more successive and adjoining
formations may be placed together in a group. Thus, the Tecovas and
Trujillo Formations have been placed in the Dockum Group. Likewise, a
formation may be subdivided into smaller units such as members, which
may also be given geographic or lithologic (rock type) names.