Part 12

Permian

Permian rocks are found in widely separated areas in Texas. The best exposed section of marine Permian rocks is found in the Glass Mountains of Brewester County, and many of these rocks are very fossiliferous. The original shell material of some of the Permian fossils of this area has been replaced by siliceous material which is very well preserved. These silicified fossils are removed from the limestone by solution in acid, and some most remarkable specimens have been recovered in this manner (Pl. 3). Brachiopods are the most common fossils, but corals, bryozoans, and mollusks have also been recovered.

Extensive Permian exposures occur also in the central part of the North-Central Plains region. These rocks were formed from sediments of both marine and continental origin and some of them are fossiliferous. The marine rocks contain a variety of invertebrate fossils including brachiopods, pelecypods, gastropods, and ammonoids. Those rocks representing terrestrial deposits contain vertebrate remains at many localities, and numerous amphibians and primitive reptiles (Pl. 40) have been collected from them.

MESOZOIC ROCKS

Mesozoic rocks occur over a wide area of Texas and include exposures of Triassic, Jurassic, and Cretaceous age. Many of the Upper and Lower Cretaceous outcrops are quite fossiliferous and easily accessible and thus of considerable interest to many amateur collectors.

Triassic

Triassic rocks crop out in parts of the High Plains, the Glass Mountains of Trans-Pecos Texas, and parts of Pecos, Crockett, Upton, Reagan, and Glasscock and other west Texas counties. These are predominantly nonmarine rocks consisting of conglomerates, sandstones, shales, and some gypsum beds.

Triassic fossils are almost exclusively vertebrates, although some poorly preserved plant and invertebrate remains have been reported. Fossil vertebrates of the Texas Triassic include phytosaurs (Pl. 42), crocodiles, amphibians, and fish.

Jurassic

In Texas, surface exposures of Jurassic rocks are known only from Malone Mountain in southwestern Hudspeth County. The rocks there are limestones, shales, sandstones, and conglomerates. Fossils reported from that locality include marine and fresh-water pelecypods, fresh-water gastropods, and ammonites.

Cretaceous

Rocks of Cretaceous age are widely distributed in Texas and represent one of the more important rock systems of the State. Cretaceous outcrops occur in central Texas, north Texas, the Edwards Plateau, parts of the High Plains, the Gulf Coastal Plain, and Trans-Pecos Texas.

As mentioned earlier, the Texas Cretaceous has been divided into the Lower Cretaceous (Comanche series) and Upper Cretaceous (Gulf series). These rocks consist primarily of marls (a type of calcareous clay), shales, chalks, and limestones, but sands and conglomerates also occur. Cretaceous rocks occur on the surface of about 28 percent of Texas, and many of the larger cities of the State are situated on Cretaceous strata.

Many of the Gulf and Comanche formations contain fossils which are of interest both to amateur and professional paleontologists. Because of their wide distribution in and near large population centers, Cretaceous outcrops can be conveniently visited by many amateur fossil collectors. The fossils are usually abundant and varied, and some are well preserved. Although numerous kinds of fossils may be collected, the more common forms are cephalopods, pelecypods, gastropods, and echinoids. Some of the more typical Cretaceous fossils are shown in Plates 16, 21, 25-28, 32, 33, 35, and 36.

Cretaceous fossils are more commonly found in shales and chalky limestones. Fossiliferous outcrops of these rocks can be found along many streams, roads, and highways of central Texas, north Texas, and the Edwards Plateau. Outcrops which have been weathered are more likely to provide good collecting. In general, collecting is poor in areas covered with heavy vegetation or recent stream deposits. Good collecting localities are outcrops which have a fairly steep slope with a covering of weathered rock material and a minimum of vegetation. One should move slowly from the base of the slope upward while searching the ground for any evidence of fossils, and particular attention should be given to any small gullies since these often contain fossils that have been washed out of upper beds in the exposure.

CENOZOIC ROCKS

Cenozoic rocks are widespread in Texas but occur primarily in a broad belt along the Gulf Coastal Plain. In addition, there are exposures of nonmarine Cenozoic strata in the High Plains, North-Central Plains, and Trans-Pecos region. There are also many exposures of Cenozoic igneous rocks in Trans-Pecos Texas.

Rocks of Cenozoic age occur in more than one-third of Texas and consist of conglomerates, sands, clays, and some limestone and lignite beds.

Tertiary

Extensive exposures of Tertiary rocks trend northeast-southwest in a broad band across the Gulf Coastal Plain area. These strata, consisting of sands, clays, and poorly consolidated limestones, are underlain by Cretaceous rocks.

Invertebrate fossils are common in certain Tertiary formations and pelecypods, gastropods, and corals are the predominant forms. In general, however, fossiliferous exposures are of local occurrence and most of the Tertiary formations are unfossiliferous. Those Tertiary invertebrates that are present, however, are often well preserved and represent many interesting types (Pls. 16, 22, 23, 29, 30, 31).

Tertiary invertebrate fossils are commonly found in sands, clays, and marls. Many of these sands and marls have a green color which is due to the presence of glauconite (a green mineral containing iron and closely related to the micas). At certain localities on the Gulf Coastal Plain the glauconite marls and sands of the Weches and Crockett formations contain large numbers of well-preserved clams, snails, and corals. Fossiliferous exposures of Tertiary rocks are sometimes found in road cuts, but better exposures may be found along the banks of rivers and creeks. Certain bluffs along the Brazos, Sabine, and Trinity rivers are well-known Tertiary fossil collecting localities. Many of these better localities are listed in some of the Bureau of Economic Geology bulletins included in the bibliography of this publication (pp. 109-110).

Quaternary

Quaternary deposits of Pleistocene age (geologic time scale, Pl. 1) are found in many parts of Texas and consist of sands, clays, and gravels.

These rocks are distributed along the Gulf Coast in a belt from 50 to 100 miles wide. They occur also as stream terraces in the Edwards Plateau and North-Central Plains regions. In addition, Quaternary sands and gravels are widely distributed over the surface of much of Trans-Pecos Texas. There are also fossiliferous Pleistocene strata in the High Plains region.

Invertebrate fossils are rare in Pleistocene rocks, but some fresh-water and terrestrial mollusks occur. Vertebrate remains, however, are abundant in many localities, and large numbers of horses, camels, mammoths, and other mammals (Pls. 46-49) have been collected. Fossil bones and teeth (figs. 25, 26, p. 104) are commonly found in the gravels and sands of many of the river terraces of the State.

MAIN TYPES OF FOSSILS

The beginning fossil collector is usually amazed by the many different plants and animals that have left some trace of their existence. In order to understand these different types of prehistoric life, it is necessary to know something about the organisms that are living today.