BUREAU OF ECONOMIC GEOLOGY
The University of Texas at Austin
Austin, Texas 78712
John T. Lonsdale, Director

Guidebook 2

TEXAS FOSSILS:
An Amateur Collector’s Handbook

By
William H. Matthews III

November 1960
Second Printing, July 1963
Third Printing, August 1967
Fourth Printing, June 1971
Fifth Printing, November 1973
Sixth Printing, April 1976
Seventh Printing, November 1978
Eighth Printing, September 1981
Ninth Printing, August 1984

Contents

Page [Introduction] 1 [What are fossils?] 3 [The study of fossils] 4 [Paleobotany] 4 [Invertebrate paleontology] 4 [Vertebrate paleontology] 4 [Micropaleontology] 4 [Preservation of fossils] 5 [Requirements of fossilization] 5 [Missing pages in the record] 5 [Different kinds of fossil preservation] 7 [Original soft parts of organisms] 7 [Original hard parts of organisms] 7 [Calcareous remains] 10 [Phosphatic remains] 10 [Siliceous remains] 10 [Chitinous remains] 10 [Altered hard parts of organisms] 10 [Carbonization] 10 [Petrifaction or permineralization] 10 [Replacement or mineralization] 10 [Replacement by calcareous material] 11 [Replacement by siliceous material] 11 [Replacement by iron compounds] 11 [Traces of organisms] 11 [Molds and casts] 11 [Tracks, trails, and burrows] 14 [Coprolites] 14 [Gastroliths] 14 [Pseudofossils] 14 [Dendrites] 14 [Slickensides] 16 [Concretions] 16 [Where and how to collect fossils] 17 [Collecting equipment] 17 [Where to look] 19 [How to collect] 20 [Cleaning and preparation of fossils] 21 [How fossils are named] 21 [The science of classification] 21 [The units of classification] 22 [Identification of fossils] 23 [Use of identification keys] 23 [Identification key to main types of invertebrate fossils] 26 [List of Texas colleges offering geology courses] 27 [Cataloging the collection] 31 [How fossils are used] 31 [Geologic history] 33 [Geologic column and time scale] 33 [The geology of Texas] 34 [Physiography] 35 [Trans-Pecos region] 35 [Texas Plains] 35 [High Plains] 35 [North-central Plains] 37 [Edwards Plateau] 37 [Grand Prairie] 37 [Llano uplift] 37 [Gulf Coastal Plain] 37 [Geology] 37 [Precambrian rocks] 40 [Paleozoic rocks] 40 [Cambrian] 40 [Ordovician] 40 [Silurian] 40 [Devonian] 40 [Mississippian] 41 [Pennsylvanian] 41 [Permian] 41 [Mesozoic rocks] 42 [Triassic] 42 [Jurassic] 42 [Cretaceous] 42 [Cenozoic rocks] 43 [Tertiary] 43 [Quaternary] 43 [Main types of fossils] 44 [Plant fossils] 44 [Classification of the plant kingdom] 44 [Division Thallophyta] 44 [Division Bryophyta] 44 [Division Tracheophyta] 44 [Animal fossils] 48 [Phylum Protozoa] 48 [Class Sarcodina] 48 [Order Foraminifera] 48 [Order Radiolaria] 48 [Phylum Porifera] 49 [Phylum Coelenterata] 49 [Class Anthozoa] 49 [Subclass Zoantharia] 50 [Order Rugosa] 50 [Order Scleractinia] 50 [Order Tabulata] 50 [Phylum Bryozoa] 50 [Phylum Brachiopoda] 54 [Class Inarticulata] 54 [Class Articulata] 56 [Phylum Mollusca] 56 [Class Gastropoda] 59 [Class Pelecypoda] 59 [Class Cephalopoda] 66 [Subclass Nautiloidea] 66 [Subclass Ammonoidea] 75 [Subclass Coleoidea] 78 [Order Belemnoidea] 78 [Phylum Annelida] 78 [Phylum Arthropoda] 78 [Subphylum Trilobitomorpha] 78 [Class Trilobita] 78 [Subphylum Crustacea] 80 [Class Ostracoda] 80 [Phylum Echinodermata] 80 [Subphylum Pelmatozoa] 81 [Class Cystoidea] 81 [Class Blastoidea] 81 [Class Crinoidea] 81 [Subphylum Eleutherozoa] 82 [Class Asterozoa] 82 [Subclass Asteroidea] 82 [Subclass Ophiuroidea] 82 [Class Echinozoa] 82 [Subclass Echinoidea] 82 [Subclass Holothuroidea] 85 [Phylum Chordata] 85 [Subphylum Hemichordata] 85 [Class Graptolithina] 85 [Subphylum Vertebrata] 86 [Superclass Pisces] 87 [Class Agnatha] 87 [Class Placodermi] 87 [Class Chondrichthyes] 87 [Class Osteichthyes] 87 [Superclass Tetrapoda] 89 [Class Amphibia] 89 [Class Reptilia] 89 [Cotylosaurs] 89 [Turtles] 89 [Pelycosaurs] 89 [Therapsids] 89 [Ichthyosaurs] 95 [Mosasaurs] 95 [Plesiosaurs] 95 [Phytosaurs] 95 [Crocodiles and alligators] 95 [Pterosaurs] 95 [Dinosaurs] 95 [Order Saurischia] 97 [Suborder Theropoda] 97 [Suborder Sauropoda] 97 [Order Ornithischia] 97 [Suborder Ornithopoda] 97 [Suborder Stegosauria] 97 [Suborder Ankylosauria] 97 [Suborder Ceratopsia] 100 [Class Aves] 100 [Class Mammalia] 100 [Subclass Allotheria] 100 [Subclass Theria] 100 [Order Edentata] 100 [Order Carnivora] 102 [Order Pantodonta] 102 [Order Dinocerata] 102 [Order Proboscidea] 102 [Order Perissodactyla] 104 [Horses] 104 [Titanotheres] 104 [Chalicotheres] 106 [Rhinoceroses] 106 [Order Artiodactyla] 106 [Entelodonts] 106 [Camels] 106 [Books about fossils] 108 [General works] 108 [Nontechnical and juvenile] 108 [Collecting helps] 108 [Reference works] 109 [Selected references on Texas fossils] 109 [Glossary] 111 [Index] 115

Illustrations

Figures— Page [1. Sketch of a coprolite—fossilized animal excrement] 14 [2. Sketch of a gastrolith—the gizzard stone of an ancient reptile] 14 [3. Dendrites—a typical pseudofossil] 14 [4. Types of symmetry in a fossil coral] 24 [5. Bilateral symmetry in fossil brachiopod] 24 [6. A brachiopod showing specimen number and accompanying label] 31 [7. Two types of micropaleontological slides] 32 [8. Typical Pennsylvanian crinoidal limestone] 41 [9. Typical Texas Foraminifera] 49 [10. Typical radiolarians] 49 [11. Morphology and principal parts of corals] 50 [12. Two types of bryozoans] 50 [13. Morphology and principal parts of articulate brachiopods] 54 [14. Lingula, a typical inarticulate brachiopod] 56 [15. Kingena wacoensis, a common Cretaceous brachiopod] 56 [16. Morphology and principal parts of gastropod shells] 60 [17. Morphology and principal parts of a typical pelecypod shell] 65 [18. Morphology and principal parts of the pearly nautilus] 75 [19. Characteristic features of the various types of cephalopod sutures] 75 [20. Types of typical fossil annelid worms] 78 [21. Morphology and principal parts of trilobites] 80 [22. Two extinct attached echinoderms, Pentremites and Caryocrinites] 81 [23. Typical modern crinoid, or “sea lily,” showing principal parts] 81 [24. Graptolites] 86 [25. Sketches of mastodon and mammoth teeth] 104 [26. Two views of a typical fossil horse tooth] 104

Plates— Page [1. Geologic time scale] Frontispiece [2. Types of fossil preservation] 8 [3. Silicified brachiopods dissolved from Permian limestones of the Glass Mountains, Brewster County, Texas] 12 [4. Dinosaur tracks in limestone in bed of Paluxy Creek near Glen Rose, Somervell County, Texas] 15 [5. Fossil collecting equipment] 18 [6-8. Fossil identification charts] 28-30 [9. Physiographic map of Texas] 36 [10. Geologic map of Texas] 38-39 [11. Geologic range of the major groups of plants and animals] 45 [12. Fossil plants—thallophytes and tracheophytes] 46 [13. Fossil plants—tracheophytes] 47 [14. Paleozoic sponges and sponge spicules] 51 [15. Pennsylvanian corals] 52 [16. Cretaceous and Tertiary corals] 53 [17. Pennsylvanian bryozoans and Cambrian and Mississippian brachiopods] 55 [18, 19. Pennsylvanian brachiopods] 57, 58 [20. Pennsylvanian gastropods] 61 [21. Pennsylvanian and Cretaceous gastropods] 62 [22, 23. Tertiary gastropods] 63, 64 [24. Pennsylvanian pelecypods] 67 [25-28. Cretaceous pelecypods] 68-71 [29-31. Tertiary pelecypods] 72-74 [32. Pennsylvanian and Cretaceous cephalopods] 76 [33. Cretaceous cephalopods] 77 [34. Fossil arthropods] 79 [35. Fossil starfishes, crinoids, and holothurian sclerites] 83 [36. Cretaceous echinoids] 84 [37. Primitive armored fish, shark teeth, and conodonts] 88 [38. Comparison of the dinosaurs] 90 [39. Comparison of Mesozoic flying and swimming reptiles] 91 [40. Pelycosaur, cotylosaur, and a primitive amphibian] 92 [41. Swimming reptiles] 93 [42. Phytosaur and flying dinosaurs] 94 [43. Skull of Phobosuchus, from Cretaceous of Trans-Pecos Texas] 96 [44. Saurischian dinosaurs] 98 [45. Ornithischian dinosaurs] 99 [46, 47. Cenozoic mammals] 101, 103 [48. Tertiary mammals] 105 [49. Cenozoic mammals] 107

Plate 1
GEOLOGIC TIME SCALE

ERA PERIOD EPOCH CHARACTERISTIC LIFE CENOZOIC “Recent Life” QUATERNARY 1 MILLION YEARS Recent Pleistocene TERTIARY 64 MILLION YEARS Pliocene Miocene Oligocene Eocene Paleocene MESOZOIC “Middle Life” CRETACEOUS 70 MILLION YEARS JURASSIC 45 MILLION YEARS TRIASSIC 50 MILLION YEARS PALEOZOIC “Ancient Life” PERMIAN 55 MILLION YEARS CARBONIFEROUS PENNSYLVANIAN 30 MILLION YEARS MISSISSIPPIAN 35 MILLION YEARS DEVONIAN 55 MILLION YEARS SILURIAN 20 MILLION YEARS ORDOVICIAN 75 MILLION YEARS CAMBRIAN 100 MILLION YEARS PRECAMBRIAN ERAS PROTEROZOIC ERA ARCHEOZOIC ERA APPROXIMATE AGE OF THE EARTH MORE THAN 3 BILLION 300 MILLION YEARS

Texas Fossils
An Amateur Collector’s Handbook

William H. Matthews III[1]

INTRODUCTION

Almost everyone has seen the fossilized remains of prehistoric plants or animals. These might have been the skeleton of a gigantic dinosaur, the petrified trunk of an ancient tree, or the shells of snails or oysters that lived in the great seas that covered Texas millions of years ago.

Each year more and more people are learning that these fossils are more than mere curiosities. Instead, they are realizing that a good collection of fossils provides much information about the early history of our earth, and that [fossil] collecting can be a most enjoyable, fascinating, and rewarding hobby. It is for these people that Texas Fossils was written.

This publication is primarily an amateur collector’s handbook and as such offers many suggestions and aids to those who would pursue the hobby of [fossil] collecting. It tells, for example, what fossils are, where and how to collect them, and how they are used. Suggestions are made as to how the specimens may be identified and catalogued, and there are discussions and illustrations of the main types of plant and animal fossils. Included also is a simplified [geologic map] of Texas and a brief review of the geology of the State.

Texas Fossils is not a comprehensive study of the paleontology of Texas. Rather, it deals primarily with the more common [species] that the average collector is likely to find. These fossils are illustrated in the plates and figures, and these illustrations should be of some help in identifying the specimens in one’s collection. Included for completeness, however, are sketches and descriptions of some of the more rare and unusual fossils, and, for general interest, there are illustrations and descriptions of many of the extinct reptiles and mammals that once inhabited this State.

In addition, a group of selected references has been included for the reader who wishes to know more about earth history and paleontology. Many of these publications provide references of a more technical nature for the more advanced or serious collector, and some of them list excellent collecting localities.

A minimum of technical terminology has been used, but terms not commonly found in dictionaries, or which have not been explained in the text, are defined in the glossary (pp. [111]-114).

Many people have helped in the planning, preparation, and completion of Texas Fossils, and their help is gratefully acknowledged: Dr. Keith Young, The University of Texas; Dr. Harold Beaver, Baylor University; and Professor Jack Boon, Arlington State College, offered helpful suggestions and information on [Cretaceous] fossils; Professors Richmond L. Bronaugh, Baylor University, and Jack T. Hughes, West Texas State College, provided information on [vertebrate] collecting localities; Professor Fred Smith, Texas A&M College, supplied data on [Tertiary] collecting localities and fossils which were used in illustrations; Dr. Saul Aronow and Professor Darrell Davis, Lamar State College of Technology; Dr. Jules DuBar, University of Houston; and Dr. Samuel P. Ellison, The University of Texas, made valuable suggestions which have been incorporated into the manuscript.

Special thanks are due Drs. John T. Lonsdale, L. F. Brown, Jr., and Peter U. Rodda, Bureau of Economic Geology, who critically read the manuscript and contributed greatly to the presentation of the material; Dr. John A. Wilson, The University of Texas, who read the section on [vertebrate] fossils and made invaluable suggestions and criticisms; Miss Josephine Casey, who edited the manuscript; and Mr. J. W. Macon, who prepared the maps and charts.

Thanks are due also to Dr. G. A. Cooper, United States National Museum, who prepared [Plate 3] especially for this publication, and to R. T. Bird and the American Museum of Natural History for photographs used in Plates [4] and [43]. Plates [38] and [39] were provided through the courtesy of Dr. J. W. Dixon, Jr., and the Geology Department of Baylor University. The other photographs were prepared by the writer. To Sarah Louise Wilson, Lamar State College of Technology, the writer gratefully acknowledges her tireless and painstaking efforts in preparing the many fine drawings which make up the balance of the illustrations.

WHAT ARE FOSSILS?

Fossils are the remains or evidence of ancient plants or animals that have been preserved in the rocks of the earth’s crust. Most fossils represent the preservable hard parts of some prehistoric organism that once lived in the area in which the remains were collected.

The word [fossil] is derived from the Latin word fossilis, meaning “dug up,” and for many years any unusual object dug out of the ground was considered to be a “fossil.” For this reason some of the earlier books dealing with fossils include discussions of rocks, minerals, and other inorganic objects.

There is much evidence to indicate that man has been interested in fossils since the very earliest times, and [fossil] shells, bones, and teeth have been found associated with the remains of primitive and prehistoric men. It is quite possible that the owners of these objects believed that they possessed supernatural powers, such as healing properties or the ability to remove curses.

During the earliest periods of recorded history, certain Greek scholars found the remains of fish and sea shells in desert and mountainous regions. These men were greatly puzzled by the occurrence of these objects at such great distances from the sea, and some of them devoted considerable time to an explanation of their presence.

In 450 B.C., Herodotus noticed fossils in the Egyptian desert and correctly concluded that the Mediterranean Sea had once been in that area.

Aristotle in 400 B.C. stated that fossils were organic in origin but that they were embedded in the rocks as a result of mysterious plastic forces at work within the earth. One of his students, Theophrastus (about 350 B.C.), also believed that fossils represented some form of life but thought that they had developed from seeds or eggs that had been planted in the rocks.

Strabo (about 63 B.C. to A.D. 20) was another important Greek scholar who attempted to explain the presence of fossils. He noted the occurrence of marine fossils well above sea level and correctly inferred that the rocks containing them had been subjected to considerable elevation.

During the “Dark Ages” fossils were alternately explained as freaks of nature, the remains of attempts at special creation, and devices of the devil which had been placed in the rocks to lead men astray. These superstitious beliefs and the opposition from religious authorities hindered the study of fossils for hundreds of years.

In approximately the middle of the fifteenth century the true origin of fossils was generally accepted, and they were considered to be the remains of prehistoric organisms which had been preserved in the earth’s crust. With the definite recognition of fossils as organic remains, many of the more primitive theories were discarded for one just as impractical—these remains were considered remnants of the Great Flood as recorded in the Scriptures. The resulting controversy between scientists and theologians lasted for about 300 years.

During the Renaissance several of the early natural scientists concerned themselves with investigations of fossils. Noteworthy among these was Leonardo da Vinci, the famous Italian artist, naturalist, and engineer. Leonardo insisted that the Flood could not be responsible for all fossils nor for their occurrence in the highest mountains. He reaffirmed the belief that fossils were indisputable evidence of ancient life, and that the sea had once covered northern Italy. Leonardo explained that the remains of the animals that had inhabited this ancient body of water were buried in the sediments of the sea floor, and that at some later date in earth history this ocean bottom was elevated well above sea level to form the Italian peninsula.

In the late eighteenth and early nineteenth centuries the study of fossils became firmly established as a science, and since that time fossils have become increasingly important to the geologist.

THE STUDY OF FOSSILS

The study of fossils is called paleontology (Greek palaios, ancient; ontos, a being; logos, word or discourse). Information gathered with the help of paleontology has greatly increased the knowledge of ancient plants and animals and of the world in which they lived.

Fossils represent the remains of such great numbers and various types of organisms that paleontologists have found it helpful to establish four main divisions within their science.

Paleobotany

Paleobotany deals with the study of [fossil] plants and the record of the changes which they have undergone.

Invertebrate Paleontology

This is the study of [fossil] animals without a backbone or spinal column. These include such forms as fossil protozoans (tiny one-celled animals), snails, clams, starfish, and worms, and usually represent the remains of animals that lived in prehistoric seas.

Because invertebrate remains are the most common fossils in Texas, this book is devoted largely to the discussion of invertebrate fossils and their method of collection.

[Vertebrate] Paleontology

The [vertebrate] paleontologist studies the fossils of animals which possessed a backbone or spinal column. The remains of fish, amphibians, reptiles, birds, and mammals are typical vertebrate fossils.

Micropaleontology

Micropaleontology is the study of fossils that are so small that they are best studied under a microscope. These tiny remains are called microfossils and usually represent the shells or fragments of minute plants or animals. Because of their small size, microfossils can be brought out of wells without being damaged by the mechanics of drilling or coring. For this reason microfossils are particularly valuable to the petroleum geologist who uses them to identify [rock] formations thousands of feet below the surface.

PRESERVATION OF FOSSILS

The majority of fossils are found in marine sedimentary rocks. These are rocks that were formed when salt-water sediments, such as limy muds, sands, or shell beds, were compressed and cemented together to form rocks. Only rarely do fossils occur in igneous and metamorphic rocks. The igneous rocks were once hot and molten and had no life in them, and metamorphic rocks have been so greatly changed or distorted that any fossils that were present in the original [rock] have usually been destroyed or so altered as to be of little use to the paleontologist.

But even in the sedimentary rocks only a minute fraction of prehistoric plants and animals have left any record of their existence. This is not difficult to understand in view of the rather rigorous requirements of fossilization.

REQUIREMENTS OF FOSSILIZATION

Although a large number of factors ultimately determine whether an organism will be fossilized, the three basic requirements are:

1. The organism should possess hard parts. These might be shell, bone, teeth, or the woody tissue of plants. However, under very favorable conditions of preservation it is possible for even such fragile material as an insect or a jellyfish to become fossilized.

2. The organic remains must escape immediate destruction after death. If the body parts of an organism are crushed, decayed, or badly weathered, this may result in the alteration or complete destruction of the [fossil] record of that particular organism.

3. Rapid burial in a material capable of retarding decomposition. The type of material burying the remains usually depends upon where the organism lived. The remains of marine animals are common as fossils because they fall to the sea floor after death, and here they are covered by soft muds which will be the shales and limestones of later geologic periods. The finer sediments are less likely to damage the remains, and certain fine-grained [Jurassic] limestones in Germany have faithfully preserved such delicate specimens as birds, insects, and jellyfishes.

Ash falling from nearby volcanoes has been known to cover entire forests, and some of these [fossil] forests have been found with the trees still standing and in an excellent state of preservation.

Quicksand and tar are also commonly responsible for the rapid burial of animals. The tar acts as a trap to capture the beasts and as an antiseptic to retard the decomposition of their hard parts. The Rancho La Brea tar pit at Los Angeles, California, is famous for the large number of [fossil] bones that have been recovered from it. These include such forms as the sabre-tooth cat, giant ground sloths, and other creatures that are now extinct. The remains of certain animals that lived during the Ice Ages have been incorporated into the ice or frozen ground, and some of these frozen remains are famous for their remarkable degree of preservation.

MISSING PAGES IN THE RECORD

Although untold numbers of organisms have lived on the earth in past ages, only a minute fraction of these have left any record of their existence. Even if the basic requirements of fossilization have been fulfilled, there are still other reasons why some fossils may never be found.

For example, large numbers of fossils have been destroyed by erosion or their hard parts have been dissolved by underground waters. Others were entombed in rocks that were later subjected to great physical change, and fossils enclosed in these rocks are usually so damaged as to be unrecognizable.

Then, too, many [fossiliferous] rocks cannot be studied because they are covered by water or great thicknesses of sediments, and still others are situated in places that are geographically inaccessible. These and many other problems confront the paleontologist as he attempts to catalog the plants and animals of the past.

The missing pages in the [fossil] record become more obvious and more numerous in the older rocks of the earth’s crust. This is because the more ancient rocks have had more time to be subjected to physical and chemical change or to be removed by erosion.

DIFFERENT KINDS OF [FOSSIL] PRESERVATION

There are many different ways in which plants and animals may become fossilized. The method of preservation is usually dependent upon (1) the original composition of the organism, (2) where it lived, and (3) the forces that affected it after death.

Most paleontologists recognize four major types of preservation, each being based upon the composition of the remains or the changes which they have undergone.

ORIGINAL SOFT PARTS OF ORGANISMS

This type of [fossil] is formed only under very special conditions of preservation. To be preserved in this manner, the organism must be buried in a medium capable of retarding decomposition of the soft parts. Materials that have been known to produce this type of fossilization are frozen soil or ice, oil-saturated soils, and [amber] (fossil resin). It is also possible for organic remains to become so desiccated that a natural mummy is formed. This usually occurs only in arid or desert regions and when the remains have been protected from predators and scavengers.

Probably the best-known examples of preserved soft parts of [fossil] animals have been discovered in Alaska and Siberia. The frozen tundra of these areas has yielded the remains of large numbers of frozen mammoths—a type of extinct elephant ([Pl. 49]). Many of these huge beasts have been buried for as long as 25,000 years, and their bodies are exposed as the frozen earth begins to thaw. Some of these giant carcasses have been so well preserved that their flesh has been eaten by dogs and their tusks sold by ivory traders. Many museums display the original hair and skin of these elephants, and some have parts of the flesh and muscle preserved in alcohol.

Original soft parts have also been recovered from oil-saturated soils in eastern Poland. These deposits yielded the well-preserved nose-horn, a foreleg, and part of the skin of an extinct rhinoceros.

The natural mummies of ground sloths have been found in caves and volcanic craters in New Mexico and Arizona. The extremely dry desert atmosphere permitted thorough dehydration of the soft parts before decay set in, and specimens with portions of the original skin, hair, tendons, and claws have been discovered.

One of the more interesting and unusual types of fossilization is preservation in [amber]. This type of preservation was made possible when ancient insects were trapped in the sticky gum that exuded from certain coniferous trees. With the passing of time this resin hardened, leaving the insect encased in a tomb of amber, and some insects and spiders have been so well preserved that even fine hairs and muscle tissues may be studied under the microscope.

Although the preservation of original soft parts has produced some interesting and spectacular fossils, this type of fossilization is relatively rare, and the paleontologist must usually work with remains that have been preserved in stone.

ORIGINAL HARD PARTS OF ORGANISMS

Almost all plants and animals possess some type of hard parts which are capable of becoming fossilized. Such hard parts may consist of the shell material of clams, oysters, or snails, the teeth or bones of vertebrates, the exoskeletons of crabs, or the woody tissue of plants. These hard parts are composed of various minerals which are capable of resisting weathering and chemical action, and fossils of this sort are relatively common.

Many of the [fossil] mollusks found in the [Tertiary] and [Cretaceous] rocks of Texas have been preserved in this manner. In some of the specimens the original shell material is so well preserved that the iridescent mother-of-pearl layer of the shell is found virtually intact. This type of preservation is less common, however, in the older rocks of the State.

PLATE 2
Types of [Fossil] Preservation

Figures— 1. Internal mold of a Texas [Cretaceous] [ammonite] (×½). 2. Internal and external molds of gastropods and pelecypods in Cedar Park limestone member of the Walnut clay of Comanchean age (×½). Specimen from quarry near Cedar Park, Williamson County, Texas. 3. Internal mold of a Texas Cretaceous [pelecypod] (×½). 4. [Fossil] worm tubes on mold of a Cretaceous ammonite (×½). 5. Petrified or permineralized mammal bone of [Tertiary] age (×½). 6. Internal mold (steinkern) of a typical Texas Cretaceous [gastropod] (×½). 7. Carbon residue of a Tertiary fish (×¼).

At certain localities in north and central Texas the Woodbine sands of Upper [Cretaceous] age (geologic time scale and geologic map, Pls. [1], [10]) contain large numbers of shark and fish teeth ([Pl. 37]), fish scales and vertebrae. The remains of these vertebrates are unusually well preserved and are prized by both amateur and professional collectors.

[Calcareous] Remains

Hard parts composed of [calcite] (calcium carbonate) are very common among the invertebrates. This is particularly true of the shells of clams, snails, and corals. Many of these shells have been preserved with little or no evidence of physical change ([Pl. 2]).

[Phosphatic] Remains

The bones and teeth of vertebrates and the exoskeletons of many invertebrates contain large amounts of calcium phosphate. Because this compound is particularly weather resistant, many [phosphatic] remains (such as the fish teeth in the Woodbine sands) are found in an excellent state of preservation.

[Siliceous] Remains

Many organisms having skeletal elements composed of [silica] (silicon dioxide) have been preserved with little observable change. The [siliceous] hard parts of many microfossils and certain types of sponges have become fossilized in this manner ([Pl. 14]).

[Chitinous] Remains

Some organisms have an [exoskeleton] (outer body covering) composed of [chitin], a material that is similar to finger nails. The fossilized [chitinous] exoskeletons of arthropods and other organisms are commonly preserved as thin films of carbon because of their chemical composition and method of burial.

ALTERED HARD PARTS OF ORGANISMS

The original hard parts of an organism normally undergo great change after burial. These changes take place in many ways, but the type of alteration is usually determined by the composition of the hard parts and where the organism lived. Some of the more common processes of alteration are discussed below.

[Carbonization]

This process, known also as [distillation] takes place as organic matter slowly decays after burial. During the process of decomposition, the organic matter gradually loses its gases and liquids leaving only a thin film of carbonaceous material ([Pl. 2], fig. 7). This is the same process by which coal is formed, and large numbers of carbonized plant fossils have been found in many coal deposits.

In Texas the carbonized remains of plants, fish, and certain invertebrates have been preserved in this manner, and some of these carbon residues have accurately recorded even the most minute structures of these organisms.

Petrifaction or [Permineralization]

Many fossils have been permineralized or petrified—literally turned to stone. This type of preservation occurs when mineral-bearing ground waters infiltrate porous bone, shell, or plant material. These underground waters deposit their mineral content in the empty spaces of the hard parts making them heavier and more resistant to weathering. Some of the more common minerals deposited in this manner are [calcite], [silica], and various compounds of iron.

[Replacement] or Mineralization

This type of preservation takes place when the original hard parts of organisms are removed after being dissolved by underground water. This is accompanied by almost simultaneous deposition of other substances in the resulting voids. Some replaced fossils will have the original structure destroyed by the replacing minerals. Others, as in the case of certain silicified tree trunks, may be preserved in minute detail.

Although more than 50 minerals have been known to replace original organic structures, the most frequent replacing substances are [calcite], [dolomite] (a calcium magnesium carbonate), [silica], and certain iron compounds.

[Replacement] by [calcareous] material

[Calcareous] [replacement] occurs when the hard parts of an organism are replaced by [calcite], [dolomite], or [aragonite] (a mineral which is composed of calcium carbonate but which is less stable than calcite). The exoskeletons of many corals, echinoderms, brachiopods, and mollusks have been replaced in this manner.

[Replacement] by [siliceous] material

When the original organic hard parts have been replaced by [silica] the [fossil] is said to have undergone [silicification], and this type of [replacement] often produces a very high degree of preservation. This is particularly true of the silicified [Permian] (geologic time scale, [Pl. 1]) fossils from the Glass Mountains in Brewster County. These fossils are embedded in limestone which must be dissolved in vats of acid, and after the enclosing [rock] has been dissolved the residue yields an amazing [variety] of perfectly preserved invertebrate fossils ([Pl. 3]).

Silicified [Cretaceous] fossils have been recovered from the Edwards limestone of central Texas. The silicified [fauna] is restricted to a few scattered localities, each of which may yield many unusually well-preserved fossils.

[Replacement] by iron compounds

Several different iron compounds have been known to replace organic matter. Many Texas limestones contain [fossil] snails and clams which have had their original shell material replaced by iron compounds such as limonite, hematite, marcasite, or [pyrite]. Certain of the [fossiliferous] [Tertiary] sandstones of the Texas Gulf Coast area contain large amounts of [glauconite] which commonly replaced organic material.

In some areas entire faunas have been replaced by iron compounds. Such is the case in the famous “[Pyrite] [Fossil] Zone” of the Pawpaw [formation] (Lower [Cretaceous]) in Tarrant County. The fossils in this part of the formation are very small or “dwarfed” and have been replaced by limonite, hematite, or pyrite. Ammonites, clams, snails, and corals are particularly abundant at this locality.

TRACES OF ORGANISMS

Fossils consist not only of plant and animal remains but of any evidence of their existence. In this type of fossilization there is no direct evidence of the original organism, rather there is some definite indication of the former presence of some ancient plant or animal. Objects of this sort normally furnish considerable information as to the identity or characteristics of the organism responsible for them.

Molds and Casts

Many shells, bones, leaves, and other forms of organic matter are preserved as molds and casts. If a shell had been pressed down into the ocean bottom before the [sediment] had hardened into [rock], it may have left the impression of the exterior of the shell. This impression is known as a mold ([Pl. 2]). If at some later time this mold was filled with another material, this produced a [cast]. This cast will show the original external characteristics of the shell. Such objects are called external molds if they show the external features of the hard parts ([Pl. 2], fig. 2) and internal molds ([Pl. 2], fig. 3) if the nature of the inner parts is shown.

Molds and casts are to be found in almost all of the fossil-bearing rocks of Texas, and they make up a large part of most [fossil] collections. It is particularly common to find fossil clams and snails preserved by this method. This is primarily because their shells are composed of minerals that are relatively easy to dissolve, and the original shell material is often destroyed.

PLATE 3
Silicified Brachiopods

All specimens from [Permian] limestones of the Glass Mountains, Brewster County, Texas

Figures— 1, 2. Avonia sp., ×2. [Ventral] and side view of two pedicle valves showing long slender spines. 3. Avonia sp., ×6. Young specimen showing attachment ring at apex. 4-6. Muirwoodia multistriatus Meek, ×4. Respectively, side and ventral view of pedicle [valve] and [dorsal] view of brachial valve. 7-9. “Marginifera” opima Girty. Respectively, ventral and side view of pedicle valve showing long stout spines (×4) and interior of brachial valve showing muscle scars and brachial ridges (×2). 10-13. Aulosteges tuberculatus R. E. King, ×4. Respectively, side and interior view of brachial valve showing muscle scars; ventral view of pedicle valve showing brush of attachment spines on ears; and ventral view of a young pedicle valve. 14. Avonia sp., ×4. Ventral view of a specimen with long spines. 15, 16. Avonia subhorrida (Meek), ×2. Ventral view of a pedicle valve and dorsal view of a brachial valve showing spines on both. 17. Avonia signata (Girty), ×2. Dorsal view of a large specimen showing hairlike spines on brachial valve. 18-20. Prorichthofenia permiana (Shumard). Respectively, side and [posterior] view of pedicle valve (×4) and interior of dorsal valve (×2) showing anchor spines and interior spines of the brachial valve. 21. Heteralosia hystricula (Girty), ×2. Cluster of individuals attached to a large Marginifera. Photograph courtesy of Dr. G. A. Cooper, U. S. National Museum.

Tracks, Trails, and Burrows

Many animals have left records of their movements over dry land or the sea bottom. Some of these, such as footprints ([Pl. 4]), indicate not only the type of animal that left them but often provide valuable information about the animal’s environment.

Thus, the study of a [series] of dinosaur tracks would not only indicate the size and shape of the foot but also provide some information as to the weight and length of the animal. In addition, the type of [rock] containing the track would help determine the conditions under which the dinosaur lived.

Some of the world’s most famous dinosaur tracks are to be found in the Lower [Cretaceous] limestones in Somervell County, Texas. These footprints, which are about 110,000,000 years old ([Pl. 4]), were discovered in the bed of Paluxy Creek near the town of Glen Rose. Large segments of the [rock] containing these tracks were collected by paleontologists of the American Museum of Natural History in New York City and the Texas Memorial Museum at Austin. Great slabs of limestone were transported to the museums, replaced in their original position, and are now on display as mute evidence of the gigantic size of these tremendous reptiles.

Invertebrates also leave tracks and trails of their activities, and these markings may be seen on the surfaces of many sandstone and limestone deposits. These may be simple tracks, left as the animal moved over the surface, or the burrows of crabs or other burrowing animals. Markings of this sort provide some evidence of the manner of locomotion of these organisms and of the type of environment that they inhabited.

Coprolites

Coprolites are [fossil] dung or body waste ([fig. 1]). These objects can provide valuable information as to the food habits or anatomical structure of the animal that made them.

Fig. 1. Sketch of a [coprolite]—fossilized animal excrement.

Gastroliths

These highly polished well-rounded stones ([fig. 2]) are believed to have been used in the stomachs of reptiles for grinding the food into smaller pieces. Large numbers of these “stomach stones” have been found with the remains of certain types of dinosaurs.

Fig. 2. Sketch of a [gastrolith]—the gizzard stone of an ancient reptile.

PSEUDOFOSSILS

Among the many inorganic objects formed by nature there are some that bear superficial resemblance to plants or animals. Because they are often mistaken for organic remains, these objects have been called pseudofossils, or “false fossils.”

Dendrites

Fig. 3. Dendrites. These thin branching mineral deposits bear a marked resemblance to plants, hence they are called pseudofossils.

Although these closely resemble the remains of ferns or other plant material ([fig. 3]), dendrites are actually thin incrustations of manganese dioxide. They are often found along the bedding planes of [Cretaceous] and [Paleozoic] (geologic time scale, [Pl. 1]) limestones in many parts of Texas.

Plate 4
Dinosaur tracks in limestone in bed of Paluxy Creek near Glen Rose, Somervell County, Texas.
Photograph courtesy of the American Museum of Natural History.
Permission to reproduce by R. T. Bird.

[Slickensides]

These are striations that are produced when [rock] surfaces move past each other while being fractured. [Slickensides] may superficially resemble certain of the [Pennsylvanian] coal plants of Texas.

Since [slickensides] are commonly at an angle to the bedding plane and plant remains lie parallel to the bedding plane, the two are usually easily distinguished.

Concretions

Many shales and sandstones contain hardened masses of minerals and [rock] that are often mistaken for fossils. These masses, called concretions, are usually found weathered out of the surrounding rock and may assume the shape of bones, flowers, vegetables, turtles, etc. Although these concretions do not represent organic remains, it is sometimes possible to find true fossils inside them.

WHERE AND HOW TO COLLECT FOSSILS

In [fossil] collecting, as in most “collecting” hobbies, the key to success lies in knowing where to look, what equipment to use, and the most effective methods of collecting.

COLLECTING EQUIPMENT

[Fossil] collecting is a relatively inexpensive hobby because it requires a minimum of supplies and equipment. However, as in almost any hobby, there are certain basic items of equipment that must be acquired.

Hammer

The hammer is the basic tool in the collector’s kit. Almost any type of hammer is satisfactory, but as collecting experience is gained it may be desirable to get a geologist’s hammer. These hammers, also called mineralogist’s or prospector’s picks, are of two types. One type has a square head on one end and a pick on the other ([Pl. 5]): the other type is similar to a stonemason’s or bricklayer’s hammer and has a chisel end instead of the pointed pick end. The square head of the hammer is useful in breaking or chipping harder rocks, and the chisel or pick end is good for digging, prying, and splitting soft rocks.

Collecting Bag

It will be necessary to have some type of bag in which to carry equipment, fossils, and other supplies. A Boy Scout knapsack, musette bag ([Pl. 5]), hunting bag, or similar canvas or leather bag is suitable.

Chisels

A pair of chisels is useful when fossils must be chipped out of the surrounding [rock]. Two sizes, preferably ½ and 1 inch, will usually suffice. A small sharp punch or awl is effective in removing smaller specimens from the softer rocks.

Wrapping Materials

Some specimens are more fragile than others, and these should be handled with special care. Several sheets of newspaper should always be kept in the collecting bag, and each specimen should be wrapped individually as it is collected. Such precautions taken in the field will usually prevent prized specimens from being broken or otherwise damaged. In addition to newspaper, it is wise to carry a supply of tissue paper in which to wrap more fragile specimens.

Map, Notebook, and Pencil

It is most important to have some method of recording where the fossils were found. It is very easy to forget where the material was collected, and one should never rely on memory. A small pocket-sized notebook is inexpensive and just the right size to carry in the field.

A highway or county map should be used to find the geographic location of each collecting locality. Maps of Texas counties can be obtained from the Texas Highway Department, File D-10, Austin 14, Texas. These maps come in three different sizes, but for most purposes the 18×25-inch sheets, with a scale of ½ inch = 1 mile, will be satisfactory. These are available for all counties and may be purchased at a nominal price.

Magnifying Glass

A magnifying glass or hand lens ([Pl. 5]) is useful for looking at small specimens and will also prove helpful in examining the finer details of larger fossils. A 10-power magnification is satisfactory for most purposes, and several inexpensive models are available.

Paper or Cloth Bags

Small bags are useful in separating specimens from different localities. Heavy-duty hardware bags for large rough material and medium-weight grocery bags for smaller specimens may be used. Locality data may be written directly on the bag or on a label placed inside with the fossils. As an added precaution some collectors do both. The more serious collector may want to use a cloth geological sample bag ([Pl. 5]).

Plate 5
[FOSSIL] COLLECTING EQUIPMENT

GEOLOGIC HAMMER (Chisel end) MAGNIFYING GLASS GEOLOGIC HAMMER (Pick end) COLLECTING BAG SAMPLE SACK

Other Useful Items

The items described above are those that are most needed and constitute the basic equipment of the [fossil] hunter. The serious amateur may wish to include certain additional items which will place his collecting on a more professional basis. Some of these accessory items are:

1. A [topographic map] of the collecting area. These are available for many parts of the State and are published and distributed at nominal cost by the United States Geological Survey, Washington, D. C., and/or Denver, Colorado. The Survey can supply an index sheet showing all such maps available for Texas.

2. A [geologic map] of the collecting area if one is available. The list of publications of the Bureau of Economic Geology should be consulted to see if a geologic report or map of the area has been published. This list may be obtained without charge from the Bureau of Economic Geology, The University of Texas, Austin 12, Texas.

3. The [geologic map] of Texas. Although a geologic map of Texas is included in this publication ([Pl. 10]), the scale is so small that its use is somewhat limited. For more detailed work a larger geologic map in color (scale: 1 inch = 31.56 miles) may be ordered from the Bureau. The sale price is 25 cents.

4. A compass for more accurate location of collecting localities.

5. Adhesive or masking tape. The locality information can be written on the tape and applied directly to the specimen.

6. Paper labels (about 3×5 inches). A properly completed label should be placed inside each bag of material.

WHERE TO LOOK

Knowing where to look for fossils is a very important part of [fossil] collecting. It has already been pointed out that igneous and metamorphic rocks are not likely to be [fossiliferous], but that most fossils are found in marine sedimentary rocks. These sediments were deposited under conditions that were favorable for organisms during life and which facilitated preservation after death. Limestones, limy shales, and certain types of sandstones are typically deposited under such conditions.

One should look particularly for areas where rocks formed from marine sediments lie relatively flat and have not been greatly disturbed by heat, pressure, and other physical or chemical changes. If the rocks appear to have undergone considerable folding and fracturing, there is great likelihood that any fossils that were present have been destroyed or damaged by this action.

Quarries are good places to look but one should be sure to obtain permission before entering. [Rock] exposures in quarries are rather fresh but have undergone some weathering. Quarries have been opened in many of the limestone formations of Texas, and large numbers of fine specimens have been collected in some of these excavations. Certain Lower [Cretaceous] limestones are useful for road metal, building stone, or in the manufacture of portland cement, and extensive quarrying has been undertaken in the Edwards Plateau region of Texas ([Pl. 9]). Bones and petrified wood are frequently found in sand and gravel quarries in many parts of the State.

Particular attention should be given to all railroad and highway cuts as rocks exposed in this way are usually still in their original position and are fairly well weathered. Cuts made by recent construction are usually more productive after they have undergone a [period] of weathering as this helps to separate the fossils from their enclosing rocks.

Gullies, canyons, and stream beds are also good places to examine. These areas are continually subjected to the processes of erosion or stream action, and new material is uncovered year after year.

If there are abandoned coal mines nearby, the dumps of waste [rock] around the mine shafts could be checked. A careful examination of such waste may reveal fine specimens of well-preserved plant fossils.

Coal has been mined in several parts of Texas, and abandoned shafts or dumps are still present in some counties. The bituminous coals of Texas are predominantly [Pennsylvanian] in age, and mining has been carried on in the following counties: Eastland, Erath, Jack, Palo Pinto, Parker, Wise, Young.

HOW TO COLLECT

When a likely collecting spot has been located, the ground should be examined very carefully to see if there are any [rock] fragments which contain pieces of shell or the imprints of leaves or other organisms.

If the fossils have been freed by weathering, they can be easily picked up and placed in the bag. Many times, however, it will be necessary to take the hammer and very carefully remove the surrounding [rock]. Smaller specimens may be more safely freed with the careful use of the proper size chisel by gently tapping the chisel and gradually chipping away the matrix—the rock that is holding the specimen. After most of the matrix has been removed, the [fossil] should be carefully wrapped and placed in the collecting bag.

Before leaving a collecting locality, one should be sure to record its geographic location and the [geologic age] of the [rock] in which the fossils were found. The place should be located on the map and the locality entered in the notebook in such a manner that it could easily be located again for additional collecting. If a county or [topographic map] is available, it is wise to mark the locality on the map. The geographic and geologic data should be written on a label placed in the bag of fossils collected at that particular locality. In addition, many collectors find it helpful to write the locality on the outside of each bag of fossils.

Material from separate localities should be kept in individual cloth or paper bags, and the collector should take every precaution to keep the labels with their respective fossils. Remember that a [fossil] without a locality is hardly worth the paper it is wrapped in.

The collector should always ask the land owner’s permission before entering or collecting on private property. One should respect all property, especially livestock and fences, and leave the area cleaner than when entered. If these precautions are observed, future collectors will probably be welcome to return for additional collecting.

CLEANING AND PREPARATION OF FOSSILS

It is usually necessary to do the final cleaning and preparation of fossils at home or in the laboratory, for most fossils brought in from the field require considerable preparation before they are ready for display.

Excess matrix should be carefully removed with hammer and chisel; blows should always be directed away from the [fossil]. Smaller tools (needles, tweezers, and awls) should be used in the final preparation stage, and one should work carefully to avoid damaging the specimen. Before starting the final cleaning, it will be helpful to place the fossils in water and let them soak overnight. This will loosen much of the excess [rock], and most of the softer material can then be removed with a small scrub brush or tooth brush. Mounted needles can be used to clean more delicate specimens or around the smaller structures of larger fossils. It may be advisable to use the magnifying glass when working with small fossils or with delicate surface structures of larger specimens.

Broken fossils can be repaired with clear plastic household cement, and specimens that are crumbling may be coated with pure white shellac, thinned collodion, or clear nail polish. The latter is preferred as it is not as likely to crack. Fragments of bone are particularly apt to crumble upon exposure to the air. This type of [fossil] is normally quite fragile and should be excavated with great care and shellaced as soon as dry.

Dilute hydrochloric acid may be used in removing silicified fossils from a [calcareous] matrix. The material to be etched should be placed in a pottery or glass container and covered with water. Acid should then be added to the water very slowly and until large numbers of bubbles are given off. Each time the bubbling ceases, more acid should be added and this process should be repeated until the [fossil] is free of matrix. This procedure should be carried on in a well-ventilated place, and the acid should be handled with extreme caution. Hydrochloric acid can cause damage or serious injury and the fumes are extremely corrosive.

HOW FOSSILS ARE NAMED

In order to get the maximum pleasure out of [fossil] collecting, most amateur paleontologists want to identify and classify the fossils that they have collected. This requires some knowledge of how fossils are classified and how they receive their scientific names.

THE SCIENCE OF CLASSIFICATION

The number of organisms, both living and extinct, is so great that some [system] of classification is needed to link them all together. Many fossils bear distinct similarities to plants and animals that are living today, and for this reason paleontological classification is similar to that used to classify modern organisms. This system, known as the system of [binomial nomenclature], was first used consistently in 1758 by Linné (or Linnaeus), an early Swedish naturalist.

Scientific names established in accordance with the principles of [binomial nomenclature] consist of two parts: the generic (or genus) name and the trivial name. These names are commonly derived from Greek or Latin words which are usually descriptive of the organism or [fossil] being named. They may, however, be derived from the names of people or places, and in such instances the names are always Latinized. Greek or Latin is used because they are “dead” languages and not subject to change. They are also “international” languages in that scientists all over the world can use the same names regardless of what language they write in. The [system] of binomial nomenclature has led to the development of the science of [taxonomy], the systematic classification and naming of plants and animals according to their relationships.

THE UNITS OF CLASSIFICATION

The world of organic life has been divided into the plant and animal kingdoms. These kingdoms have been further divided into larger divisions called phyla (from the Greek word phylon, a race). Each [phylum] is composed of organisms with certain characteristics in common. For example, all animals with a spinal cord (or notochord) are assigned to the phylum Chordata.

The [phylum] is reduced to smaller divisions called classes, classes are divided into orders, orders into families, families into genera, and each genus is divided into still smaller units called [species]. A species may be further reduced to subspecies, varieties, or other subspecific categories, but these need not concern us in a publication of this nature.

The following table illustrates the use of [binomial nomenclature] in the classification of man, a clam, and a dog.

The generic name and the [trivial name] constitute the scientific name of a [species] and according to this [system] of classification the scientific name of all living men is Homo sapiens. It is obvious that there are many variations among individual men, but all men have certain general characteristics in common and are therefore placed in the same species.

In a scientific name, the generic name is always started with a capital letter and the [trivial name] with a small letter. Both names must be italicized or underlined.

The name of the author (the person who first described the [fossil]) usually appears following the scientific name. The date of the scientific publication containing the original description of the fossil is often placed after the author. For example:

Turrilites worthensis Adkins and Winton 1920

With the large numbers of plants and animals that are living today, plus those of the past, random naming would result in much confusion. For this reason scientists have established strict rules that must be followed when a specimen is named. The strict application of these rules enables scientists in all parts of the world to assign scientific names without fear of duplication.

IDENTIFICATION OF FOSSILS

The beginning collector is usually content to know if his specimen is a clam or a snail or a fern or a palm leaf. But as the collection grows, it becomes increasingly desirable to know the scientific name of each [fossil].

When he starts to identify fossils it may be helpful to show them to a geology teacher if a college or university is nearby. Most teachers are glad to be of help and will probably have similar specimens in their own collections. As all colleges do not have geology departments, a list of institutions with geologists on their faculties is included at the end of this section of the handbook ([p. 27]). In addition, many of the science teachers in the public schools are familiar with fossils and can give helpful suggestions as to how to classify material.

Museums are also good places from which to get help. If the museum has a geological collection, it will be most helpful to compare specimens with the fossils in their collections and to ask the museum personnel for advice. In addition to the above sources of information, local professional geologists are usually familiar with the geology of the local area and the paleontological literature of the region.

Possibly local librarians can recommend books, encyclopedias, or other publications that will be of help. Members of a local [rock] and mineral club, if one is available, are another source of information. Many times these collectors can pass along good ideas and tell exactly which books to consult.

After books or journals describing the fossils of the area have been located, the collected specimens should be closely compared with any illustrations that are shown. Each [fossil] should be examined carefully, its more characteristic features noted, and it should again be compared with the illustrations and descriptions in the book. The [phylum] or class to which the specimen belongs should be determined first. For example, the genus and [species] of a certain fossil may not be known, but it looks like a snail and accordingly it is named a [gastropod] (for class Gastropoda, the snail class), and this is, at least, a start in determining the scientific name of that particular fossil. The descriptive material in the text of each reference will usually point out the more detailed features which will be diagnostic of the genus or species.

The illustrations and descriptive material in this publication will also be of considerable help in identification. Many illustrations of the more common invertebrate fossils have been included, but the publication was not designed primarily for use in [fossil] identification. Rather, it is intended to guide the amateur or student who is interested in fossil collecting, and to furnish suggestions as to how collecting may be more effectively pursued.

USE OF IDENTIFICATION KEYS

[Fossil] identification keys may be useful in helping the beginning collector identify specimens. The collector compares a fossil with the key description and eliminates those characters that do not fit the specimen.

The key used in this handbook is based primarily on [symmetry]—the orderly arrangement of the parts of an object with reference to lines, planes, or points. The shape of the shell or body, presence or absence of coiling, and presence or absence of body partitions are also useful criteria in identifying fossils. To use the key the beginner should know something about symmetry. Two major types of symmetry are used in this key.

1. [Radial symmetry]—the symmetrical repetition of parts around an axis. This is the [symmetry] of a wheel, and any vertical section through the center of the object divides it into symmetrical halves ([fig. 4]a).

2. [Bilateral] [symmetry]—the symmetrical duplication of parts on each side of a plane ([fig. 5]). The plane divides the object into two halves that are mirror images of each other. This is the symmetry of a plank.

It should be noted that many objects may have both kinds of [symmetry]. For example: A cone when viewed from the top has [radial symmetry] and when viewed from the side shows [bilateral] symmetry ([fig. 4]a, b).

Fig. 4. Types of [symmetry] in a [fossil] [coral]. (a) Radial symmetry. (b) [Bilateral] symmetry.

Fig. 5. [Bilateral] [symmetry] as displayed by a typical [fossil] [brachiopod].

An illustration of the use of the key on pages [26]-27 follows. Assuming that a specimen displays [radial symmetry], this means that it belongs under Part I on the key. If the [fossil] has a tapering, cylindrical, cone-shaped shell (“A” on the key), the subheadings under the “A” part of the key are examined. Should the specimen have a shell which is round, tapering at one end, with [transverse] septa or sutures (number 2 under “A”), it is probably a [cephalopod]. This is indicated on the right hand side of the page. Number 1 under “A” is eliminated because the fossil did not have [longitudinal] radial partitions within the shell.

Some fossils display no apparent [symmetry] and such a [fossil] would be referred to Part III of the key. If this fossil had internal [transverse] partitions “A” would be eliminated. If the fossil was not a coiled fossil “B” would also be eliminated and we would proceed directly to “C”—uncoiled fossils. If the specimen is a branching twig-like fossil, numbers 1, 2, and 3 would be eliminated and the specimen referred to number 4 (Branching twig-like fossils). Should the specimen have evenly distributed relatively large openings with radial [longitudinal] partitions or septa, the specimen is probably a [colonial] [coral] (“b” under number 4 on the key). The “a” part of number 4 would be eliminated because the coral had large openings and radial longitudinal septa.

Once a tentative identification has been made from the key, pictures and descriptions of this [fossil] group are examined to establish a more precise identification. It should be remembered that keys are not perfect, and the collector should not expect to be able to identify every specimen with this key.

IDENTIFICATION KEY TO MAIN TYPES OF INVERTEBRATE FOSSILS

(Instructions on pages [23]-25 for use of key)

I. Fossils displaying radial [symmetry]—symmetrical repetition of parts around a central axis A. [Fossil] tapering, cylindrical, cone-shaped: 1. Fossil with [longitudinal] radial partitions or septa; cone-shaped[Coral] 2. Shell with [transverse] septa or sutures; tapering at one end[Cephalopod] 3. Shell without internal septa or partitions: a. Shell large, heavy; usually with external longitudinal ribs. Occur only in [Cretaceous] rocks[Rudistid] b. Shell small (usually less than 2 inches long), tusk-shaped, open at both ends. Rare in [Paleozoic] and [Mesozoic] rocks[Scaphopod] B. Fossil disk-shaped or flattened dome-shaped: 1. Fossil with radiating star pattern on top[Echinoid] 2. Fossil [subconical] to hemispherical, dome-shaped; base concave or flat; minute pits or pores covering surface; typically small (less than 3 inches across)Bryozoa 3. Fossil small (less than ½ inch); generally disk-shapedForaminifera (orbitoidid) 4. Fossil disk-shaped or button-like; with longitudinal, radial partitions or septaCoral C. Fossil composed of segments or plates: 1. Fossil composed of circular segments, disks, or chambers; when united form cylinder: a. Tapered shellCephalopod b. Non-tapered, segments small and of relatively uniform thickness with hole in center; individual columnals disk-shapedCrinoid stem 2. Fossil composed of many-sided plates: a. Bud-shaped fossil of 13 wedge-shaped plates[Blastoid] b. Cup-shaped fossil of many curved plates surrounded by branching armsCrinoid II. Fossils displaying [bilateral] symmetry—symmetrical duplication of parts on each side of a plane A. Fossil coiled in a single plane: 1. Shell divided by internal transverse partitions or suturesCephalopod 2. Shell without internal partitions or sutures[Gastropod] 3. Shell small; spindle-shaped; resembles wheat grain. Common in [Pennsylvanian] and [Permian] rocksForaminifera (fusulinids) B. Fossil not coiled: 1. Shells or valves similar to clams: a. Plane of symmetry parallel to hinge; [equivalved][Pelecypod] b. Plane of symmetry (almost bilaterally symmetrical) at right angles to hinge line; mostly [inequivalved]; strongly ribbed. “Scallop-like” with “ears.” Rare in Paleozoic rocksPelecypod c. Plane of symmetry at right angles to hinge line; inequivalved; not “scallop-like” and without “ears.” Larger [valve] commonly has an opening in beak. Common in Paleozoic rocks[Brachiopod] 2. Fossil tapering, cylindrical, cone-shaped: a. Fossil with internal longitudinal, radial septa or partitions; cone-shapedCoral b. Shell with internal transverse partitions or sutures; tapering at one endCephalopod c. Shell without internal septa or partitions. (1) Shell large, heavy; usually with external longitudinal ribs. Occur only in Cretaceous rocksRudistid (2) Shell small (usually less than 2 inches), tusk-shaped, open at both ends. Rare in Paleozoic and Mesozoic rocksScaphopod 3. Fossil heart-shaped, domed or flattened; radial star pattern on topEchinoid 4. Fossil segmented: a. Fossil divided into 3 lobes; may be curled up. Not found in Mesozoic or [Cenozoic] rocks[Trilobite] b. Fossil flattened or elongate; resembles shrimp, crab, or crayfishCrustacean III. Fossils displaying no apparent symmetry A. Shell without transverse internal partitions or sutures: 1. Shell coiled like ram’s horn, low spired, opening of shell very large; surface has [concentric] ridges. Shell has two valves; smaller, flattened valve not often found. In Texas found only in Cretaceous rocksPelecypod (Note: Some Paleozoic gastropods, “2,” closely resemble larger valve of these pelecypods) 2. Shell tightly coiled; most have higher spire than “1.” Opening of shell smaller than “1”; shell not as rough as “1” and has only one valveGastropod B. Coiled fossils; coiling not in one plane: 1. Shell with transverse internal partitions or sutures: a. Partitions always smooth; thick shelled; loosely and irregularly coiled, usually in large compact masses of many individual shells. Occur only in Cretaceous rocks[Caprinid] b. Partitions (sutures) usually wrinkled; relatively thin shelled; mostly regularly and tightly coiled; occur as separate individual specimensCephalopod 2. Shell without transverse internal partitions or suturesGastropod 3. Solid spiral ridge around central axis; resembles a corkscrewBryozoa C. Uncoiled fossils: 1. Fossil resembles a narrow saw blade; typically found as thin film of carbon. Not found in Mesozoic or Cenozoic rocks[Graptolite] 2. Fossil irregularly cone-shaped; longitudinal radial partitions or septaCoral 3. Shell resembles a clam or oyster shell but valve or shell not symmetricalPelecypod (mostly oysters) 4. Branching twig-like fossils: a. Fossils covered with minute pores or openingsBryozoa b. Fossils with evenly distributed, relatively large openings with longitudinal radial partitions or septa[Colonial] coral 5. Lace-like fossils; occur as thin sheets or filmsBryozoa 6. Fossils composed of radiating masses of [polygonal] or circular tubes containing radial septaColonial coral 7. Irregular fossils; typically cylindrical with rough surface: a. Fossil has large axial opening and thick wall; usually has external longitudinal ribs. Occurs only in Cretaceous rocksRudistid b. Fossil solid with no large axial opening; surface with small pits or pores (fewer than in Bryozoa). In Texas, occurs most commonly in Pennsylvanian and Permian rocksSponge

LIST OF TEXAS COLLEGES OFFERING GEOLOGY COURSES

A.&M. College of Texas, College Station Amarillo College, Amarillo Arlington State College, Arlington Austin College, Sherman Baylor University, Waco Blinn College, Brenham Corpus Christi, University of, Corpus Christi Del Mar College, Corpus Christi East Texas State College, Commerce Hardin-Simmons University, Abilene Henderson County Junior College, Athens Houston, University of, Houston Howard County Junior College, Big Spring Kilgore College, Kilgore Lamar State College of Technology, Beaumont Lee College, Baytown McMurry College, Abilene Midwestern University, Wichita Falls North Texas State College, Denton Odessa College, Odessa Pan American College, Edinburg Rice University, Houston St. Mary’s University, San Antonio San Angelo College, San Angelo San Antonio College, San Antonio Southern Methodist University, Dallas South Texas College, Houston Southwestern University, Georgetown Stephen F. Austin State College, Nacogdoches Sul Ross State College, Alpine Tarleton State College, Stephenville Texarkana College, Texarkana Texas Christian University, Fort Worth Texas College, Tyler Texas College of Arts and Industries, Kingsville Texas Technological College, Lubbock Texas Western College, El Paso The University of Texas, Austin Trinity University, San Antonio Tyler Junior College, Tyler West Texas State College, Canyon

Plate 6
[Fossil] Identification Chart
I [RADIAL SYMMETRY]

A. Tapering, cylindrical cone-shaped fossils 1. Cone-shaped with [longitudinal] partitions or septa[Coral] 2. Fossils with septa or sutures; tapering at one end[Cephalopod] 3. Shell without internal partitions or sutures a. Shell large heavy, external longitudinal ribs. [Cretaceous] only[Rudistid] b. Shell small, tusk-shaped open at both ends. Rare in [Paleozoic] and [Mesozoic][Scaphopod] B. Disc or dome-shaped fossils 1. Star pattern on top[Echinoid] 2. [Subconical] small pits or pores on topBryozoan 3. Small disc-shaped (less than ½ inch)Orbitoid Foraminifera 4. Disc-shaped or button-like, with longitudinal partitions or septaCoral C. Fossils composed of segments or plates 1. Circular discs or chambers; when united form cylinder a. Tapered shellCephalopod b. Not tapered, segments small of uniform thickness, hole in centerCrinoid Stem 2. [Fossil] composed of many-sided plates a. Bud-shaped, 13 wedge-shaped plates[Blastoid] b. Cup-shaped, many curved plates branching armsCrinoid

Plate 7
[Fossil] Identification Chart
II [BILATERAL] [SYMMETRY]

A. [Fossil] coiled in a single plane 1. Shell divided by internal [transverse] partitions or sutures[Cephalopod] 2. Shell without internal partitions or sutures[Gastropod] 3. Shell small, spindle-shaped; resembles wheat grain. [Pennsylvanian] and [Permian]Foraminifera [fusulinid] B. Fossil not coiled 1. Shells or valves similar to clams a. Plane of [symmetry] parallel to hinge; [equivalved][Pelecypod] b. Plane of symmetry almost at right angles to hinge; strongly ribbed; “Scallop-like” with “ears”, [inequivalved]Pelecypod c. Plane of symmetry at right angles to [hinge-line]; without “ears”, not “Scallop-like”; commonly with opening in beak, inequivalved[Brachiopod] 2. Fossil tapering, cylindrical or cone-shaped a. Cone-shaped, internal [longitudinal] partitions or septa[Coral] b. Tapered, internal transverse partitionsCephalopod c. Shell without internal septa or partitions (1.) Shell large heavy, longitudinal ribs. [Cretaceous] only[Rudistid] (2.) Shell small, tusk-shaped, open at both ends, rare in [Paleozoic] and [Mesozoic] rocks[Scaphopod] 3. Fossil heart-shaped, domed or flattened; star pattern on top[Echinoid] 4. Fossil segmented a. Divided into 3 lobes, may be curled up. Paleozoic only[Trilobite] b. Flattened or elongate, resembles shrimpCrustacean

Plate 8
[Fossil] Identification Chart
III NO APPARENT [SYMMETRY]

A. Shell without [transverse] partitions or sutures 1. Shell coiled like ram’s horn, low spired; shell has two valves, smaller flattened [valve] often missing. In Texas exclusively [Cretaceous][Pelecypod] 2. Shell tightly coiled, most have higher spire than 1, shell smaller and not as rough as 1, has only one valve[Gastropod] B. Coiled fossils, coiling not in one plane 1. Shell with transverse internal partitions or sutures a. Partitions always smooth, thick shelled, loosely and irregularly coiled, in Texas exclusively Cretaceous[Caprinid] b. Partitions (sutures) generally wrinkled, regularly and tightly coiled[Cephalopod] 2. Shell without transverse internal partitions or suturesGastropod 3. Solid spiral ridge around central axis, resembles corkscrewBryozoan C. Uncoiled fossils 1. [Fossil] resembles narrow saw blade. [Paleozoic] only[Graptolite] 2. Fossil irregularly cone-shaped, [longitudinal] partitions or septa[Coral] 3. Shell resembles clam or oyster, nonsymmetricalPelecypod (mostly oysters) 4. Branching twig-like fossils a. Covered with minute pores or openingsBryozoa b. With evenly distributed larger openings with septa[Colonial] coral 5. Lace-like fossils, occur as thin sheets or filmsBryozoa 6. Masses of circular or [polygonal] tubes with septaColonial coral 7. Irregular fossils, cylindrical with rough surface a. Large axial opening with thick wall, external longitudinal ribs. Cretaceous only[Rudistid] b. Solid, no opening, small pits or pores. [Pennsylvanian] or [Permian]Sponge

CATALOGING THE COLLECTION

After the fossils have been cleaned and tentatively identified, they should be cataloged. This is necessary to enable the collector to have a record of his collection and to furnish as much information as possible about each individual [fossil].

The collecting data can be taken from the labels that were placed in each bag of fossils as they were collected, or from the field notebook. Actually, it is wise to check one against the other. This information should then be entered in some type of record book and also placed on a more permanent label which is put in the tray or box with the [fossil]. The catalog and label should contain such pertinent data as (1) the scientific name of the fossil, (2) the geologic [formation] from which the specimen was collected, (3) the exact geographic location of the collecting locality, (4) the name of the collector, (5) the date the fossil was collected, and (6) the catalog number of the specimen. The latter is usually placed in the upper right hand corner of the label ([fig. 6]) and corresponds with a like number in the record book.

Specimen No. P-185 NAME Spirifer rockymontanus [FORMATION] Big Saline (Penn.) LOCALITY Little Brady Creek, McCulloch Co., Tex. (1000′ NE of Smith ranch House) COLLECTOR F. B. Plummer DATE July 1937

Fig. 6. A [brachiopod] showing the catalog number on it, and the accompanying label that pertains to the specimen.

The entries in the catalog should be numbered consecutively, and all specimens from the same locality should bear the same number. This number should be written on the [fossil] with India ink, preferably on any remaining matrix or on some inconspicuous part of the specimen ([fig. 6]). If the surface of the fossil is too coarse or porous for ink, the catalog number can be written on a small patch of white enamel or clear nail polish painted on the specimen. After the ink has dried it should be coated with a dab of clear shellac or clear nail polish to help preserve the number. If each specimen is numbered, it can easily be identified even if it should become separated from its label.

HOW FOSSILS ARE USED

Fossils are useful in a number of different ways, for each specimen provides some information about when it lived, where it lived, and how it lived.

Fossils are very important, for example, in tracing the development of the plants and animals of our earth. This is possible because the fossils in the older rocks are usually primitive and relatively simple; but a study of similar specimens that lived in later geologic time shows that the fossils become progressively more complex and more advanced in the younger rocks.

Some fossils, for example, the reef-building corals, appear to have always lived under much the same conditions as they live today. Hence, it is reasonably certain that the rocks containing [fossil] [reef] corals found in place (that is, where they were originally buried), were deposited in warm, fairly shallow, salt water. By studying the occurrence and distribution of such marine fossils, it is possible to outline the location and extent of prehistoric seas. Moreover, the type of fossils present will frequently give some indication as to the bottom conditions, depth, temperature, and salinity of these ancient bodies of water.

Probably the most important use of fossils is for purposes of [correlation]—the process of demonstrating that certain [rock] layers are closely related to each other. By correlating or “matching” the beds containing specific fossils, it is possible to determine the distribution of geologic units of similar age. Some fossils have a very limited vertical or [geologic range] and a wide horizontal or geographic range. In other words, they lived but a relatively short [period] in geologic time but were rather widely distributed during their relatively short life. Such fossils are known as index fossils or guide fossils and are especially useful in correlation because they are normally only associated with rocks of one certain age.

Fig. 7. Sketches of two types of micropaleontological slides. (a) Multiple space faunal slide. (b) Single-hole slide.

Microfossils are often very valuable as guide fossils for the petroleum geologist. The micropaleontologist washes the well cuttings from the drill hole and separates the tiny fossils from the surrounding rocks. The specimens are then mounted on special slides ([fig. 7]) and studied under the microscope. Information derived from these fossils often provides valuable data on the age of the subsurface [formation] and the possibilities of oil production. Microfossils are particularly valuable in the oil fields of the Gulf Coast region of Texas. In fact, some of the oil-producing zones in this area have even been named for certain key genera of microfossils. For example, the “het” zone of [Oligocene] age (geologic time scale, [Pl. 1]) is named for the genus Heterostegina, which is a tiny one-celled animal. Other microfossils, such as fusulinids, ostracodes, spores, and pollens, are also used to identify subsurface formations in many other parts of the State.

Plant fossils are very useful as climatic indicators but are not too reliable for purposes of age determination. They do, however, provide much information about the development of plants throughout geologic time.

GEOLOGIC HISTORY

The geologic history of our earth has been recorded primarily in marine sedimentary rocks, and this record indicates that our earth is very old and that life has been present for many millions of years. The earth is not only extremely old (more than 3½ billion years of age), but it has also undergone many changes which have taken place slowly but steadily and have greatly affected both the earth and its inhabitants. The earth’s physical features have not always been as they are seen today. Geologic research has shown that mountains now occupy the sites of ancient seas, and that coal is being mined where swamps existed millions of years ago. Furthermore, there is much evidence to indicate that plants and animals have also undergone great change. The trend of this organic change is, in general, toward more complex and advanced forms of life, but some forms have remained virtually unchanged and others have become extinct.

In order to interpret geologic history, the earth scientist must attempt to gather evidence of the great changes in climate, geography, and life that took place in the geologic past. The record of these changes can be found in the rocks, and here is found the story of the various events in earth history.

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.

THE GEOLOGY OF TEXAS

The geologic history of Texas, like the geologic history of the rest of the earth, is recorded primarily in marine sedimentary rocks. These rocks provide some knowledge of the early geography and the first inhabitants of what is now the State of Texas. Most of these rocks were formed from sediments deposited in shallow seas which covered parts of the State at various times in earth history.

By studying these rocks and their relations to each other, geologists have established a geologic column for Texas.

Physiography

In order to discuss the distribution and exposures of the rocks of Texas, it is helpful to be familiar with the physiography of the State. Physiography deals with the study of the origin and description of land forms, such as mountains, valleys, and plains. [Plate 9] is a map of Texas which shows the major physiographic provinces within the State.

The majority of the land forms in Texas have been produced by the processes of erosion attacking the structural features of an area. Certain other land forms may be related to the effects of igneous activity which resulted in the accumulation of large masses of igneous rocks. The Davis Mountains are an example of surface features produced in this manner.

In discussing the physiography of Texas, three major physiographic provinces will be recognized. These are (1) the Trans-Pecos region, (2) the Texas Plains, and (3) the Gulf Coastal Plain ([Pl. 9]).

TRANS-PECOS REGION

The Trans-Pecos region, located in the westernmost part of the State, is an area of mountains and plateaus with broad basins between the major mountain ranges. Many different types of rocks are exposed in Trans-Pecos Texas and these include marine, fresh-water, and terrestrial deposits. In many areas igneous rocks flowed out on the surface and now overlie sedimentary rocks. There are also many places where igneous rocks have been injected into the surrounding rocks, and these igneous rocks have been exposed by later erosion.

Included within this area is the Van Horn uplift of southern Hudspeth and Culberson counties, the Solitario uplift of southern Presidio and Brewster counties, and the Marathon uplift of northeast Brewster County. This region also includes the Big Bend area of Texas, a part of which has been set aside as a National Park where many interesting and important geological features may be seen.

The Trans-Pecos region is one of rugged [topography] with elevations as high as 8,700 feet, at Guadalupe Peak in the Guadalupe Mountains of northern Culberson County, and as low as 1,500 feet, in the Rio Grande valley.

Numerous invertebrate fossils occur in the [Cretaceous] limestones and shales of the Trans-Pecos region and in the [Paleozoic] rocks of the Marathon uplift. The Gaptank [formation] of [Pennsylvanian] age and the [Permian] [reef] limestones of the Glass Mountains are especially [fossiliferous]. In addition, many [vertebrate] fossils have been collected in Trans-Pecos Texas, particularly in and around Big Bend National Park.

TEXAS PLAINS

The plains of Texas are broad expanses of country with very little surface relief. Most of the plains support grasses and some have wooded areas, particularly along stream valleys.

The plains of the northwestern part of the State have been subdivided as follows.

High Plains

This area ([Pl. 9]), often called “the caprock,” is an elevated plateau which rises above the rolling plains which surround it. The High Plains are bounded by the Pecos River valley on the south, southeast, and west and by the North-Central Plains on the east.

The surface of the High Plains is very flat and characterized by a sparse cover of grasses and few trees. The surface strata consist largely of unconsolidated deposits of sands and gravels of [Quaternary] and [Tertiary] age, with remnants of Lower [Cretaceous] limestones along the southern margin. The rocks of the High Plains are mostly unfossiliferous, but mammalian remains have been found at several localities.

Plate 9
Physiographic map of Texas.