Geological Survey Bulletin 845 Guidebook of the Western United States: Part F. Southern Pacific Lines

SHEET No. 16 (click in image for an enlargement in a new window)

LOBO TO EL PASO, TEX.

Three miles beyond Lobo State Highway 3 crosses the Southern Pacific tracks and goes to Van Horn, a small town on the Texas & Pacific Railway, dimly visible down the valley, about 8 miles to the north. The old military trail from the east followed the Lobo Flats to Van Horn Wells, thence north and west to Eagle Spring, Fort Quitman, and San Elizario on its way to El Paso, whence a trail continued on into Chihuahua.

East of Fay are the Wylie Mountains,⁷⁵ a deeply dissected elevated plateau of limestone of Permian age cut off on its west side by a fault, as shown in Figure 21.

---

⁷⁵The limestone of this range is the same as that which caps the Sierra Diablo north of Van Horn. The schist, with quartz schist exposed on the west face of the Wylie Mountains is composed of light-pink quartz-muscovite schist, quartz biotite-muscovite schist, chlorite schist, and amphibolite veins and lenses. The strike is nearly east and the dip 20°-30° S. (Baker.)

---

FIGURE 21.—Section across the western part of the Wylie Mountains north of Lobo siding, Tex. Cp, Permian limestone; €vh, Van Horn sandstone (Cambrian)

Many chapters of geologic history are indicated by the rocks of western Texas. Although some of the conditions and events are clearly shown, some intervals of geologic time are not represented by sediments. The old basement of schist and granite of pre-Cambrian time appears in the Van Horn and El Paso regions, where in places it is overlain by sandstone and limestone probably of Algonkian age. The relation of land and sea and the extent to which Algonkian deposits were laid down can be only vaguely pictured. Late in Cambrian time there was extensive marine submergence, with shores on which accumulated the sand of the Blisa and Van Horn sandstones. In the next period (Ordovician) there were widespread marine conditions from time to time, separated by intervals of general uplift in which doubtless some deposits were removed by erosion. This oscillation of submergence and emergence continued through the Paleozoic era, but representatives of part of the Silurian, Devonian, Mississippian, and Pennsylvanian are all indicative of widespread marine seas, most of the shores of which can not be located. In these times the uplifts were general and in greater part not attended by flexing until late Pennsylvanian time, when arching and considerable crustal fracturing occurred in many if not all parts of the general region. The erosion that ensued removed considerable material and reduced the surface to a rolling and in places rugged upland, in which rocks of pre-Paleozoic to Pennsylvanian age were exposed. Some of this old upland surface is revealed in the Marathon and Van Horn regions. In widespread and long-continued marine submergence in Permian time southwestern Texas and the adjoining regions were covered with limy sediments, which in part of the area had a thickness of 5,000 feet or more. In places the deposition was long continued, subsidence keeping pace with the accumulation of the fine sediments. Local conditions varied greatly. In some places there were extensive barrier reefs near the borders of the sea and persisting throughout the deposition of thousands of feet of strata. These reefs consist of massive limestones or dolomites, such as those in the Guadalupe, Diablo, Apache, and Glass Mountains, and contain remains of algae, bryozoans, sponges, crinoids, and other fossils of reef habitat. Behind the reefs were wide lagoons in which thinly stratified beds were laid down, some of them porous limestones and locally clays, red beds, and gypsum. (P. B. King.) A great interval of sand deposition is indicated by the sandstone of the Delaware Mountain formation. In part of the region extensive mud fiats of red clay were built on lowlands subject to overflow, and in some of the shallow intervening basins there was a thick accumulation of salt and gypsum. On the adjoining lands lived many animals, largely strange reptiles, of which many remains have been found in the Permian red beds. One of the most peculiar of these animals, the finback lizard, is shown in Plate 16, A.

PLATE 16 A (top), FINBACK LIZARDS OF PERMIAN TIME. Restoration from bones of Dimetrodon incisivus and Naosaurus claviger found in the "Red Beds" of Texas. They range in length from 3 to 10 feet and are among the earliest forms of saurian life. B (bottom), CARLSBAD CAVERNS, 20 MILES SOUTH OF CARLSBAD, N. MEX. Deposits of calcium carbone. (National Park Service.)

After Permian time there was widespread uplift with considerable flexing of the strata and extensive erosion. In certain local basins limy sediments were laid down, as shown by the thick mass of Jurassic limestone of the Malone Mountains, the product of a sea or marine estuary. Through the Cretaceous period there were several marine occupations of wide extent and long duration, in which the Comanche strata Cretaceous) and the clays and chalks of the Upper Cretaceous were accumulated. Late in Cretaceous time, however, western Texas was elevated above the sea, and it has been an upland ever since. Volcanic action began at this time, with the ejection of tuffs and ash and thin flows of acid lava, the earliest of which were buried by sand. Later there were tremendous eruptions of lavas of many kinds, with the building of high volcanic cones, some of cinder and scoria, which continued into Tertiary and later time. There was in late Tertiary time a widespread uplift in which the lavas were tilted, flexed, and faulted. Since then they have been widely removed and sculptured by erosion to their present forms, and thick mantles of alluvium have been deposited in some of the valleys.

The Lobo Flats support much tobosa grass, a plant that carries its moisture a long time and is therefore in high favor for pasture. This wide valley was a favorite rendezvous for Apache Indians and outlaws, who committed many depredations. At Fay siding a 2,012-foot boring found but little water. Beyond Fay siding the railroad passes around the north end of the Van Horn Mountains, an outlying knob of the Permian limestone reaching the railroad at milepost 702, 2 miles beyond the siding. At Collado siding, 2 miles farther on, the railroad deflects around a knob of the same limestone at the south end of the Carrizo Mountains.⁷⁶

---

⁷⁶These mountains consist mainly of Carrizo Mountain schist (pre-Cambrian), with small overlapping areas of Van Horn sandstone (Upper Cambrian), capped by limestone of Permian age. The Van Horn sandstone varies in thickness but in places is more than 200 feet thick, consisting of red micaceous sandstone with a basal conglomerate of schist and quartz. The Carrizo Mountain schist extends north to the gap in which the Texas & Pacific Railway crosses the range west of Van Horn. According to Baker, the schist is well exposed in Bass Canyon, north of Dalberg siding, where it strikes northwest and includes mica schist, quartz schist, and chlorite schist, with lenses and veins of quartz and dikes of dark-green basic intrusive. Some of the schists contain garnet.

The Carrizo Mountains are near the center of a large, irregular uplift in which the strata and underlying crystalline rocks are raised to their maximum elevation in Texas. In Hackett Peak, near the center of these mountains, the Carrizo Mountain schist has an elevation of 5,280 feet in an area that has been considerably denuded by erosion. The uplift is traversed by many large faults, some crossing nearly north and south and others east and west, and on its southwest side there is an overthrust of considerable amplitude. The movements have taken place at intervals from late Cretaceous to middle Tertiary time. A fault of 300 or 400 feet displacement along the west side of the Wylie Mountains (see fig. 21) and the fault at the mica mine south of Dalberg siding are good examples of north-south displacements, and there are several transverse faults west and southwest of Van Horn. The south slope of the uplift is well exhibited in the northern part of the Van Horn Mountains, in which limestones of Permian age and the overlying Cox sandstone rise gradually to the north.

---

From Collado siding there is a branch railroad south to the Microlithic quarry, 5 miles distant, where mica and feldspars of various colors are obtained from a coarse pegmatitic rock in Carrizo Mountain schist of pre-Cambrian age in the northwestern slope of the Van Horn Mountains.⁷⁷ The occurrence of this old rock at this place is due to a north-northwesterly fault that brings up a block of the schist, Van Horn sandstone (150 feet thick), and limestone of Permian age on the west against limestones of Trinity age (Lower Cretaccous) on the east. To the south from Dalberg siding and vicinity there is a good view down the wide valley between the Van Horn and Eagle Mountains to the Rio Grande, 40 miles distant. Some years ago there were in this valley at a place about 8 miles south of the railroad, some deep cracks in the ground that were attributed to an earthquake. They trended north and south and cut clays in the arroyo and gravel on the benches. From Collado siding to Hot Wells and beyond there are fine views of the high, craggy Eagle Mountain to the west, a huge pile of lavas and other volcanic rocks of Tertiary age,⁷⁸ in which the highest point is 7,510 feet above sea level, or about 3,000 feet above the valley followed by the railroad. It was in this range that after the death of Victorio at Tres Castilos, Mexico, the survivors of his band of Apaches had their last hiding place; they were finally caught by Captain Baylor and the Texas Rangers near Victorio Peak, 25 miles north of Van Horn.

---

⁷⁷The pegmatites at this place present great variety in color and composition, and some of the dikes are 100 feet wide. They range from graphic granite, an intimate intergrowth of feldspar and quartz, to crystals of feldspar and mica a foot or more in size. The feldspar varies in shade from flesh-color to pearly white. Large tabular masses of black tourmaline occur in places and some crystalline hematite. The schists are of many kinds; the prevailing type is a finely foliated aggregate of muscovite and flesh-colored or white feldspar, but some carry feldspar, biotite, and garnet. The average general dip of the foliation appears to be east, but it swings in various directions. There is considerable vein quartz, mostly milky white. (Baker.) The various minerals are separated at this quarry and used for facing concrete blocks and other construction materials.

⁷⁸The volcanic rocks of Eagle Mountain lie on an irregular platform of sandstone and limestone of Trinity age (Comanche) with Permian limestone and pre-Cambrian schist in the lower northeasterly slopes. According to Baker, these pre-Cambrian rocks are. quartz and amphibole schists, quartzites, and a very basic dark-green intrusive cut by quartz veins, one of which contains some copper minerals. The strike is N. 70° E., and the dip is steep to the south and southeast. A cross fault, trending west-southwest, cuts off the strata to the north. On the north foot of the mountain are exposed a considerable body of Trinity strata overlain by Washita and Upper Cretaceous beds.

---

At Hot Wells hot water obtained from borings 1,000 feet deep in the valley fill is used for the treatment of rheumatism and other ills. The water must come from a considerable depth, probably along a fault plane under the valley fill, which is thick in this valley.⁷⁹ Eagle Spring, at the foot of Eagle Mountain, about 5 miles west-southwest of Hot Wells, is a noted watering place for cattle, and in earlier days for travelers on the old trail.⁸⁰

---

⁷⁹The ordinary rate of temperature increase underground is 1° for every 60 feet. At most places in this region a temperature of about 140° is to be expected at a depth of about 4,000 feet. The presence of volcanic rocks not yet fully cooled, or of special conditions in the earth's crust, however, may greatly increase the rate, which is found to vary somewhat from place to place.

⁸⁰Eagle Spring is on a fault between the Campagrande formation and Finlay limestone. The Campagrande beds at this place, according to Baker, comprise about 800 feet of strata, mainly sandstones with a minor amount of conglomerate and several thin beds of limestone, one of which near the base contains the Trinity fossil Porocystis pruniformis. (For table of these formations see p. 100.) Several sills of intrusive rock are present, and a great dike of rhyolite half a mile north of Eagle Spring has changed the limestone (Permian), which it penetrates, into marble for a distance of 25 feet from the contact.

The east slope of Eagle Mountain south of Hot Wells presents the entire Trinity group lying on Carrizo Mountain schist. The Finlay limestone here forms a great platform on which is piled the thick succession of volcanic rocks constituting the upper part of Eagle Mountain. These rocks are mostly rhyolite, with some interbedded tuffs and conglomerates lying on a great thickness of tuff breccias. Bones of land tortoises occur here in rhyolite tuffs.

A sandy limestone weathering brown, about 500 feet above the base of the succession, is regarded as Finlay. It carries Exogyra quitmanensis and includes some conglomeratic sandstone and some sandy shale members. In the anticline between the overturned strata at the north end of the mountain there are ledges of dove-colored cherty limestone, then a debris-covered interval to 200 feet of dark-gray clays with interbedded limy clays, extending to nodular earthy limestones. The cap rock of the hogback ridges north of the main fault is heavy-bedded Finlay limestone, and this caps nearly all the hogback ridges extending northward to Sierra Blanca and Etholen except the main Devil Ridge. Most of these ridges are conspicuous from the railroad. The Cox sandstone appears to be absent here, or perhaps is faulted down. (Baker.)

A thin bed of coal occurs in sandstone and shale members of the Finlay limestone half a mile south of Eagle Spring, southeast of the fault. It is 3 feet thick, dips 80° N. 15° E., and has been altered to a semianthracite by the heat of a great intrusion of igneous rhyolite which has also changed some of the limestones into marble. A short distance up the arroyo, above the old coal pit, heavy beds of limestone carry Exogyra quitmanensis and Orbitulina texana. Farther up is the tuff breccia of the great volcanic mass. A mile to the northwest are exposures of shale, sandstone, and Orbitulina-bearing limestone, which crop out at intervals for 3 miles west along the south side of the fault.

At the north end of Eagle Mountain there are exposures of about 550 feet of limestones regarded as Georgetown, overlain by 100 feet of hard brown sandstone, probably Del Rio. This is capped by fine-grained chalky limestone referred to the Buda, and dark blue-gray and drab shale with some sandy and limy layers, the latter containing Inoceramus labiatus and other fossils of the Eagle Ford formation.

At Carpenter Spring, 4 miles south-southwest of Eagle Spring, considerably below the volcanic materials and probably on the other limb of a syncline, is several hundred feet of shale underlain by limestone, all believed to be Finlay. This limestone also occurs on the northeast side of Eagle Mountain, where it contains some brown sandstone and shale and has yielded, according to Baker, the fossils Orbitulina texana, Enallaster texanus, Pecten subalpina, and Tylostoma sp.

---

Near Torbert siding the valley is very wide, extending north to and beyond the Texas & Pacific Railway. It contains much grass and many yuccas. Far to the north is a long line of cliffs of limestone (Permian) forming the southern margin of the Sierra Diablo.⁸¹

---

⁸¹Just south of these cliffs is a long west-east fault south of which is exposed a thick succession of limestones and red shales of Algonkian(?) age (Millican formation) dipping steeply in various directions. Some of the relations are shown in Figure 22. This formation is believed to overlie the Carrizo Mountain schist, and it is overlain unconformably by the Van Horn sandstone (Cambrian) and the Permian limestones. These limestones also cap high mesas and buttes north of Bola siding, one of the most prominent of which is Eagle Flat Butte, just north of the Texas & Pacific Railway. Southwest of Bola siding are ridges of strata of Comanche age, mostly limestone and sandstone of the Trinity group, constituting the Devil Ridge, which parallels the railroad for 15 miles or more.

FIGURE 22.—Section from the southern edge of the Diablo Plateau to Eagle Flat, east of the Sierra Blanca, Tex. By P. B. King. K, Sandstone and limestone (Cretaceous); Cp, limestone (Permian); €vh, van Born sandstone (Cambrian); Am, Millican formation (Algonkian), including breccia (br) ad diabase (b); Ac, Carrizo Mountain schist; Q, Quaternary sand and gravel.

---

Sierra Blanca is at the junction of the Southern Pacific and Texas & Pacific lines (p. 293) and on State Highway 1, which crosses southwest Texas. In the vicinity are extensive cattle ranches, and considerable prospecting has been done on the adjoining, mountains, but without very encouraging results. The dominant feature of the landscape is the high conical mountain about 8 miles distant, called Sierra Blanca through a perversion of the Mexican name "Cerro Blanco" (white hill). Its elevation is about 6,970 feet. It consists of a huge body of rhyolite, a fine-grained, nearly white rock that has welled up as a viscous mass from a fissure in the earth's crust. The rhyolite lies on a platform of limestone of Washita age, which on the west side is penetrated by a large sill of a darker, coarser intrusive rock (trachyte), probably older than the rhyolite and similar in character to the large lens-shaped intrusive mass that uplifts limestone of Trinity age in Triple Hill. There are two other conical masses of rhyolite a short distance north and northwest of the large one, but they are of much less height.⁸²

---

⁸²The rock of Sierra Blanca and the similar smaller cones is classed as rhyolite, as it is composed mainly of quartz, orthoclase, and sodic plagioclase. The phenocrysts are orthoclase, oligoclase, and small amounts of quartz and hornblende. The intrusive trachyte is a porphyritic rock composed mainly of sodic plagioclase and hornblende with phenocrysts of oligoclase and hornblende in a groundmass of essentially the same minerals. These rocks were determined by C. S. Ross, of the United States Geological Survey.

---

Just north of Sierra Blanca is a long ridge of limestones and sandstones of Trinity age penetrated by small bodies of intrusive rocks and apparently cut off on the east side by a fault. This ridge extends north to Triple Hill.

South of Sierra Blanca are other high ridges of the same limestones and sandstones.⁸³

---

⁸³High on the ridge 3 miles southwest of Sierra Blanca the limestones contain an abundance of Orbitolina texana, a a small disk-shaped fossil characteristic of the upper part of the Trinity group. A mile southwest of Sierra Blanca the limestone has been metamorphosed by intrusive sills, and secondary minerals, such as garnet, hematite, and specularite, have been developed. The strata (Cox) underlying the limestones (Finlay) in the hogback ridges in this vicinity are light green-brown medium-grained sandstones with thin limestone members at intervals, passing into medium-bedded gray limestones with interbedded greenish sandstones, in all 750 feet thick. In the large ridge 3 to 5 miles south of Sierra Blanca the cap of Finlay limestone is underlain by 1,000 feet of limestones, sandstones, and conglomerates, some thin bedded, of the Cox formation. At the northeast base of the ridge just south of Sierra Blanca are sandstones with a few interbedded limestones, all Cox. (Baker.)

The large mesa 5 miles south of Etholen is capped by massive limestone (Finlay) containing Orbitolina texana, and about 200 feet above the base of this limestone is an interbedded brown sandstone with green chert and silicified wood near its base. Bluff Mesa, to the east of this feature, is also capped by heavy limestone (basal Finlay) about 200 feet thick, with a thick breccia at the top containing Exogyra quitmanensis. Other fossils in this limestone, according to Baker, are Orbitolina texana, Pecten, Pholadomya and Alectryonia.

---

There has been difficulty in obtaining an adequate water supply for Sierra Blanca, but in 1930 a well about a mile east of the town struck water, apparently either in a sandstone low in the Trinity group or coming from a fault fissure that appears to cross the valley in that vicinity. About 31 miles to the south, on the bank of the Rio Grande, are the Indian Hot Springs, a noted resort which utilizes the hot water for remedial purposes.

Near Etholen siding there are many interesting geologic features. Not far north is the huge cone of the Sierra Blanca, rising nearly 2,200 feet above the adjoining valley. Near its south slope passes a west-northwest fault, on the south or upthrown side of which limestone of Permian age abuts against strata of the Washita group, which constitute the platform on which the mass of Sierra Blanca rises. In the gap on the north side of the peak a large variety of interesting fossils have been found in these strata.

The Quitman Mountains are very conspicuous southwest of the railroad near Etholen. They consist of a mass of intrusive granite at the north and south ends and a huge central intrusion of quartz syenite. These rocks are cut by dikes of diabase and augite porphyry. The granite and syenite intrusions have lifted and deformed the Cretaceous strata over an area of considerable extent. A most interesting example of the alteration of sedimentary rocks by an igneous intrusive mass is presented on the north side of the Quitman Mountains 5 miles south-southwest of Etholen siding. Here part of the limestone (Finlay) has been changed to marble by the heat of the intrusive quartz syenite, and a great zone of garnet (grossularite) has been developed at the contact. There is also considerable vesuvianite and actmolite. Sandy beds are altered to hornfels, and much silica has been deposited, together with iron, manganese, copper, zinc, and silver minerals. Many of these minerals are incrusted with chalcedony. The principal materials added to the sedimentary rocks are silica and iron oxide, much of the latter in the form of hematite. There are many mineral deposits in the altered rocks near the Quitman Mountains which have been extensively prospected but have not yet developed economic importance. Besides iron, lead, copper, and silver, small amounts of nickel, tungsten, uranium, gold, and molybdenum have been reported.

FIGURE 23.—Section from the north end of the Quitman Mountains to the east slope of the Sierra Blanca, Tex., crossing the railroad just west of Etholen station. After C. L. Baker. Kdr, Del Rio formation (Cretaceous)

In the small ridge west of Etholen are limestones and shales of upper Washita age (probably Del Rio) carrying "Nodosaria" texana. Southwest of the siding is a high rough ridge of a syenitic intrusive rock that also constitutes the north end of the Quitman Mountains, just beyond. In these ranges are the Bonanza and Alice Ray mines, formerly productive. Nearly all the surfaces in this general region are bare and rocky, and the vegetation has the wide spacing characteristic of arid regions. In the valleys and on gentler slopes, however, there is considerable grass and other forage for stock.

The knobs just northwest of Etholen consist of a conglomerate (probably the same as the Campagrande formation of Richardson, the basal formation of the Comanche series), and this rock also occurs in near-by hills south of the railroad, in the small hill three-quarters of a mile east of the siding, and at some other localities in the same general region. It consists of coarse sandstone and conglomerate of brown color and represents an old beach or river deposit. Underlying limestone of Permian age appears on the north side of the hills 2 miles north of Etholen, but the contact is covered by talus. (Turn to sheet 17.)