Virtual Landscapes of Texas

3

Physical Geography of Texas —3.

Relief expression of the Mesozoic and later rocks. —The existence of the early Mesozoic (Tri assic) is doubtful, although possible. Rocks referred to this period overlie the Permian along the western part of the Central Province and appear in small areas around the border of the Plateau of the Plains, but are of no topographic significance. Jurassic limestone strata of the Mexican type have been found in only a limited area in the barren ranges west of the Cordilleran front and are not known on the Atlantic slope or the eastern Front Ranges.

The later group of formations, those of the Coast ward Slope, consists of sheets of sea-made sediments, from Cretaceous to Pleistocene age, inclusive, and of aggradational deposits of upland wash and stream and lacustral alluvium of Tertiary and later age, all of which, except the Lower Creta ceous, are mostly unconsolidated terranes of clay, sand, marl, and loam. Of this later group, the marine Cretaceous, Tertiary, and Pleistocene rocks are the chief formations, especially east and south of the Central Province. They were unconform ably deposited across the upturned edges of the older formations above described. They may be subdivided into two general series, producing two broad variations of low relief: (1) the older Cre taceous formations, and (2) the newer Cretaceous, Tertiary, and Pleistocene formations. These rocks occur in belts, each underlying a broad strip of country.

The Cretaceous rocks are divisible into an older or lower and a newer or upper series. They occur in the Trans-Pecos Mountains and in the Coast ward Slope plain. The older formations consist of hard limestones alternating with clays, and are underlain by sands; they produce dip plains, cut plains, and low scarps. The Upper Cretaceous strata consist largely of unindurated clay marls, with a few indurated scarp-making strata, all underlain by sands and weathering into low, undu lating areas.

Of all the Texas formations influencing the relief the Upper and Lower Cretaceous rocks have the largest areal development. They extend from southern Indian Territory, where their horizontal strata abruptly end against the upturned Paleozoic strata of the Ouachita Mountains, southwest toward the Trans-Pecos and Mexican cordilleras, which are largely composed of their crumpled sheets. The upper and younger Cretaceous rocks, being softer and yielding more readily to disinte gration, usually underlie level plains. The lower and older Cretaceous formations, composed of hard, resisting limestones, weather into sharp relief features — scarps, plateaus, and mountains — and, with the exception of the high volcanoes of southern Mexico, compose most of the high relief of Texas and Mexico. They are also cavernous (see fig. 30, Sheet 111, Special Illustrations). The newer Cretaceous rocks form the Black Prairie of the East-Central Province and the interior por tion of the Southern Province. The older Creta ceous rocks form the southern part of the Plateau Province, the Callahan Divide, the Lampasas Cut Plain, and many of the Trans-Pecos Mountains.

The marine Tertiary and Pleistocene sediments are found only to the coastward of the later Cre taceous formations in the Eastern and Southern provinces, making, with portions of the later Cre taceous, the formations of the Coastal Plain. The Eocene Tertiary strata are mostly unconsolidated alterations of sand and clay, with exceptional local indurations of ferruginous sandstone. The later marine sedimentaries of the Coast Prairie district (Miocene to Recent) have not been classified by age, but collectively they make a great thickness of unconsolidated sands and clays of late Tertiary and Pleistocene age. According to well borings at Galveston, they are over 3000 feet thick. They produce a remarkably level topography — appar ently a new plain lately reclaimed from the sea.

Sedimentary Rocks of Other than Marine Formation.

The non-marine deposits of Texas are lacustral deposits, sheets of upland gravel and alluvial wash, " tepetate," * and wind-made formations, all of which were laid down at local deposition levels. Of these formations the alluvium is found through out the whole region, in the valleys of all stream

ways, and even over the uplands of much of the Southern and East-Central provinces. The lacus tral deposits are found in valleys of the Cordilleran region, partially filling the desert basins. The upland formations (the "wash" 2) are flood sheets of gravel, sand, and marl, sometimes consolidated in the calcareous matrix known as " tepetate." They cap the Llano Estacado and occur along the interior margin of the Southern Province at the foot of the Balcones escarpment, consisting of the worked-over talus of the mountains and the debris of the Edwards Cut Plain. These deposits, initiated in Tertiary time, are in constant process of formation. The wash and tepetate occur on most of the slopes of the semi-arid and arid regions, where streams evaporate and sink into the plains, in the wind-made formations on the Llano Esta cado, and in the valleys of the Great Plains and Central provinces.

Relief Features of the Igneous Rocks.

The igneous rocks, in respect of occurrence, are of three classes: (1) the older granites accompany ing the pre-Cambrian or fundamental rocks, upon which the whole superstructure of fossiliferous rocks may have been laid down; (2) intrusive rocks, pushed up through and between the other rock sheets and necessarily of later age than the rocks which they intrude; and (3) extrusive rocks, which have flowed or fallen over the sur face. The oldest igneous rocks of Texas, included in class 1, and herein called fundamental, strictly speaking are not such, inasmuch as some of them at least are intruded into schists which are the lowest of this portion of the earth's crust visible to our inspection.

The old granites outcrop in the Llano country of the Central Province, in the Franklin, Hueco, and Cornudas mountains of the Trans-Pecos Province, and in the central and western half of the Ouachita Mountains. They are not of exten sive topographic importance.

The intrusive rocks occur extensively in the Trans-Pecos Province, and exceptionally along the interior border of the Rio Grande embayment.

Extruded rocks of Cretaceous and Tertiary age occur chiefly in the Cordilleran region, as necks, dikes, flows, and cones. Volcanic necks of rhyolite form in part the extensive mountain groups known as the Chisos, Corazones, and Davis. Extensive flows of basalt and rhyolite make the indurated cap rocks of such features as the scarped cut plains of the Davis Mountains and the Mesa de Maya. Extrusive sheets of lava known as " malpais " are also found in the floor of the Hueco Basin of the Trans-Pecos Province. Volcanic craters, or cinder cones, which are exceptional features, occur in northeast New Mexico east of the Rocky Moun tain front and in central New Mexico.

The marine sedimentary rocks of the Cordilleran region are of various ages prior to the Tertiary, representing in part the survival of structural features of pre-Cretaceous time, mostly buried by later deposits of Cretaceous rocks.

The Ouachita group is made up of the same Paleozoic beds that form the foundations of the non-mountainous regions, where they are buried beneath the later rock sheets. In the mountains the Algonkian, Cambro-Silurian, and Carbonif erous rocks are the survivals of ancient land masses that were not base-leveled in pre-Creta ceous time. Their arrangement in long anticlinal folds more or less influences the present relief, and, assisted by erosion, produces a type of con figuration quite different from that of the moun tains of the Cordilleras.

The mountains of the Trans-Pecos Province are composed not only of the older sedimentary rocks found in the Ouachita uplift and the floor of the Coastward Slope plain, but also of the Cretaceous rocks which make much of the surface of the latter region. Here they are folded and tilted into

1 The word " tepetate," also spelled "telpetate," is a term used throughout Mexico and Central America for secondary non-marine deposits, either chemical or volcanic, forming a superficial coating over the country rock or impregnating the regolith. In Texas and northern Mexico the tepetate is always a chemical precipitate of lime, formed on calcareous soils or transported in solution and deposited through evap oration around the margins of desert basins.

8 For a description of the process of distribution of the wash, see Nueces folio, No. 42, United States Geological Survey.

mountain structure, while sheets and necks of hard eruptive rock, produced in Mesozoic and Cenozoic time, furnish further relief-making ele ments. The marine Tertiary and Pleistocene for mations of the Coastal Plain are missing, but in the flats and basins between the mountains are extensive unconsolidated non-marine deposits, prob ably of synchronous age.

The Mountains.

Within the Greater Texas region are two moun tain systems — the Ouachita system of Arkansas and Indian Territory, and the Trans-Pecos Moun tains. These systems are of different structural types, ages, and configuration, and trend approxi mately at right angles to each other. The Ouachita system is Appalachian in structure and general resemblance, and is thereby related to the eastern half of the United States; the Trans-Pecos Mountains, on the other hand, are a part of the great Cordilleran system which dominates the western half of the continent between the Great Plains and the Pacific.

Ouachita System.

This system extends east and west between the ninety-third and one hundredth meridians, from the Mississippi embayment of the Coastal Plain to the plateau of the Great Plains, through west ern Arkansas, Indian Territory, and Oklahoma. The system as a whole is a narrow line of old mountains, whose summit nowhere exceeds 3000 feet. It is composed of three principal groups, of different types of relief and rock composition; these are the Massern Ranges on the east, the Arbuckle Hills inutile center, and the Wichita Mountains on the west.

The Massern Ranges were so named by Thomas Nuttall on a map accompanying his book entitled A Journey of Travels into the Arkansas Territory during the Year 1819: Philadelphia, 1820. These consist of elongated ridges of vertically folded clays and sandstones with some limestone, mostly of Carboniferous age. They extend east and west to longitude 95° 30', where they change to a southerly direction, ending in a manner as yet not satisfactorily explained, at the northern edge of the Grand Prairie and against the eastern end of the Arbuckle Hills. The southward continua tion of these folds was planed off and buried beneath the Cretaceous rocks of the Grand Prairie of Texas (see fig. 45, Sheet VI, Types of Mountains).

The Arbuckle Ranges extend from the ninety sixth to the ninety-eighth meridian, in a series of low limestone ridges and granitic hills which strike in a direction north of west. These are old mountains composed of vertically folded limestone strata with a granitic base, exposed toward their eastern end, in the vicinity of Tishomingo. They have been so degraded that they have lost that magnitude which is usually associated with moun tains.

The Wichita Range is the western end of the Ouachita system, and forms a rugged sierra betw reen

Fig. 3.—Granite ridges of the Wichita Range.

longitudes 98° 30' and 100°. The highest peak, Mount Sheridan, rises 2500 feet — about 1300 feet above the surrounding plains, which are com posed of old granitic and volcanic rock projecting through flanks of Silurian limestone (see fig. 3).

The mountains of this system in general are old, and represent the remnants of once more lofty and

extensive ranges which have undergone degrada tion since early Mesozoic time.

Mountains of the Trans-Pecos Province.

The Trans-Pecos Province is a peculiar combina tion of mountains and stretches of plateau plain and bolsons, surrounding, bordering, and lying between mountain ranges. The plains will be discussed later under a separate head.

General Features.

The Trans-Pecos Mountains of Texas and New Mexico lie between the Pecos and the Rio Grande south of latitude 35° 30. They represent the eastern Front Ranges of that portion of the North American Cordilleras between the southern end of the Rocky Mountains, in northern New Mexico, and the northern end of the eastern sierras of Mexico. These mountains are called by some the Continental Divide, but erroneously, for the Cordilleran region has no single dividing ridge in this latitude, but is a canoe-shaped area, bordered toward the Pacific and Great Plains by broken crests between which are basin plains and low mountain ridges (see figs. 25 and 28, Sheet 111, Special Illustrations).

By origin these mountains are of three distinct types, as follows:

1. True mountains of deformation, composed of structural folds or tilted fault blocks of sedimen tary rocks, the mountain forms corresponding in trend to the strike of the structure.

2. Plateau mountains, consisting of high sub horizontal plateaus void of serious deformation, occurring either as summits or as shoulders and attending higher relief features.

3. Mountains of igneous material, of three sub types — old igneous vents (such as dikes and necks), craters, and summits of circumdenudation capped by sheets of ejecta.

Portions of these mountains have been elevated at different geologic epochs by various orogenic processes, but the group as a whole has also been elevated by general regional movements, so that its eastern base in the United States, adjacent to the western border of the Coastward Slope plain, now stands from 4000 to 5000 feet above the sea, the height of this line increasing northward; between the mountains and the coast is found the long and gentle descent of the Coastward Slope.

Although conspicuous and sharply defined relative to the plains from which they rise, these mountains, as a rule, with the exception of the Sierra Blanca of the Sacramento Range of New Mexico, do not rise to the heights of the Rocky Mountains to the north or of the Sierra Madre of Mexico to the south. This is due to the fact that they occur along the lowest belt of the Cordilleran platform which crosses the continent along the southern boundary of the United States.

The highest altitudes attained are in the moun tains of the eastern Front Ranges. Sierra Blanca, of the Sacramento Range in southern New Mexico, reported to be 13,000 feet high, is the highest summit. The highest mountain of the system in Texas is Guadalupe Peak, near the New Mexican line — 9000 feet, or 5000 feet above the interior margin of the Coastward Slope plain. Southward the mountains do not attain high altitudes until they cross the Rio Grande in Mexico. Li verm ore Peak (8250 feet) and Mount Emory (9000 feet) are the highest summits south of Guadalupe Peak. Immediately upon crossing into Mexico the ranges again rise to higher altitudes —10,000 feet or more.

The Trans-Pecos Mountains lack continuity and exhibit many irregular and eccentric forms of relief. In general the individual mountains pre sent sharp and rugged outlines. They are usually barren of timber, except a few summits of the Sacramento, Davis (see fig. 46, Sheet VI, Types of Mountains), and Chisos mountains that rise above the base of the timber zone, which is about 6000 feet along the Rio Grande.

The individual mountains may be primarily classified as sierras, disconnected peaks (see fig. 25, Sheet 111, Special Illustrations), and groups of peaks. The various mountain forms, whose linea ments are so clear in the arid atmosphere, have generally been given individual and descriptive names by the former explorers and inhabitants of Spanish speech. Thus we find them called by