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

SHEET No. 9 (click in image for an enlargement in a new window)

West of the San Marcos River the railroad passes over a wide lowland of the Indio formation to a point beyond Sullivan siding, where it ascends about 100 feet onto a high terrace. This terrace is underlain by the Indio formation but covered and preserved by a compact deposit of gravel and sand carrying much chert evidently derived from the Edwards Plateau. This deposit extends north of the railroad for some distance as a cap on the Mill Creek Hills and undoubtedly was originally deposited by an earlier San Marcos River in late Tertiary time.⁴³ In the descent off this terrace west of Kingsbury there are exposures of the Indio formation, mostly soft sandstone and a few hard layers. Beyond this down grade is a wide lowland extending to Seguin (say-gheen').

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⁴³Most of the interstream divides on the higher portion of the Coastal Plain are capped by gravel deposits of this character. They are remnants of an old surface, called the Uvalde plain because it is very extensive in the Uvalde region. This plain has been largely removed by streams, which in general have cut valleys 200 feet or more deep, many of them along courses differing materially from the drainage ways that crossed the old Uvalde surface. Notable remnants of this plain remain about San Antonio and south and east of New Braunfels, San Marcos, and Austin.

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Seguin is on the alluvial plain of the Guadalupe River, here underlain by lower shaly beds of the Indio formation. It is a prosperous town, with cotton mill, sugar refinery, and various other industries. Water power is generated from the Guadalupe River a short distance north of the town. Seguin was founded while Texas was a republic and was named from two Spanish settlers who lived in the vicinity during the stirring days of the Texas revolution. One was Don Erasmo Seguín, of San Antonio de Bexar (bay'har), who had a share in creating the constitution of Texas, and the other Juan Seguín, who commanded a small body of Mexicans who fought effectively under General Houston in the battle of San Jacinto.

A gravel pit about 2 miles west of Seguin, south of the railroad, shows about 10 feet of gravel grading up into sand, the upper part of which consists of caliche. This material occurs in most parts of the West and is frequently mistaken for stratified limestone. Much of it is hard and white or nearly white, and it consists largely of calcium carbonate with a greater or less admixture of sand. It owes its origin to water rising through the porous materials by capillary attraction and depositing calcium carbonate on evaporation at and near the surface. Doubtless the rate of accumulation is very slow, and ordinarily the caliche or at least the thicker bodies of it are found only on old plains or terraces in regions of low precipitation. The thickness ranges from a few inches to 10 feet or more. The pit west of Seguin gives a fine exhibit of this material and its relations to the gravel and sand.

Just west of Seguin the Midway group,⁴⁴ or base of the Tertiary system, is brought to the surface from under the Indio formation by the regular rise of the strata to the west. It consists of shale with thin layers of sandstone and clay and is sufficiently hard to constitute ridges in places along its outcrop zone. Such ridges attain considerable prominence in the Mill Creek Hills, west of Seguin, which, however, are gravel-capped, and in the hills that extend southwestward to San Antonio.

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⁴⁴The Midway consists, for the most part, of dark-gray joint clays with associated cannon-ball, turtle-back, or cone-in-cone concretions and locally carries a microfauna. No large fossils have been recorded from the upper Midway clays of this area. The thickness is between 250 and 300 feet. In Guadalupe County the lower Midway strata crop out characteristically only along a slender faulted tongue southwest of Staples, on the San Marcos River. Most of the Midway surface material is a very heavy dark-gray clay which weathers into a soil indistinguishable from the river silts, and the usual vegetation is stunted mesquite. Guadalupe and Bexar Counties must have formed a synclinal basin during the early Eocene, for marine conditions apparently persisted, at least intermittently, well into Indio time. The waters were for the most part shallow, for Ostrea and Cerithium are the most common species at the lower Indio outcrops. Even these, however, record the continuance of the general marine conditions of Midway time and indicate that there was a near-by retreat for the very considerable number of Midway species that persisted into Wilcox time.

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The contact between the Indio and Midway formations on the northwestern margin of Seguin and that between the Midway formation and the Cretaceous strata, 3 to 4 miles to the west, are covered by alluvium in the lowlands along the Guadalupe River. There is, however, a good outcrop of a fossiliferous and ferruginous sandstone of a horizon high in the Midway near the old ferry 2-1/2 miles above the Seguin power house.

The Guadalupe River, which is crossed just east of Hilda station (McQueeney village), is a large and beautiful stream that drains a broad area of the Edwards Plateau (p. 74) and south-central Texas and empties into the Gulf of Mexico at the head of San Antonio Bay. Although there is a wide alluvial plain adjoining the river, formations of Upper Cretaceous age are extensively exposed at intervals in its banks and adjoining slopes. The uppermost of these is the Navarro, which consists almost entirely of dark clay from 400 to 500 feet thick, with some beds of sand and layers of calcareous concretionary sandstone and impure limestone. In this general region the Navarro formation dips somewhat less than 1° E. The clay is quarried extensively in pits near the river bank about a mile south of the bridge across the Guadalupe River at the village of McQueeney and is made into brick and tile used in San Antonio and other places. Ordinarily in the manufacture of such products a pure clay, which melts at a moderate temperature, has to be tempered by mixing with sand, but at this place there is sufficient sand or sand admixture to afford suitable composition to withstand the requisite heating in the kilns. The exposure comprises a bank about 100 feet high and a pit 50 feet deep. Fossils⁴⁵ are abundant in the lower part of the pit.

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⁴⁵A few of the better-known fossils that occur in the Navarro formation in this area are Leda longifrons, Gryphaea mutabilis, Exogyra costata (variety with narrow costae), Gryphaeostrea vomer, Pecten argillensis, Lima acutilineata, Pulvinites argentea, Crenella serica, Liopistha protexta, Veniella conradi, Cyprimeria alta, Legumen ellipticum, Gyrodes petrosus, and Sphenodiscus (two or more species). (L. W. Stephenson.)

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From the Guadalupe River to San Antonio the railroad traverses a region of slightly rolling plains of low relief developed in the soft clays of the Navarro and Taylor formations, here about 1,300 feet thick and dipping gently to the southeast. Outcrops are few and small, for soil and superficial materials cover most of the surface, and much of the land is under cultivation. The mesquite is very prominent in the untiled fields. Southeast of Schertz a large gravel pit in the alluvial deposits on the Cibolo River is visible from the railroad. To the west is a hilly country of Austin chalk, and beyond is the highland known as the Balcones scarp (bal-co'nace), formed of the hard limestones of the Lower Cretaceous (Comanche series), which there rise to the surface on the general southeast dip of all the strata of the Coastal Plain region, the uplift increased in places by faulting. The Upper Cretaceous strata (Gulf series) crop out in a wide zone along the inner margin of the Coastal Plain, which extends through Fort Worth, Dallas, Waco, Austin, and San Antonio. They are underlain by the formations of the Comanche series, which constitute the high uplands to the west and north. The table on page 65 shows the general succession and principal features.

Cretaceous formations of the Coastal Plain in central Texas

The Cibolo River (Spanish, buffalo), a small stream crossed at Schertz, drains portion of the Edwards Plateau west of San Antonio. On the plain to the west, south of the railroad, is the great aviation establishment of the United States Army, known as Randolph Field. It was completed in 1931, with an area of 2,320 acres. Its capacity is 4,000 men. The high tower of its central administration building is conspicuous from afar, as are also the great hangars with checkerboard roofs. This establishment cost $25,000,000; the site was presented to the Federal Government by the city of San Antonio. The smooth, broad plain at this place has been developed on the soft clay of the Taylor formation.

Not far beyond the small village of Converse some of the high buildings of San Antonio are visible in the distance, and beyond Kirby siding part of Fort Sam Houston is in sight on a high terrace north of the railroad.

San Antonio is a metropolitan center for a wide area in south-central Texas and until a few years ago was the largest city in the State. Its growth in population from 1920 to 1930 was 43.5 per cent. It has numerous manufactures, large educational institutions, and a variety of business interests. It is an old settlement dating back to Spanish mission days and, as San Antonio de Bexar, was long the capital of Tejas in New Spain under Spanish and Mexican rule, with a history marked by many sanguinary episodes.

It is a city of much charm, combining the old with the new in a setting of natural beauty. The San Antonio River⁴⁶ winds through the city with many curves and is crossed by bridges that afford pleasing glimpses of its greenswarded banks, even in the heart of the business district. There are many large edifices of architectural merit, and a fine municipal auditorium that seats 6,500 and cost $1,500,000. The mild winter climate is an attractive feature. Excellent water for domestic use and for many manufacturing establishments is supplied by artesian wells 900 to 1,200 feet deep from strata in the Coastal Plain sediments. According to the local chamber of commerce, the output of its 1,175 factories had a value of $85,000,000 in 1930. They have the advantage of cheap natural gas, oil, and lignite for fuel. San Antonio has many churches and clubs, several libraries and theaters, and many educational institutions. A women's college, Our Lady of the Lake, on the western edge of the city, is visible from the railroad a few minutes after leaving the depot.

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⁴⁶This river, the outlet for the drainage of an extensive hilly area northwest of the city, sometimes has freshets which on some occasions have done considerable damage. To prevent these floods the great Olmos Dam has been built across the valley. Occasionally there are cloudbursts in this region and other parts of the West in which a large amount of rain falls in a very short time. There is a record of a succession of these at Taylor, Tex., on Sept. 9-10, 1921, in which 23.11 inches of rain fell.

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On the outskirts of the city to the north is Fort Sam Houston, an Army post of 4,378 men and 211 officers, and to the south are extensive Army flying fields and schools. In Brackenridge Park, in the valley of the San Antonio River, are numerous features of interest, including a large zoological collection.

Although San Antonio has a large proportion of sunshiny days, its precipitation averages about 27 inches a year, or about the same as in much of the west-central United States. This is usually sufficient to produce fair crops and excellent forage, but there is considerable irrigation from ditches and from artesian and pumped wells. From 45 years of observation by the United States Weather Bureau it has been found that the mean annual temperature is 69°, the winters averaging about 60° and the summers 80°, and the average humidity is 68 per cent. The springs and autumns are long, but the summer heat at most times is tempered by breezes and low humidity.

San Antonio probably owes its origin to two great springs, with a total average volume of about 58,000,000 gallons a day,⁴⁷ that supply the flow in the San Antonio and San Pedro Rivers. It is believed that the water is derived from the Edwards limestone along a fault.

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⁴⁷See Meinser, O. E., Large springs in the United States: U. S. Geol. Survey Water-Supply Paper 557, pp. 37-38, 1927.

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Probably the first Spanish visitor was Cabeza de Vaca, who crossed central Texas about 1535. In 1718 a garrison was stationed here, and a mission was moved from the Rio Grande and renamed San Antonio de Valero in honor of the Viceroy of New Spain. In 1730 a presidio was erected here, and early in 1731 a colony of 16 Spanish families from the Canary Islands sent out by the King of Spain came through Mexico and established themselves with a few local people at the springs of the San Antonio River under the name San Fernando.

This was finally merged into the presidio of San Antonio de Bexar and for many years called simply Bexar. Three years later this settlement was made the seat of government for the general region. In 1821 it passed into the possession of Mexico when she became an independent nation. In 1835-36, in the revolt against Mexico, San Antonio was a center of strife, culminating in the siege of the Alamo in February of that year. Many of the old-time buildings remain, including the Alamo, the 200-year-old palace of the Spanish governor on the west side of Military Plaza (pl. 7, B), San Fernando Cathedral, and several old missions. There is a large Mexican population, part of it in an extensive Mexican quarter south and west of the old plaza.

PLATE 7. A (top), THE ALAMO, SAN ANTONIO, TEX. Here in 1836 a band of 182 Texans were beseigned by the Mexican Army. B (bottom), PALACE OF THE SPANISH GOVERNOR, MILITARY PLAZA, SAN ANTONIO, TEX. Recently restored.

San Antonio was an important station on the old Camino Real from Monclova, Mexico, which crossed the Rio Grande below Eagle Pass. The mission San Francisco de la Espada, just south of the town, was built on this road. From San Antonio it led north and east to the vicinity of Natchitoches and thence to New Orleans.

The most notable feature in San Antonio is the famous Alamo (pl. 7, A), where 182 heroes, nearly all of them volunteers from different parts of the United States, chose to die rather than to surrender to twenty times their number of Mexican soldiers under General Santa Ana. At that time San Antonio was on the southeastern bank of the San Antonio River and consisted of well-fortified houses in a rectangle; on the opposite bank was the walled inclosure of the Alamo. The assaults lasted from February 23 to March 6, 1836, when the Mexicans overwhelmed the defenders, all of whom were killed. Now the Alamo is a museum exhibiting many relics of the glorious past of Texas and bearing this stirring inscription: "Thermopylae had its messenger of defeat—the Alamo had none." It was the war cry "Remember the Alamo" that spurred the Texans to victory at San Jacinto a few weeks later. (See p. 44.) Tablets near by mark the sites of the funeral pyres of the Alamo heroes. Though later used as a military post, the Alamo⁴⁸ was apparently first a chapel (established on a new site in 1744) of the mission of San Antonio de Valero (1718), the first of several missions established by Franciscan friars in the general vicinity.

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⁴⁸The word álamo is Spanish for cottonwood tree, but it is considered likely that the name was applied to the chapel after its occupation by a company of Mexican troops known as the "Alamo de Parras."

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Mission San José de Aguayo, founded in 1720 by Padre Antonio Margil and named in honor of the Marquis Miguel de Aguayo, governor of Tejas, is 6 miles south of the center of San Antonio. Its south window, a fine example of stone carving, was exhibited at the World's Fair in St. Louis in 1904. Its beautiful old altar, ornamented by a noted Spanish sculptor, is now in the Cathedral of San Fernando, on the east side of the Military Plaza. This cathedral was begun in 1734, completed by a grant from King Ferdinand of Spain in 1744, and reconstructed in 1868. The mission La Purísima Concepción de Acu&ntidle;a, 2 miles south of the center of the city, originally established in eastern Texas, was moved to San Antonio in 1731 and is still in use. Near it in 1835 James Bowie and a party of 92 Texans won a fight against five times their number of Mexicans. Bowie died in 1836 in the defense of the Alamo. The mission San Juan de Capistrano, which was established in 1731 near the San Juan ford of the San Antonio River, was also formerly near Nacogdoches. The mission San Francisco de la Espada was originally in eastern Texas, having been the first mission established there. Founded in 1690 under the name San Francisco de los Tejas, it was abandoned three years later, reestablished in 1716 under the name San Francisco de los Neches, and transferred in 1731 to the west bank of the San Antonio River, 9 miles south of the center of the city. Aqueducts built by monks and Indians two centuries ago still irrigate the gardens at this place.

The older part of San Antonio is built on a plain of alluvial deposits, but the northern, western, and eastern parts extend onto rolling hills of Upper Cretaceous rocks. A fault with downthrow on the east side passes through the northwestern part of the city. On its west side are hills of Austin chalk, consisting largely of a soft chalky limestone⁴⁹ which has been quarried extensively for use in building, especially for houses in the older part of the city. Exposures of this material extend through part of Brackenridge Park, notably near the Sunken Gardens and Monkey Island, and in the rolling hills to the northwest, but much of the rock weathers into soil on the sloping surfaces. To the west this formation dips beneath the clays of the Taylor formation,⁵⁰ on which the western part of the city is built.

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⁴⁹This chalky material contains many shells of Foraminifera, minute organisms that lived in the sea water that covered this area during most of Cretaceous time. There are also many shells, including Gryphaea aucella, Mortoniceras texanum, Inoceramus undulatoplicatus, Exogyra ponderosa (upper beds), and other marine species. A 1-foot layer composed of the shells of Gryphaea and Aucella is conspicuous along the north edge of San Pedro Park. (See pl. 8, A.)

⁵⁰This formation, which is about 475 feet thick, contains many fossils including various oysters such as Exogyra ponderosa, E. laeviuscula, and Ostrea aff. O. diluviana.

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In the slopes of the gravel-capped ridges extending south from Fort Sam Houston there are exposures of clay of the Navarro formation, which extends southward to an overlap of clay and sand of the Midway formation of Tertiary age.

The most notable physiographic features in the San Antonio region are the wide plains and terraces which have been developed by erosion and deposition by streams on the surface of the soft clays of the Tertiary and Cretaceous formations. The largest plain, which lies a short distance above the narrow alluvial strips bordering the streams, extends from the San Antonio River to León Creek and is occupied by Kelly Field and other aviation stations. It is covered in greater part by a sheet of gravel and loam, mostly from 10 to 20 feet thick, and has an elevation of 600 to 700 feet, with gentle slope to the south. A smaller but similar plain lies between Salado (sa-lah'do) and Rosillo (ro-see'yo) Creeks, east of the city. The highest plain, which occupies the ridge between the valleys of Salado Creek and the San Antonio River and is about 100 feet higher than the adjoining area, represents the Uvalde Plain referred to on page 62 (footnote). It is capped by a sheet of gravel and loam which is well exposed in the long cut of the Missouri-Kansas-Texas Railroad in the southeastern part of the city, as shown in Plate 8, B, and Fort Sam Houston is also built on its smooth surface. There are many outliers of this plain farther north and west.

PLATE 8. A (top), AUSTIN CHALK IN SAN PEDRO PARK, SAN ANTONIO, TEX. The thick hard layer in the middle consists largely of Gryphaea aucella, an oyster reef of Comanche time (Lower Cretaceous). (Stephenson.) B (bottom, QUATERNARY AND EOCENE DEPOSITS IN CUT OF MISSOURI-KANSAS-TEXAS RAILROAD, IN SOUTHEASTERN PART OF SAN ANTONIO, TEX. White caliche, 10 feet, underlain by gravel, 10 feet, and Midway shaly clay, 8 feet. (Stephenson.)

Not far north of San Antonio there are excellent exposures of slabby Eagle Ford limestone, Buda limestone, and Del Rio yellow clays with abundant Exogyra arietina, and in the hills of the Edwards Plateau, of Georgetown, Edwards, and Glen Rose limestones, which carry many distinctive fossils.

Westward from San Antonio the railroad goes south for 1-1/2 miles and then, turning abruptly west, crosses the San Antonio River and the wide alluvial plain and skirts the east side of Kelly Field. This plain is wide and level because it is developed on the soft clays of the Upper Cretaceous. It is capped by a sheet of alluvial loam deposited by Leon Creek and other streams in relatively recent geologic time. Leon Creek is crossed just beyond Leon siding, and the low rolling hills of the Midway formation capped by gravel of the higher terrace level are traversed between Leon Creek and Medio Creek.⁵¹

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⁵¹In the bank of Leon Creek about a mile north of the railroad is an excellent exposure of the unconformable contact of the Midway on Navarro sandy clay or Escondido (p. 71), a relation which is revealed at intervals up the creek to the fault that crosses the stream about 4 miles above the railroad.

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A mile west of Leon Creek the railroad bends around the south end of a ridge, showing ledges of buff sandstone of the Midway formation, which with low dip to the west passes under the Indio formation at Medio Creek. All the lower lands from this point westward past Macdona and Lacoste (turn to sheet 10) are underlain by alluvial sand and gravel deposited by the Medina River, which is crossed a mile east of Macdona. This stream rises in the Edwards Plateau, and at the head of a deep canyon about 25 miles west-northwest of San Antonio it is dammed to make Lake Medina, a large storage reservoir and an attractive resort. The dam is 164 feet high and 1,580 feet long. The water diverted into a canal some distance below the dam is carried along the ridge west of Castroville to be used for irrigation in the region south. This canal is crossed by the railroad at Pearson siding, 4 miles west of Lacoste. The Medina River along the railroad carries but little water because of the dam 15 miles above that diverts most of the flow into the irrigation canal. In an exposure of the Escondido formation in a cut about 5 miles west of Macdona there is yellow limestone at the top, 2 feet, with a thin fossiliferous layer; yellow shale, 5 feet; yellow impure sandstone, 4 inches; and yellow-brown shale, 3 feet to the base. The beds dip about 1° SW. The railroad is deflected to the south to carry it around the south end of a high ridge of Escondido shale which extends along the west side of the Medina River Valley in the Castroville region,⁵² north of the railroad. The higher part of this ridge is capped by gravel of a high-level terrace, the Uvalde Plain, of which there are many remnants in the region west of San Antonio.

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⁵²Castroville was founded by and named for Count Henri de Castro, who in 1844 brought there a colony of French and Alsatians. The architecture is largely of French rural type, with sloping roofs, small windows, and batten blinds.

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