Texas has a great variety of rocks and minerals—some are common and others are not. This book is designed to acquaint you with some of them and to tell you in a nontechnical way what they are like, some of the places where they are found, and how they are used. Although we do not know exactly how all of the rocks and minerals formed, some of the ideas about their origin are mentioned.
If you would like to learn more about rocks and minerals in general, the names of several reference books are listed on page 100. In addition, scientific reports that describe in detail many of the rocks and minerals of Texas have been published by the Bureau of Economic Geology of The University of Texas, the United States Geological Survey, and other organizations. A selected list of these reports is given on pages 100-101.
Rocks and minerals are familiar objects to all of us. We pick up attractive or unusual pebbles for our collections, we admire rocky mountain peaks, we speak of the mineral resources of our State and Nation. Rocks and minerals enter, either directly or indirectly, into our daily living. From them come the soils in which grow the grains, the fruits, and the vegetables for our food, the trees for our lumber, and the flowers for our pleasure. The iron, copper, lead, gold, silver, and manganese, the sulfur and salt, the clays and building stones, and the other metals and nonmetals that we require for our way of living were once a part of the earth’s crust.
Earth’s Outer Crust
Rocks and minerals make up most of the outer layer or crust of our earth—the actual ground beneath our feet. The crust is approximately 18 to 30 miles thick beneath the continents. In general, the outermost part consists of many layers of stratified rocks, one above another. The older rocks normally make up the bottom or the deeper layers, and the younger rocks form the upper layers. Not all the layers are perfectly flat and parallel—some are lenticular (lens-shaped), some are tilted, some are partly eroded away, and some are present in one place and absent in another. Beneath the continents, the layers of rock rest on ancient metamorphic rocks and on great masses of igneous rock such as granite. These lower rocks are known as the basement.
Earth’s outer crust (thickness not drawn to scale).
Over much of the land surface of the earth, the outermost layer is made up of layers of rock
On the continents, the layers of rock rest on metamorphic rocks and on igneous rocks such as granite
Geologists
Those who study the earth’s crust—its origin, history, rocks, minerals, fossils, and structure—are known as geologists. The geologists who are especially interested in a particular phase of geology, as this science is called, are given special names: those who study fossils are called paleontologists; those who study minerals are called mineralogists; those who study rocks are called petrologists.
Time and Rock Units
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.
- 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)
Plate 10. GENERALIZED GEOLOGIC MAP OF TEXAS
Modified from Geologic Map of Texas, 1933
This map in a higher resolution
- 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.
Geologic Map
The geologic map (pp. 4-5) shows the rocks that are found at the surface in Texas. Some of these are extremely old. Some, geologically speaking, are very young.
