Texas Fossils: An Amateur Collector's Handbook

by William Henry Matthews

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.

HIGH PLAINS NORTH-CENTRAL PLAINS GRAND PRAIRIE TRANS-PECOS TEXAS VAN HORN UPLIFT THE BIG BEND AREA SOLITARIO UPLIFT MARATHON UPLIFT EDWARDS PLATEAU LLANO UPLIFT BALCONES [FAULT] ZONE GULF COASTAL PLAIN

North-Central Plains

Surface strata of the North-Central Plains ([Pl. 9]) are westward-dipping [Pennsylvanian], [Permian], and [Triassic] rocks. Present also are extensive exposures of [Quaternary] sands and gravels which trend north-south across the central portion of the region. The area is bounded on the west by the High Plains, on the east by the Grand Prairie, and on the south by the Edwards Plateau and Llano uplift. Many [vertebrate] fossils have been collected from the Permian and Triassic rocks of this area. There are also many excellent outcrops of [fossiliferous] Pennsylvanian formations in the North-Central Plains region.

Edwards Plateau

The Edwards Plateau ([Pl. 9]) is located in south-central Texas and is bounded on the south by the Balcones [fault] zone and on the north by the North-Central Plains. The surface of the area is typically flat with a gentle slope to the south. The rocks of the Edwards Plateau consist primarily of Lower [Cretaceous] limestones and shales, many of which are very [fossiliferous].

Grand Prairie

This area ([Pl. 9]) has a relatively flat surface but there are areas of gently rolling hills. The eastern boundary of the Grand Prairie is marked partly by the Balcones [fault] zone. North of McLennan County, however, the Balcones fault zone is not expressed at the surface and in this area the eastern boundary is defined by the western edge of the Woodbine exposures. Upper and Lower [Cretaceous] rocks occur at the surface and [dip] to the southeast; many of these rocks contain a large number of invertebrate fossils.

Llano Uplift

The Llano uplift ([Pl. 9]) is located in the central part of the State where [Precambrian] igneous and metamorphic rocks and sedimentary rocks of early [Paleozoic] age occur on the surface. The area, which now appears as a basin-shaped depression, was at one time covered by Lower [Cretaceous] rocks and perhaps also by [Devonian], [Mississippian], and [Pennsylvanian] strata. These have since been removed by erosion. The east, south, and west sides of the uplift are surrounded by Lower Cretaceous rocks, and the northern margin is marked by the Mississippian and Pennsylvanian formations of the North-Central Plains. The area is, in general, composed of unfossiliferous rocks, but some invertebrate fossils (primarily trilobites and brachiopods) have been collected.

GULF COASTAL PLAIN

The Gulf Coastal Plain ([Pl. 9]) is composed of [Cretaceous], [Tertiary], and [Quaternary] rocks and includes the eastern, southeastern, and southern portions of the State. The rocks of the area consist of sands, clays, shales, and limestones. The Texas Gulf Coastal Plain is bounded on the north and west by the Balcones [fault] zone, on the south and southwest by the Gulf of Mexico, and extends eastward into Arkansas and Louisiana.

The region has broad river valleys and uplands of low relief, but there is an increase in relief toward the interior of the State. The surface of the area slopes gradually toward the Gulf and successively younger formations are encountered gulfward.

The rocks of the Texas Gulf Coastal Plain are relatively unfossiliferous, but many of the Upper [Cretaceous] rocks contain fossils. In the central portion of the region some marine formations of [Tertiary] age locally contain well-preserved invertebrate fossils.