Geological Survey Professional Paper 215 Geology of the Southern Guadalupe Mountains, Texas

EL CAPITAN

Bartlett,⁴ in 1850, when journeying by wagon from San Antonio to El Paso, wrote:

Our road led in a direction nearly west, towards the bold head of the great Guadalupe Mountain, which had been before us some eight or ten days. This is a most remarkable landmark, rising as it does far above the surrounding plain. The sierra which ends with it comes from the northeast. It is a dark, gloomy-looking range, with bold and forbidding sides, consisting of huge piles of rocks, their debris heaped far above the surrounding hills. As it approaches its termination the color changes to a pure white, tinted with buff or light orange, presenting a beautiful contrast, with the other portions of the range, or with the light blue of the sky beyond, for in this elevated region the heavens have a remarkable brilliancy and depth of color.

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⁴Bartlett, J. R., Personal narrative of explorations and incidents in Texas, New Mexico, California, Sonora, and Chihuahua, connected with the United States and Mexican Boundary Commission, during the years 1850, 1851, 1852, and 1853, vol. 1, pp. 117-118, New York, D. Appleton & Co., 1854.

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(Views of the southeast side of the Guadalupe Mountains, appearing much as Bartlett saw them, are shown on plate 4).

The "head of the great Guadalupe Mountain" is still as impressive an object to the traveler as when Bartlett first saw it. Soon after leaving Carlsbad, Van Horn, or El Paso, the motorist discovers it in view before him, 50 miles or more away. When at length he draws closer, his road, following the course of the old caravan road that came into existence at the close of the Mexican War, winds through the hills and canyons of Guadalupe Pass with the headland rising above it to the north. From the road, tier after tier of flat-lying sandstone beds extend upward on barren slopes. On them, as on a pedestal, reposes a monumental crag of white limestone, forming a sheer cliff a thousand feet high (pl. 1). To the modern traveler, as to the Mexicans of the last century who dug for salt in the flats west of the headland, the crag is truly El Capitan, the leader or landmark.

The high peaks at the south end of the Guadalupe Mountains have been given a number of names at different times, the use of which has been indefinite and conflicting. The terminology followed here, which is that adopted by the U. S. Geographic Board, is to call the headland El Capitan, and the higher peak a short distance to the north Guadalupe Peak. However, Richardson (1904) and Girty (1908), in their geological reports, called the headland Guadalupe Point and used El Capitan for the higher peak to the north. Their terminology has been followed in most subsequent geological writings. In addition, the higher peak is commonly known to the local residents as Signal Peak, a term that appears to be of relatively recent origin. Use of the name El Capitan for the headland rather than for the higher peak seems to agree better with the original Spanish meaning of the term.⁵

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⁵For a discussion of geographic terminology, see Lang, W. B., The Permian formations of the Pecos valley of New Mexico and Texas: Am. Assoc. Petroleum Geologists Bull., vol. 21, pp. 839-844, 1937.

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GUADALUPE MOUNTAINS

El Capitan lies near the center of the area here described, and is the southern extremity of the Guadalupe Mountains, a limestone upland that expands like a wedge toward the north (fig. 2). The eastern side of the upland is the forbidding escarpment with northeast trend described by Bartlett, and is appropriately termed the Reef Escarpment.⁶

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⁶King, P. B., The Permian of west Texas and southeastern New Mexico: Am. Assoc. Petroleum Geologists Bull. vol. 26, p. 553, 1942.

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FIGURE 2.—Map of Guadalupe Mountains and vicinity, showing topographic features. Compiled from various sources, including maps by U. S. Geological Survey and U. S. Forest Service. (click on image for a PDF version)

The western side of the wedge, whose trend is somewhat west of north, has an even more impressive face (as shown on plate 5). It is only from this direction, Shumard⁷ observed

that these mountains can be contemplated in all their grandeur. Here extends an unbroken line of vertical precipices from two to three thousand feet in height, the faces of which are so smooth as to be accessible only a few hundred feet above the base. The abrupt faces of these cliffs pursue a general course parallel to the axis of upheaval of the mountains, which present the appearance of having cleft vertically through their centers and the western halves removed.

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⁷Shumard G. G., Observations on the geological formations of the country between the Rio Pecos and Rio Grande, in New Mexico: St. Louis Acad. Sci. Trans., vol. 1, p. 280, 1858 [1860].

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Between the two escarpments, the interior of the wedge is a pine-covered, rolling upland, divided into many parts by deeply incised canyons. In the southern end of the wedge, the uplands exceed 8,000 feet in altitude above sea level, and culminate in Guadalupe Peak, which rises to 8,751 feet. This is the highest point in the State of Texas. Beyond the Texas-New Mexico boundary, about 7 miles north of Guadalupe Peak, the summits are lower, and at some distance farther north and northeast the range fades out in the Pecos Valley.

The Guadalupe Mountains form the northern half of a great, eastward-tilted block of the earth's crust more than 100 miles long and about half as wide (fig. 2). The southeast-facing Reef Escarpment, which extends diagonally across the tilted surface, follows an ancient tectonic and stratigraphic axis, along which the limestones of the Guadalupe Mountains come to an end. To the southeast, where the limestones are absent, the tilted block forms a lower series of broken sandstone plateaus, known as the Delaware Mountains.

On the west side of the tilted block, the mountains break off in steep escarpments, of which the precipices described by Shumard are a part. The escarpments slope toward the Salt Basin, a depression with no outlet to the sea, whose lower part stands at an altitude a few feet above 3,600 feet, or nearly a mile below the summit of Guadalupe Peak not far away. Extending westward from the lowest benches of the escarpment toward the saline lakes and alkali flats that dot the central floor of the basin, is a great alluvial apron composed of detritus washed down from the mountains. Rising from the alluvium in places are low rock ridges, such as the Patterson Hills southwest of El Capitan (pl. 5, A). The rocks in the ridges are the same as those high in the mountains to the east, but instead of dipping gently eastward as in the mountains, they dip more steeply westward beneath the basin.

The main tectonic feature of the Guadalupe and Delaware Mountains is thus a great arch whose steepest dip is on its west flank. The archlike form, however, is greatly complicated by faulting (as may be seen in the structure sections of plate 3). The west base of the mountains is followed in most places by one of several major faults, whose presence is shown in part by outcrops of down-dropped rocks to the west, and in part, where alluvium buries the down-thrown side, by the even base line of the mountains. Between the west-tilted rocks of the Patterson Hills and the east-tilted rocks in the mountains near El Capitan are fault blocks in which the strata are more deeply depressed than in those on either side. The crest of the arch has thus collapsed by the sinking of its keystone. The rocks within the southern Guadalupe Mountains for several miles east of the major faults at the west base of the mountains also are faulted, but still farther eastward, the only sign of disturbance is the gentle tilting of the rocks to the east.

The surface configuration of the region, with its mountains, foothills, and flanking basin on the west, is thus closely related to the tectonic configuration produced by uplift and faulting. The original tectonic configuration has been somewhat modified by erosion of the higher parts of the area and by deposition in the lower parts, but these modifications have been so small that they suggest the uplifting and faulting are of relatively recent age. Some of the movements are certainly of Quaternary age, for unconsolidated deposits of the alluvial apron are disturbed and faulted near the base of the mountains. However, the alluvial apron is composed of fragments washed from high mountains, and these mountains were formed by movements older than those just noted. How old these earlier movements are is a matter for conjecture; they may be of later Tertiary age.