The earth’s crust is believed to be at least 3¼ billion years old. In
order to deal with this vast stretch of time, geologists have divided
the billions of years into various time units and have given each unit a
name. The great divisions of geologic time, called _eras_, are Early
Precambrian, Late Precambrian, Paleozoic, Mesozoic, and Cenozoic. These
eras are divided into smaller units of time called _periods_, and the
periods are divided into _epochs_. The _[xx time scale]_ shows the
geologic time divisions. Earliest geologic time is shown at the bottom
of the scale; most recent is shown at the top.
By examining and studying the different rocks and rock layers,
geologists try to discover in which unit of geologic time these rocks
formed. Those rocks that formed during a _period_ of geologic time are
called a _system_ of rocks; those that formed during an _epoch_ are
called a _series_. For example, the Cambrian System of rocks formed
during the Cambrian Period; the Cretaceous System of rocks formed during
the Cretaceous Period; the Tertiary System of rocks formed during the
Tertiary Period. We are now in the younger epoch (called Recent) of the
Quaternary Period of the Cenozoic Era. The rocks that are forming now
are the Recent Series of rocks.
[Illustration: Geologic time scale]
ERA
PERIOD
EPOCH
CENOZOIC
QUATERNARY (lasted 0-1 million years)
Recent
Pleistocene
TERTIARY (lasted 62 million years)
Pliocene
Miocene
Oligocene
Eocene
Paleocene
—63 million years ago—
MESOZOIC
CRETACEOUS (lasted 72 million years)
JURASSIC (lasted 46 million years)
TRIASSIC (lasted 49 million years)
—230 million years ago—
PALEOZOIC
PERMIAN (lasted 50 million years)
PENNSYLVANIAN (lasted 30 million years)
MISSISSIPPIAN (lasted 35 million years)
DEVONIAN (lasted 60 million years)
SILURIAN (lasted 20 million years)
ORDOVICIAN (lasted 75 million years)
CAMBRIAN (lasted 100? million years)
—600? million years ago—
LATE PRECAMBRIAN
EARLY PRECAMBRIAN
These time estimates are from the paper, Geologic Time Scale, by J.
Lawrence Kulp, published in Science, Vol. 133, No. 3459, April 14, 1961.
(The time divisions are not drawn to scale)
[Illustration: Plate 10. GENERALIZED GEOLOGIC MAP OF TEXAS
Modified from Geologic Map of Texas, 1933]
EXPLANATION
CENOZOIC
1 Quaternary
2 Tertiary (Oligocene, Miocene, and Pliocene)
3 Tertiary (Eocene)
4 Volcanic (extrusive) igneous rocks
MESOZOIC
5 Upper Cretaceous (Gulf series)
6 Lower Cretaceous (Comanche series)
7 Jurassic
8 Triassic
PALEOZOIC
9 Permian
10 Mississippian and Pennsylvanian
11 Cambrian, Ordovician, Silurian, Devonian and undivided
Paleozoic
12 Rocks (Precambrian) older than Paleozoic
13 Intrusive igneous rocks (Precambrian, Mesozoic or Cenozoic)
These rocks are found either at the surface or directly beneath the
soils and subsoils which cover most of Texas.
Geologists also subdivide rocks into lesser units. One of these, called
a _group_, is made up of two or more _formations_. A _formation_
comprises rocks or strata (layers of rock) that are recognized and
mapped as a unit. Some formations consist of layers of one particular
type of rock, such as limestone or shale. Formations are named after a
nearby geographic locality, and in some formation names, the type of
rock is included. For example, three of the Texas geologic formations
are called Buda Limestone, Del Rio Clay, and Eagle Ford Shale.