DESCRIPTION OF THE EL PASO DISTRICT.
By G. B. Richardson.
INTRODUCTION.
LOCATION.
The El Paso quadrangle extends from latitude 31° 30' to 32° 00' and from longitude 106° to 106° 30', occupying an area of 1014 square miles in western Texas and the adjacent part of Mexico. The Texas-New Mexico boundary forms the northern limit of the quadrangle and the Rio Grande, flowing across the southwestern quarter of the area, defines the international boundary. The Mexican portion, occupied chiefly by the river valley, has not been surveyed. To complete the description of the Franklin Mountains a tract about 6 miles wide just west of the quadrangle is included in the maps and descriptions of this folio and the whole area will be referred to as the El Paso district. The surveyed portion of the district has an area of 894 square miles. It is situated in the extreme northwestern part of trans-Pecos Texas.
TRANS-PECOS TEXAS.
Trans-Pecos Texas, that part of the State which lies west of Pecos River, is distinctly different from other parts of Texas in topography, climate, and geology. The greater portion of the State is occupied by plains, but west of the Pecos the plains are succeeded by mountains, which mark the boundary between the Great Plains and the Cordillera. The portion of the Cordillera included in trans-Pecos Texas is the southern continuation of the central mountainous area of New Mexico, and is characterized by an assemblage of diverse topographic forms which individually resemble features of the Rocky Mountain province on the north, the Basin Range province on the west, and the Mexican Plateau province on the southwest. Topographically the trans-Pecos region is a transition area adjoining these provinces.
The region is one of mountains and intermontane plains. The dominant topographic trend is northwest and southeast except near the New Mexico-Texas boundary, where a north-south trend is developed. This region lies in the lowest belt of country extending across the interior of the continent, Paisano, the highest pass on the "Sunset route" of the Southern Pacific system, has an elevation of 5082 feet, and the altitude of Allamore, at the summit of the Texas and Pacific Railway, is 4603 feet. Only two peaks rise above 8000 feet and the lowlands commonly range between 3500 and 4500 feet in elevation. In general the trans-Pecos highlands lack continuity and exhibit a variety of forms, including isolated peaks, groups of peaks, plateaus, narrow tilted blocks, and broad monoclines. The intermontane plains have been named "bolsons," a term derived from the Spanish bolsón, a purse. These are broad, almost level constructional plains built up by wash derived from the adjacent highlands. Bolsons generally slope toward a central axis; some of them are entirely surrounded by a rim and constitute closed basins, but the greater number have outlets, although in this arid climate they are free from surface drainage except where crossed by the few perennial streams of the region.
The arid climate of the trans-Pecos region accentuates its peculiarities. The annual precipitation on the greater part of the area is about 15 inches and in places amounts to less than 10 inches. The common type of rainfall is the occasional heavy summer shower of short duration and limited extent, giving rise to torrential floods. Accordingly there is no permanent run-off, and the short-lived streams that gather in the highlands disappear by absorption and evaporation shortly after reaching the lowlands. Under more humid conditions the bolson plains could not exist, for the débris, instead of accumulating, would be carried away by streams. Vegetation throughout the region is of the desert type. The lowlands and most of the uplands are bare of trees and only the highest mountains support a stunted forest growth.
The rocks of trans-Pecos Texas reveal a long and varied history, which began in pre-Cambrian time. Almost all the systems from the Algonkian to the Quaternary are represented by sediments, and locally this area has been at different times the seat of igneous activity by which a variety of molten magmas were intruded into preexisting rocks or extruded on the surface.
OUTLINE OF THE TOPOGRAPHY AND GEOLOGY OF TRANS-PECOS TEXAS EAST OF THE EL PASO QUADRANGLE.
Light can be thrown on the general geology of the El Paso district by briefly reviewing the conditions in a zone across the northern part of trans-Pecos Texas, immediately south of the Texas-New Mexico boundary. (See fig. 2.) This zone is characterized
The dominant topographic feature of the eastern Cordillera in this latitude is the highland belt comprising the Guadalupe and Delaware Mountains, which extend southward from New Mexico and separate the lowlands of Toyah Bolson on the east from Salt Flat on the west. These mountains constitute an eastward-sloping monocline and present a steep scarp to the Salt Flat, above which they rise from 1000 to almost 5000 feet. The Guadalupe Mountains extend across the State boundary about 45 miles west of Pecos River, where they are 10 miles wide, but they converge in a wedge-shaped form, and about 10 miles south of the boundary terminate abruptly in a precipitous cliff known as Guadalupe Point. El Capitan Peak, one-fourth of a mile north of Guadalupe Point, has an elevation of 8690 feet and is thought to be the highest point in Texas. The Delaware Mountains are the southern continuation of the Guadalupe Mountains. They extend southeastward uninterruptedly for about 40 miles, but beyond this stretch are considerably dissected and form an irregular highland mass that reaches almost to the Texas and Pacific Railway. These mountains constitute a typical cuesta. (See figs. 1 and 2.) Their southwestward-facing scarp is between 1000 and 2000 feet high, and from its crest the surface slopes gradually northeastward, conforming approximately with the dip of the underlying rocks. The rocks of the Guadalupe and Delaware mountains consist of about 4000 feet of sandstone and limestone containing an abundant and unique Permian fauna. a These rocks outcrop eastward along the slope of the mountains in a belt about 15 miles wide, beyond which the inclination of the surface decreases to a gentle eastward slope, in a plain averaging 15 miles in width, which is underlain by bedded gypsum. The gypsum lies unconformably upon the Permian (?) strata just mentioned, and its thickness, as indicated by well records, is at least 300 feet. On the east a narrow range of low hills intervenes between the gypsum plain and the Toyah Bolson. These hills are capped by gray magnesian limestone and local beds of buff sandstone of unknown age, which overlie the gypsum and are folded into a series of gentle arches and troughs.
The Salt Flat is one of the prominent bolsons of the trans-Pecos country. It has the prevailing northwest-southeast trend, is more than 150 miles long, averages possibly 15 miles in width, and is a closed basin with no drainage outlet. It occupies a structural trough, and in Texas it is bounded on the east by the Guadalupe and Delaware mountains and on the west by the Sierra Diablo. The center of the basin is 3600 feet above sea level. Low, marshy areas, commonly floored with gypsum, occur near the State boundary, where there is also a salt deposit of commercial importance, but the greater part of the basin is underlain to an unknown depth by gravel, sand, and clay derived from the adjacent highlands.
Southwest of the Salt Flat and south of the Sierra Diablo, about 100 miles southeast of El Paso, there is an area of early Paleozoic and pre-Cambrian rocks which outcrop in low hills near the Texas and Pacific Railway. The rocks of pre-Cambrian age are separable into two distinct formations; one consists of metamorphic rocks including quartzites, slates, and various siliceous schists of sedimentary origin, with some altered basic intrusives, and the other is composed of fine-textured red sandstone, limestone and breccia. These rocks have been much disturbed and are unconformably overlain by 500 feet of conglomerate and coarse red sandstone of probable Cambrian age; these in turn are succeeded by a great thickness of limestone whose lower part is Ordovician and upper part Carboniferous (Pennsylvanian), the intervening systems being absent. There is abundant evidence here of profound pre-Pennsylvanian erosion.
North of this area the Sierra Diablo rises abruptly almost 3000 feet above the Salt Flat. This range constitutes the dissected southeastern escarpment of the Diablo Plateau, a rather flat-topped upland having an area approximating 2500 square miles. The surface of this plateau is not flat over wide areas; it slopes gently eastward in the western part and westward in the Sierra Diablo, yet the plateau nature of the area as a whole is distinct. It is underlain by flat-lying or gently inclined strata of upper Carboniferous and Lower Cretaceous age. The eroded escarpments of the Diablo Plateau are known by different names. North of the Sierra Diablo, adjacent to the State boundary, the northeastern border of the plateau is marked by the Cornudas Mountains and the Sierra Tinaja Pinta, which are groups of isolated igneous peaks and lava-capped mesas flanked by Carboniferous and Cretaceous sediments. The western border of the Diablo Plateau north of the Texas and Pacific Railway is known as the Finlay Mountains, and farther north, adjacent to the Texas-New Mexico boundary, as the Hueco Mountains. These two mountainous areas are separated by an abrupt escarpment, about 500 feet high and 20 miles long, capped by horizontal limestone of Lower Cretaceous age. At the south end of the escarpment the rocks are bent into a rude dome which has been dissected into a group of hills known as the Finlay Mountains. In the center of the dome upper Carboniferous limestone and shale outcrop and are unconformably overlain by Lower Cretaceous sandstones and limestones that extend far to the north and cover a considerable part of the southwestern surface of the Diablo Plateau. The Finlay Mountains have been intruded by a number of dikes which suggest that the dome structure has resulted from the upward movement of igneous magmas. The Hueco Mountains lie partly in the El Paso quadrangle and will be described in another section.
CULTURE.
The fertile Rio Grande valley has long been inhabited. When the Spaniards entered the region, over three hundred years ago, they found the Indians practicing irrigation, and the Mexican towns of Juarez, across the river from El Paso, and Ysleta and San Elizario, on the American side, are of ancient origin. The city of El Paso has an estimated population of 30,000 and is the commercial center of a large area in southwestern United States and Mexico where mining, agriculture, and grazing are important industries. Five railway systems enter El Paso—the Southern Pacific, Texas and Pacific, Santa Fe, El Paso Southwestern, and Mexican Central—and one of the largest smelters in the United States is situated a short distance northwest of the city. Irrigated farms in the valley below El Paso support a thriving population and there are a number of cattle ranches on the Hueco Bolson.