DESCRIPTION OF THE AUSTIN QUADRANGLE

By Robert T. Hill and T. Waylaiid Yaughan.

GEOGRAPHY. with the outcrop of these formations as shown on thicket of live-oak and post-oak brush and timber. spots of forest, growing upon the Eagle Ford and the geologic map. The slopes of the hills are terraced or benched by Bud a formations, the former bearing mostly hack- Geographic position and relations. The Austin the outcrops of the subhorizontal strata. Streams, berry and the latter live oak, as seen in the Sixth TWO TYPES OF TOPOGRAPHY. quadrangle embraces one-quarter of a square especially the Colorado, have cut deep valleys into Ward of the city of Austin and along the eastern degree of the earth's surface, and contains 1029.84 Notwithstanding the partial destruction of this plateau. (See fig. 2.) These valleys, imme­ bank of Shoal Creek. West of these areas are square miles. It is bounded by parallels 30° and original surfaces by erosion, causing the present diately adjacent to their courses, are bordered by small spots of prairie land, with black soil, under­ 30° 30' N., and meridians 97° 30' and 98° W. diversified relief, it is evident that the general steep bluffs of banded yellow or white limestones lain by the greenish-yellow subsoils of the Del Rio The adjacent quadrangles that have been topo­ configuration of the area is that of a greatly of great beauty, crowned above by terraced strati­ clay and covered with a thin growth of mesquite graphically mapped are the Bastrop on the east, dissected plain, which, leading from the Cordil- fication slopes leading to the summits. The terraces bushes. The surfaces of these small areas are Georgetown on the north, and Blanco on the west. leran region to the sea, has been termed the of these hills are often covered with a growth of also marked by hogwallows. The various fea­ To the northeast is the Taylor quadrangle, to the Regional Coastward Slope. This broader feature jumper and Texas laurel (Sophora), while the tures of this belt are complicated, owing to the northwest the Burnet, and to the southeast the is sharply divisible, within the area of the quad­ high summits are usually covered with a dense excessive "faulting along their area of occurrence. .Flatonia. The Austin quadrangle lies southeast rangle, into two types of country a higher dis­ growth of scrub oak and live oak. That portion North of the Colorado the belt narrows to less of the center of the State, at the interior border trict in the western third which is a part of the of the Edwards Plateau which is north of the than half a mile in width, or even is absent in physiographic province of Colorado has been termed the Lampasas Cut places. A unique feature of the Manchaca belt the Texas region known as Plain. are the cliffs of Buda limestone seen along the the Edwards Plateau, and a creek valleys. COASTAL PLAIN. lower district to the east White rock country. A broad belt of country which | ph ysi ographically The area of that part of the quadrangle which extending north and south through the center of belongs to the interior is situated to the east of the Balcones scarp, the quadrangle, marked by the outcrop of the margin of the great A tlantic although presenting within itself several diverse Austin chalk, may be known as the White Rock Coastal Plain. (For a de­ features, constitutes a portion of the great physio­ country. The formation is especially well exposed tailed description of the graphic feature known as the Atlantic Coastal in Fiskville, Austin, South Austin, Manchaca physiography of the Texas Plain. This portion of the plain represents its Springs, and Mountain City. This belt is a dis­ region see Topographic older, higher, and more eroded interior margin, trict of gently rolling, hilly land like the English folio No. 3, U. S. Geological and in general, except along the southeastern downs, and is characterized by glaring white Survey, 1900.) margin and western border, is largely a region of outcrops of the Austin chalk on the slopes and in undulating upland prairie, mostly of the type the bluffs of the creeks. Graceful clumps of live EDWARDS PLATEAU. which is known in Texas as rolling prairie land oak and sometimes groves of jumper are found. Balcones scarp. A note­ and which has been called the Black Prairie. The residual soil of this belt, which is usually worthy topographic feature The highest points of this plain rarely rise over thin, is black and calcareous. of the quadrangle is a high 750 feet above the sea. This feature is subdivis­ Taylor Prairie. Immediately east of the White escarpment along the border ible, in the Austin quadrangle, into several dis­ Rock belt is the main district of the Black Prairie between the Edwards Pla­ tinct northeast-southwest belts of country, which lands of the country underlain by the Taylor and teau and the Rio Grande may be enumerated, in the order of their sequence Webberville formations. This is an area of rolling Plain. It runs northeast and from west to east, as follows: Bear Creek coun­ prairie land, consisting of a deep mantle of black Fig. 1. Provinces and minor subdivisions of the Texas region. southwest from the vicinity try, Manchaca belt, White Rock country, Taylor waxy soil which is exceedingly fertile. The west­ of the Coastal Plain and at the junction of the of McNeil to near Driftwood post-office (Blanco Prairie, Littig Prairie, and Lytton Springs coun­ ern portion is called the Taylor Prairie. Manor Central, East Central, Southern, and Great Plains quadrangle), passing by the eastern foot of Mount try. Besides the above. there are exceptional and Creedmoor are situated on its eastern edge; provinces of the Texas region. (See fig. 1.) It Bonnel and Oatmanville, and separates the two minor features within the area of the Coastal Pflugerville and Manchaca Springs are near its embraces the greater portion of Travis County major provinces mentioned. This escarpment is Plain which will be described as alluvial plains western edge. and parts of William son, Hays, Bastrop, and Cald- called the Balcones, and in its extension south- and terraces and volcanic hills. Littig Prairie. In the extreme eastern part well counties. westward to the Rio Grande is a structural fea­ Bear Greek country. Immediately adjacent to of the quadrangle, to the east of Manor and ture known as a fault scarp. It rises from 100 to the foot of the Balcones fault and extending Creedmoor, and extending to Texas Hill, there is TOPOGEAPSY. 300 feet above the lower country of the Coastal approximately eastward toward the International an area marked by a rather sterile, sandy, clay GENEKAL FEATUBES. Plain, and properly belongs to the Edwards Pla­ and Great Northern Railway is a narrow plain of soil and covered by a growth of chaparral, large In general the topography of the quadrangle is teau. Its front is not a vertical cliff, as one brush-covered country which ranges in altitude opuntias, and mesquite. This country, known as varied and picturesque, consisting of a diversified might infer from a too literal interpretation of from TOO to 900 feet. This country corresponds the Littig Prairie, is underlain by the Webber- landscape of rugged hills, rolling plains, and level the word scarp, but rather an indented line of with the outcrop of the geologic formations else­ ville formation, so far as can be determined, thinly areas, broken by frequent streams and presenting sloping hills leading up from a lower plain to a where described as the Edwards and George­ veneered by Pleistocene surficial deposits. a pleasing alternation of timber and prairie. The plateau summit, as seen along the eastern front of town limestones, shown on the map east of the Lytton Springs country. The area covered by highest altitudes of the quadrangle are found the ridge northwest of Austin. Balcones fault. Owing to the stony character this division lies in the southeast corner of the along the western margin, the greatest upland The country west of the Balcones scarp line, of the surface, the shallowness of the soil, and the quadrangle and extends from one and one-half altitude being 1200 feet, at the extreme northwest which is locally known as "the mountains," con­ thick growth of brush, the region has been locally miles northwest of Lytton Springs to near Ely­ corner, and the least about 600 feet, along the sists of bold hills, many rising 500 feet above the termed "hard scrabble." This plain is cut in sium. This area consists of rather low, gently eastern margin. The valley depressions as meas­ drainage valleys, and some of them so symmetrical places by steep creek gorges, while its surface is rounded hills having an altitude of from 400 to ured along the Colorado vary from 600 feet at in contour and stratification that they appear as usually broken by projecting limestone ledges and 650 feet above the sea. The soil is sandy and the northwestern edge of the quadrangle to if they had been turned in a lathe. The summits covered with a thick growth of post oak and live subject to rapid erosion. The timber consists 375 feet where the Colorado leaves the eastern of the highest of these, hills, which are usually oak. In some places the surface is covered with mostly of post oak and other trees, such as char­ border. flat topped and composed of a single geologic for­ vast numbers of oxidized residual flints, which are acterize the southern extension of the East Texas mation the Edwards limestone -have a nearly suggestive of alluvial deposits, but which are the timber belt, of which it is a part. RELATION BETWEEN FORMATION AND RELIEF. constant topographic level. These hills are the residua of the flint-bearing layers of the under­ Alluvial plains and terraces. Extending along Nowhere is there a more intimate relation remnants of a vast dissected plateau or cut plain, lying Edwards limestone. The hard scrabble the margins of the Colorado and the larger creeks between topography and stratigraphy than in the and stretch across Texas from the Brazos to the belt as a whole is a downfalien fault block. are wide valleys of alluvial bottom land, subject

CEDAR VALLEY PLATEAU JOLLYVILLE PLATEAU Ke Ke K£,~ ____ Kr.n-.iv _ Ke Kcpw COLORADO RIVER

400 feet above sea level 400 feet above sea level

Fig. 2. Northeast-southwest section crossing the Colorado River 8 miles west of Austin, showing the dissection of the Edwards Plateau by the Colorado River and its tributaries, and the benches formed by the harder beds. Horizontal scale, 1 inch = 1| miles; vertical scale, 1 inch = 2250 feet. Natural profile is shown in shaded drawing below section.

Texas region, and the Austin quadrangle is a Rio Grande. In the Austin quadrangle the rem­ While not well adapted to agriculture, this por­ to overflow; above these there is usually a series very fine illustration of this relation. In fact, nants of this .plateau stand at altitudes of from tion of the country is utilized for grazing pur­ of more or less connected terraces, now standing nearly all the natural features, especially those 1100 feet along the western border to poses, and contains an occasional small area of above the line of overflow; and still higher, upon closely related to human activities, such as topog­ about 975 feet at the edge of the escarp- sect^d Pia-" tillable land. some of the divides, there are areas of gravelly raphy, soil, flora, and the occurrence of structural ment, having a slope of about 20 feet Manchaca belt. Immediately east of the Bear soil. All of these deposits are composed of allu­ material and underground wTater, are determined to the mile as measured along a westward line Creek belt is another narrow strip of exceptional vial material which has been brought down by or influenced by the geologic formations. Each from the Lone Tree triangulation station. In country, underlain by the Del Rio, Buda, and the streams. of the geographic subdivisions is the superficial places, such as the area termed the Jollyville Eagle Ford formations. (See Historical Geology The Colorado, being the oldest and most deeply expression of some of the geologic formations to Plateau on the map, considerable .remnants of the sheet.) This consists of alternations of wooded incised stream, has more extensive bottom lands be described later, and is coincident in extent old plateau exist. These are covered with a dense hills and prairies. Along its eastern margin are and more numerous higher terraces;1 The latter are

\fUfci^^ somewhat similar in character of soil and vegeta­ and intermittent. The Colorado is perennial or In the extreme southwest corner of the quad­ farms; and country roads, originally made by wood tion, being usually composed of red loam and constant in flow, although variable in volume, rangle is found a small section of the Blanco cutters, which meander through the region of the gravel derived from the central provinces of the being subject to great floods. Other streams, like River, a tributary of the San Marcos, which Edwards Plateau. Many of the first-class high­ State and covered by a growth of post-oak timber. Barton and Onion creeks, are interrupted, present­ enters and leaves the quadrangle from the south. ways are macadamized in places, and commendable The bottoms and terraces of the larger creeks are ing alternate stretches of running water and dry The portion of this stream here shown is embedded interest has been taken by the people of Travis1 usually composed of calcareous soils and debris bottom. Still others, such as the secondary in a steep and almost impassable vertical canyon County in their improvement. Three substantial derived from the Cretaceous formations of the streams of the eastern margin, are intermittent, nearly 300 feet deep. bridges span the Colorado, the westernmost of adjacent Edwards Plateau. having no water except in time of rainfall. The secondary streams in the eastern half of which is devoted solely to railway traffic, and the The old terraces may be divided into two The streamways have certain distinctive char­ the quadrangle, such as Cottonwood, Wilbarger, others, at Austin and Montopolis, to vehicles. classes: (1) high gravel terraces, which rise from acters in each of the two greater topographic Gilleland, Walnut, Cedar, and Mahard, are usu­ It is interesting to note on the map how the 100 to 200 feet above the level of the streams, divisions. In the Edwards Plateau they are ally dry arroyos which have an occasional pool of courses of the principal railways and highways and which are now largely of a stony nature; usually encased in deep and narrow canyons, like standing water and are incased in valleys with are governed by the geologic structure. (2) wide, flat terraces elevated only from 40 to 50 those of the Colorado and Barton Creek west of rounded clay or gravel slopes. feet above the streams, usually of good soil and Austin. In the Coastal Plain the valleys are These stream courses show peculiar and inter­ FOREST AND WOODLAND. devoted to agriculture. These terraces, which have usually broad and terraced by older alluvial plains esting adaptation to the geologic structure and In the Austin quadrangle there is a diversified their greatest development within the Coastal leading up to the rounded divides described under changesO7 of lev:l, elsewhere^ discussed in Antecedent. , . , flora, which, like other features, varies with the Plain east of the Balcones scarp line, increase in a separate head. this paper. In technical terms the qSentonse° underlying geologic formations. The highest area down the stream and in places attain a width It is interesting to note that the courses of the Colorado River is antecedent to the streams- summits of the Edwards Plateau, where patches of 5 miles or more. larger streams are sinuous in all the topographic Edwards Plateau and consequent to the Coastal of the Edwards limestone are ^preserved, are Westward, in the Edwards Plateau country, divisions. Thus the Colorado River flows in great Plain. Barton Creek and the Blanco relative to covered with a thick growth of ptist<>ak, live-oak, the terraces narrow in width and are reduced in sweeping curves, sometimes almost join- the plateau are also antecedent and inherited, but and scrub-oak timber. The numerous slopes of number, although they continue to be a marked inar, in iboth j *i the11 country abovei thej"i yonSinuous of the cHn=* of later origin than the Colorado. Onion Creek this region were formerly covered with a dense T> i Colorado. feature in the canyon of the Colorado. Balcones escarpment and in the plains and the numerous smaller tributaries are still growth of juniper, locally called cedar, which for Onion Creek basin. .Along Onion Creek, country southeast of Austin. The meanders are newer consequent streams which have adjusted nearly fifty years has furnished the principal extending from a little above Buda to Pilot as thoroughly established in the canyon part of themselves to the regional slope and to the deep­ domestic fuel of Austin. In the region of the Knob, is a wide, flat constructional valley plain the Colorado as in the prairie country in the east­ ening of the Colorado Valley as the land was Coastal Plain the Bear Creek belt is forested with standing about 100 feet above the present stream- ern portion of the quadrangle. elevated. Many of the streams which we have post oak similar to that capping the high sum­ way, terminating in a deep gorge through the Pilot The Colorado, a great through flowing river called consequent are really antecedent in the mits of the Edwards Plateau. The Del Rio soils Knob hills. The surface of the plain is very level, (i. e., one that rises beyond the province and runs western part of the quadrangle and, in their lower of the Manchaca belt sustain a growth of mesquite its elevation rising from about 500 feet at Pilot through it), crosses the quadrangle from, north­ courses, especially as they enter the old alluvial trees, while the outcrops of the Buda formation Knob to a little over TOO feet at Buda. This west to southeast. It rises in the breaks of the plains of the Colorado, are controlled by the flood are generally covered with live oak. The principal plain appears once to have been an old lake basin, Llano Estacado and empties into the Gulf of plains of the larger river. Such is the case with arborescent growth of the Eagle Ford formation which has been drained by the canyon later cut Mexico. It meanders across the quadrangle in Barton Creek in South Austin and Onion Creek consists of hackberry trees. The White Rock through the northern side of the Pilot Knob great oxbows, and occupies a deep valley some near its mouth, each of which, on reaching the belt presents an open, park-like aspect, with here dome. 500 feet below the present summit level of the alluvial bottom of the old river, changes its course and there a clump or u motte" of gigantic live Upland gravel plains. There are several high Edwards Plateau and 250 feet below the summit down the valley and runs parallel with the Colo­ oaks, while upon the rocky slopes, as in South divides and small upland plateaus within the levels of the Coastal Plain. This stream, the rado for some distance. Austin, a thick growth of juniper may also occur. Coastal Plain area which are covered with old most important drainage channel of the region, Some of these streams in portions of their The Manor belt is usually devoid of arborescent gravel deposits. These are so distributed as to gathers little perennial water from the area, courses have also adapted themselves to structural growth, except occasionally a mesquite bush. The indicate that nearly all of the area of the Coastal although it receives water from many intermittent features, following the joints and fissures of the Taylor Prairie is occupied by a peculiar flora of Plain in the Austin quadrangle was once veneered streams in time of rainfall, from a few gravity Balcones fault zone. Such is the case with that chaparral, consisting of tall mesquite bushes with alluvial debris from the Edwards Plateau. springs in the northwestern portion of the quad­ portion of the stretch of the Colorado between the between which are large clusters of the Mexican One of these peculiar gravel deposits is in the rangle, and from some fissure springs between the foot of Mount Bonnel and the city dam. Barton nopal, a species of cactus belonging to the genus vicinity of St. Elmo post-office, and hence has been foot of Mount Bonnel and the city of Austin. Creek just above the springs, Onion Creek west Opuntia. The western margin of the upland named the St. Elmo Plateau. Its surface is level The principal secondary streams are Barton of Buda, near the western margin of the quadran­ Atlantic timber belt, consisting of a growth of and the soil consists of black, waxy, calcareous and Onion creeks, which rise along the western gle, and Shoal Creek in the city of Austin show post-oak and black-jack trees, mantles the Littig clay containing gravel. It is mostly open prairie, border of the quadrangle south of the Colorado similar adaptation to fault lines. country. In the bottoms of the Colorado and except a, few mesquite trees. The elevation of and enter the latter stream to the eastward. alluvial valleys of the other creeks grow stately CULTURE. this plateau is from 550 to 700 feet. Another Barton Creek is peculiar in that it runs closely pecans, as well as other large trees. high divide that reaches across from the rolling parallel to the Colorado River and has incised The Austin quadrangle has a comparatively black prairies and into the sandy Lytton Springs deep canyons into the Edwards Plateau, Barton Creek dense population, but nine-tenths of its inhabit­ GENERAL GEOLOGY. area is Mustang Kidge, which stretches from the almost equal to those of the Colorado Cany°n- ants are found on the Coastal Plain, east of the confluence of Mahard and Cedar creeks, in the itself. Furthermore, this stream, except in time Balcones scarp, the Edwards Plateau country to The geology of this quadrangle is comparatively southeast section of the quadrangle, in a westerly of flood, is eccentric in the intermittent character the west being but sparsely populated. simple. It is an area of evenly bedded rocks direction to the International and Great Northern of its drainage. Within the Edwards Plateau its The densest rural population is found in the which, in common with all the strata of the Railroad about 2 miles south of Buda. This ridge bed is at first a dry arroyo for several miles, which White Rock and Taylor prairies and the Colorado Regional Coastward Slope, have been uplifted is covered with gravel, which in the areas of the is succeeded by 6 or 7 miles of running water bottoms, nearly the entire areas of which are without serious deformation except faulting. It Taylor and Webberville formations is embedded derived from gravity springs; then follows a dry devoted to the cultivation of cotton, with some presents typical examples of the geologic structure in calcareous clay; in the Eocene area, in loose stream bed for 5 or 6 miles, to within a mile of minor crops. The few people inhabiting the of the eastern margin of the Edwards Plateau sands or loamy clays. The vegetation consists its mouth, where magnificent artesian springs Edwards Plateau are engaged in raising cattle and the interior margin of the Coastal Plain, as mostly of mesquite, or the land may be prairie in (Barton Springs) arise from fissures within its and cutting cedar timber from the hills to supply seen between the Brazos and the Rio Grand e and the Black Prairie region. In the Eocene area bed and supply a beautiful and constant stream the city of Austin with fuel. Occasionally farm­ as shown in the Nueces and Uvalde folios (folios post-oak timber may be abundant. The summit which empties into the Colorado. Barton Creek ers cultivate small areas of alluvial soil in the 42 and 64). of the ridge is very nearly level, the elevation is also notable for the relatively small amount of valleys of this district. Classification. The rocks found within this being from 650 to 800 feet. Bald Knob, a con­ country drained by it laterally. There are a number of towns. Austin, the quadrangle are of three kinds sedimentary, sur­ spicuous hill west of Manor, is also capped with Onion Creek rises in the Blanco quadrangle, capital of the State, occupies both banks of the ficial, and igneous. The sedimentary rocks occupy this material. west of the Austin quadrangle, and flows nearly Colorado near the center of the quadrangle. It the greater portion of the area and the surficial rocks are of considerable extent, whereas the Volcanic hills. An exceptional feature of the the entire distance across the latter,' Onion_ . Creek_ , is a city of 30,000 inhabitants and contains many country is the small group of volcanic hills known emptying into the Colorado River near Valley- handsome public buildings, including the capitol igneous rocks are of only occasional occurrence. as Pilot Knob, about 7 miles southeast of Austin. the eastern margin. It is somewhat different in and university. Manor, about 12 miles north­ The sedimentary rocks were deposited in the sea These occupy an area of about 3 square miles and physiographic character from Barton Creek in east of Austin, is a prosperous rural village of and subsequently elevated so as to form a land consist of three low summits rising to a height of that it is apparently not so deeply incised and about 5000 inhabitants. McNeil, Manchaca, and surface! They now consist of evenly bedded hori­ about 100 feet above the adjacent prairie. They receives the floods of many laterals, such as Bear, Buda are small towns along the line of the Inter­ zontal limestones with occasional beds of clay, and are covered by a peculiar species of grass, which Slaughter, and Williamson creeks, thereby drain­ national and Great Northern Railroad. The other constitute the foundation of the country. The is apparently limited to their surface, growing on ing a very large area of the quadrangle. Its towns on the map, such as Oak Hill, Watters, surficial rocks consist of material derived from the a black soil from which protrude many fragments headwater portion is normally a long dry arroyo, Pflugerville, Sprinkle, Bluff Springs, New Sweden, marine sedimentary rocks and the igneous rocks, of hard, basaltic rock. with occasional pools rising in the hills of the and Fiskville, are small villages with one or more brought down from the northwest by streams and At various other places in the region, as shown Edwards Plateau of Blancb County. As it enters stores and a few houses. deposited as a veneer over the uplands, as terraces on the geologic map, there are small outcrops of the Austin quadrangle Jt shows some long and The principal railways are the International and along the streams, and in occasional temporary this material, constituting exceptional features beautiful pools of permanent water derived from Great Northern, running approximately north lake basins. They are fluviatile or lacustrine within their areas. gravity springs. Crossing the Bear Creek country and south across the quadrangle; the Houston sediments. The igneous rocks consists of solidi­ it is a dry and stony arroyo inclosed in a rocky and Texas Central, extending from Austin east­ fied molten rock in fissures (and necks and associ­ DRAINAGE. canyon. From Buda to the Pilot Knob uplift it ward ; and the Austin and. Northwestern Branch ated lava and volcanic ash. The waters of the Austin quadrangle reach occupies a more modern valley within the higher of the Houston and Texas Central Railroad, run­ SEDIMENTAEY EOCKS. the sea principally through the Colorado Eiver, and older alluvial plain elsewhere described as ning northward nearly to the northern edge of CRETACEOUS PERIOD. although a few streams on the northern edge lead the Onion Creek Basin. On reaching the .western the quadrangle, when it turns westward toward ^ to the Brazos,i and some on the southern edgeO to side of the Pilot Knob uplift it enters a steep- Burnet. All of the marine sedimentary rocks of this" the San Marcos. The Colorado drains over nine- walled canyon cut directly across the uplift for a Highways radiate from Austin in all directions. quadrangle except the Lytton formation belong tenths of the total area. distance of about 3 miles. After passing the lat­ They may be classified under three heads: county to the Cretaceous system. These rocks also are The streams, except in time of flood, are not ter obstacle it continues its way through the roads of the first order, which lead from Austin most important from the standpoint of economic copious; in fact, they present peculiarities of flow Taylor Prairie and the Colorado bottom to the to the neighboring county seats; lanes leading geology, sinqe they yield the richest soils, furnish which may be classified as perennial, interrupted, Colorado River. from the main high ways'to small communities or the best material for road and building purposes,' and determine the distribution of the greater por­ grains of silica varying in size from that of a pea to The basal beds of the Comanche series have many of the canyons and along the margin of the tion of the underground water. that of microscopic particles, and small subangular been described as being predominantly of an are­ Edwards Plateau from the East Fork of the These rocks are classified into two greater cate­ fragments of clay in the cement of lime. Only naceous character. The Glen Rose formation may Nueces to the Colorado, where the streams have gories, termed " series," each divisible into a num­ the upper 80 feet of the formation are exposed in be distinguished as being essentially calcareous. cut downward through the Edwards limestone. ber of units termed "formations," which in turn the quadrangle. A complete section down to the Each of these subdivisions, however, is accom­ The beautiful bluffs of the Colorado as seen from may sometimes be divided into beds and layers. Paleozoic rocks, as exposed in the Colorado bluff panied by material similar to that of the other as Mount Bonnel westward to the Burnet County For convenience two or more formations of the west of the quadrangle, is presented on Columnar an accessory. Thus the calcareous Glen Rose for­ line are of this character. series, especially in cases where they are not Section sheet 2. mation is slightly arenaceous at its base, the arena­ The accompanying detailed section of the entire readily separable, are collectively discussed as Fossils occur in these beds as low down as the ceous material consisting of siliceous grains so thickness of the beds of the bluff on the south subordinate parts of the series under the term contact conglomerates, but they are neither plen­ triturated that they are reduced to an almost side of the Colorado in the vicinity of Lohmann "division" (or "group"), as shown in the follow­ tiful nor well preserved. The upper or coquina- impalpable powder. This siliceous matter gradu­ Ford, Travis County, is typical. (See Columnar ing table: like beds are full of casts and molds, among which ally diminishes in quantity upward, and the lime Section sheet 2.) It coincides almost exactly and clay proportionately increase. with Mr. Taff's Sandy Creek section on the Cretaceous rocks of the Austin quadrangle. are Ammonites justince and undetermined species of Cucullcea, Trigonia, Plioladomya, and Cyrena. The Glen Rose formation outcrops on the slopes opposite side of the river. (Third Ann. Rept.

SERIES. DIVISION. FORMATION. In these beds also appears the first of the several of all the numerous hills of the Edwards Plateau Geol. Surv. Texas, 1892, pp. 298-299.) (GROUP.) oyster agglomerates of the Comanche and forms the bluffs of Mount Bonnel and the Webberville formation. Section from top of high hill south of Round Mountain and Montana. Taylor marl. series. This is composed of a solidified oyst«=sheii canyon of the Colorado. (See fig. 9.) The rocks east of road from See Caves to Lohmann Ford on the Gulf series. Colorado River. (Upper Austin chalk. mass of large oyster shells, forming a consist of beds of white and yellow limestone Colorado. Cretaceous.) Eagle Ford formation. stratum 7 or 8 feet in thickness, which outcrops alternating with softer beds of limestone or clay. Comanche Peak limestone: Feet. Dakota. (Missing.) just below the junction of Post Oak and Cow The thickness is from 500 to 'TOO feet. In the 40. Limestone, breaking easily; firm slabs at top..... 5 Walnut clay: Buda limestone. creeks. northwestern part of the quadrangle, near the 39. Yellow calcareous clays with large Exogyra tex- Washita. Del Rio clay. Accompanying the oyster breccia there is epsom western boundary, the formation is thinner than Georgetown limestone. ana; forms a shelf...... 10 salts, or magnesium sulphate. The oyster-shell at Austin, thickening southeastward. Grlen Rose formation: Comanche Edwards limestone. series. Comanche Peak lime­ bed effloresces into a powdered earthy substance The lowest beds of the Glen Rose formation 38. Shaly limestone; not very fossiliferous...... 10 (Lower Fredericksburg. 37. Alternating hard and soft strata of limestone; Cretaceous.) stone. accompanied by the epsom salts. Magnesium and are marked by the appearance of strata of homo­ Walnut clay. some thin slabs about base; not fossiliferous ... 15 pyritiferous layers occur in other horizons higher geneous texture, such as " magnesian " marls and 36. Alternating hard and soft yellowish limestone; Glen Rose formation. Trinity. in the division, and are no doubt in part the cause hard layers in which the fossil jRequienia occurs. not very fossiliferous ...... 35 Travis Peak formation. 35. Shaly limestone, fossiliferous; contains a few indi­ of the mineral character of some of the artesian The term magnesian has long been applied to viduals of Cardium mediate and a few other One of the most complete sequences of the waters, especially those of wells which are not certain yellow strata in these beds. The writers species...... 4 34. White limestone; breaks easily ...... 15 rocks of the Cretaceous period to be found in our drilled into the basement sands below these layers. can not state positively whether these strata are 33. Marly material, forming a terrace...... 10 country is revealed in the outcrops of the Austin At the top of the sandy beds in the Colorado or are not magnesian in composition. The name 32. Alternations of soft argillaceous or marly lime­ quadrangle, especially in the section along the section a yellow, arenaceous, fossiliferous lime­ "Caprotina Horizon No. 1" has also been applied stone with harder thin layers of purer limestone (4 hard and 3 soft layers)...... 30 valley of the Colorado. Hence these rocks will stone appears. This marks the first or lowest to these beds, because in the earlier geologic litera­ 31. Slope and shelf; fossils at top...... 15 be described in detail. appearance of the peculiar fossils Monopleura ture the fossils now called Requienia were termed 30. Hard, nodular limestone; contains Nerinea frag­ ments...... 5 The Cretaceous formations succeed one another (Caprotina) and Requienia, and indicates the Caprotina. 29. Slope and shelf...... 14 to the eastward in more or less regular belts, the beginning of the conditions which finally pro­ In nearly all complete sections the Glen Rose 28. Thin, hard ledge...... 1 outcrops of the oldest formations being found duced the Glen Rose formation. formation shows three marked subdivisions. The 27. Slope, very gentle rather a shelf...... 15 26. Bed of Monopleura in hard, yellowish limestone along the western margin of the quadrangle and The Travis Peak formation records a subsidence lower and upper thirds are composed of thin- thin, a foot or two. successively newer ones being encountered to the of the land during its deposition. As the waters bedded alternating marl and flags, usually weath­ Hard, perforated limestone...... 2 24. Alternating thin, hard layers and soft, thick layers; eastward. The continuity of this sequence is deepened the deposits changed from coarser to ering into terraced slopes; the middle third is the thin layers 6 inches to 1 foot, the soft 3 to 4 broken, however, at the Balcones fault line, as is finer material, becoming more comminuted and made up of thicker and more massive beds, which feet...... 20 further explained under the heading "Geologic calcareous at the top of the beds, until the sand weather into bluffs. Some of the beds near the 23. Soft, chalky, argillaceous stuff, only a few feet. 22. Ledge of hard, yellowish, perforated limestone, 2 structure." grains are so fine as to be almost imperceptible to base of the thicker layers are chalky in texture feet; hard ledges of limestone, 8 feet...... 10 the eye, the whole mass becoming chalky and and carry many peculiar fossils, especially note­ 21. Small, hard ledge, 1 or 2 feet. COMANCHE SERIES. worthy being a foraminifer, Orbitulina texana, 20. Soft, argillaceous limestone, marly; forms a slope 10 "magnesian" in appearance. 19. Shelf above, ledge below, rises...... 10 This series is composed largely of limestones, The following section, by Mr. J. A. Taff (Third besides many casts of large mollusks. The lower 18. Soft, chalky, argillaceous limestone with Exogyra although there are arenaceous beds at its base and Ann. Rept. Geol. Surv. Texas, 1892, p. 295) will portion of the formation carries much fine arena­ texana at base, with harder layers that form shelves 11 hard ledges. Twenty feet from the many beds of marly clay interspersed through it. give an idea of the sequence and composition of ceous material in addition to the calcareous top of these beds the hard ledge is honeycombed One formation of the series, the Del Eio, is com­ the formation as exposed in the valley of the material, and its indurated and unindurated beds by solution, and is arenaceous. In the lower 20 posed almost entirely of clay. do not occur in %uch uniform alternations as do feet numerous fossils occur: Tylostoma peder- Colorado between Travis Peak post-office and nalis, Cardium mediate, " Groniolina," etc.; The rocks of the Comanche series are limited Smithwick Mill, Burnet County. (See Columnar those of the upper third. For instance, there also horizon of E. texana. Thickness of series.. 60 in outcrop to the western half of the quadrangle, Section sheet 2.) will be 10 or 12 feet of soft, friable material and 17. Hard ledges of honeycombed (perforated) lime­ occupying all of the country of the Edwards then a thin layer (less than a foot) of indurated stone. The limestone, hard, yellowish, con­ Hickory Creek section of the Trams Peak formation, begin­ tains many poorly preserved calcitized fossil Plateau, the Bear Creek belt, and most of the ning at the top of the divide between Hickory and Cow stone. In weathering this results in wide terraces shells, largely the remains of nerineas ...... 30 Manchaca belt. The eastern border of their out­ creeks and continuing to the Colorado River level at the with steep slopes. 16. Hard ledge of limestone; many Cardium mediate 5 mouth of Hickory Creek, Burnet County. 15. Soft, argillaceous, chalky limestone...... 5 crop passes near Round Rock, beyond the north The yellow " magnesian " strata also increase in 14. Ledges, 6 inches to 1 foot thick, with soft, shaly Feet. layers between...... 20 border of the quadrangle, through Austin, to the 13. Bands of conglomeratic and calcareous sandstone, thickness in ascending series, and become very east of Davis Hill, and to the west of Mountain alternating with beds of arenaceous limestone, the conspicuous in the middle portion, often 13. Soft limestone ...... 20 arenaceous limestone predominating...... 40 12. Hard ledge ...... 2 City, as shown by the boundary between the Buda being from 5 to 15 feet in thickness, as yeilowCHme= 11. Soft, chalky, argillaceous layer...... 10 12. Marly magnesian limestone...... 40 i i -i nr» i» ityr -r~» i stone layers. and Eagle Ford formations on the geologic map. 11. Calcareous sand at base, grading upward to a siliceous seen in the blutts oi M.ount Bonnel 10. Ledge of hard, brownish or yellowish limestone, limestone at the top, barren of fossils ...... 55 containing embedded sand grains ...... 5 near Austin. These limestones are soft and of a Soft, chalky, argillaceous limestone, with an occa­ TRINITY DIVISION. 10. Yellow calcareous sand, stratified ...... 15 9. Conglomerate similar in character to No. 2, with the cream or brownish-yellow color, and alternate with sional hard ledge. Hard ledge 2 feet thick 15 Travis Peak formation. These beds are espec­ exception that the pebbles are smaller and more strata of marls similarly constituted, and are some­ feet above base. In the upper part of this worn, grading into sand below and into calcareous marly bed fossils are very abundant. Cardium ially well exposed on both slopes of the valley of sand above ...... 25 times accompanied by pockets or nodules of calcite, mediale, Tylostoma pedernalis, many echino- Colorado River between the mouths of Sycamore 8. Red sand, unconsolidated...... 3 aragonite, strontianite, celestite, and epsomite. derms, Pseudodiadema texana, Nerinea, Ostrea, 7. Friable yellow sand...... 5 etc...... 35 and Cypress creeks, in Burnet and Travis counties. The upper third of the formation, as seen at the Ledge of hard, yellowish limestone ...... 5 6. Cross-bedded shell breccia, containing many small They were so named because they are well rounded grains and pebbles of quartz, flint, and top of Mount Bonnel, presents alternations of fri­ 7. Slope, underlain by soft, chalky limestone...... 25 exposed in the vicinity of Travis Peak post-office. granite sand. Fossils: Trigonia and small bivalves, able marls and hard limestone strata. The lime­ 6. Arenaceous ledge, a few feet. and Ammonites justince ...... 7 5. Soft ledge with many Monopleura, a few feet. (See First Ann. Rept. Geol. Surv. Texas, 1890, 5. Ostrea beds, magnesian lime cement, fossils en masse 3 stone strata usually average less than a foot in Travis Peak formation: p. 118.) 4. Brecciated grit, composed of worn fragments of oyster thickness. These alternations occur with great 4. Rather hard ledge, with poorly preserved fossils; While these beds are arenaceous in composition shells and shells of other Mollusca, with sand and regularity and persistence. Clay is the chief appear to be oysters...... 2 fine pebbles, stratified in false beds...... 5 3. Soft, chalky limestone...... 20 and porous in texture, like the basement beds of 3. Bands of friable bluish shale and calcareous sand, accessory of the calcareous beds. The marls are 2. Ledge of yellowish limestone, 2 feet, and 40 feet of the Comanche in general, they differ considerably stratified. Fragments of oyster shells are common soft and laminated and are composed largely of the section covered by river alluvium...... 42 in the calcareous sandstone...... 15 1. Yellowish calcareous sandstone at river level, from the allied beds to the northward. They 2. Basal conglomerate of pebbles of limestone,, quartz, minute shell fragments, giving the beds a dis­ thickness not obtainable. consist in part of conglomerate, composed of chert, granite, and schist, well rounded in a cement tinctly granular, oolitic character. They have Totals of above section. rounded pebbles of Silurian and Carboniferous of ferruginous yellow and red gritty sand. Some of little clay and imbibe moisture very freely. the pebbles at the base are from 4 to 6 inches in Comanche Peak and Walnut formations (in part)...... 15 limestones, granite, Llano schists, quartz derived diameter. They decrease in size, however, .upward "While possessing no great agricultural possi­ Grlen Rose formation (entire) about...... 455 from the adjacent Paleozoic rocks, beds of finely from the base, until a false-bedded calcareous shell bilities, the basal or alternating beds are capable Travis Peak formation (in part)...... 64 grit appears at the top...... 50 cross-bedded pack sand, white siliceous shell of producing valuable building material. Some Total of section, about...... 534 breccia resembling the Florida coquina, and some Total thickness of Travis Peak beds...... 263 of these have rich " magnesian" buff- 1. Laminated, flaggy, Carboniferous sandstones and fri­ yellow colors, while some of the white suitable "or FREDERICKSBURa DIVISION. clay. The base is formed usually of conglomer­ able light-blue clay of the Carboniferous (Coal unlaminated limestones suitable for Walnut clay. At the top of the Glen Rose ate,' and is succeeded byJ coarse,' angular,o J KBasal , con= Measures) formation, from the Colorado River level cross-bedded sand, which becomes more fj tftfiower upward to the base of the Cretaceous conglomerate, carving resemble the Caen stone of France, which formation a bed of yellow, calcareous clay always the laminated sandstones containing prints of ferns, finely triturated until it reaches the nearly ...... 100 is imported into this country. Some of the beds occurs, which is extremely rich in two species of may also be valuable for the manufacture of oysters: Exogyra texana Roemer and Grryplicea condition known in Texas as "pack sand" i. e., Total thickness of section...... 363 «/ a very fine-grained, loosely consolidated sand, hydraulic cements, although at present they are marcoui Hill and Vaughan. Its thickness is from cemented by carbonate of lime. Glen Rose formation. The Travis Peak forma­ not utilized. The formation also contains unde­ 10 to 15 'feet. This is an extremely persistent The sands show occasional patches of red and tion grades upward into the Glen Rose without any veloped beds of epsom salts, strontianite, and bed both in its lithologic and its paleontologic greenish-white clays, whose tints resemble very abrupt change in the character of the sediments, as other materials. characters. To it the^name Walnut clay has been much the characteristic colors of the Potomac beds can be seen in the high bluff of Cow Creek imme­ The alternations of horizontal beds of soft marls given. Above these clays is a soft, chalky lime­ of the Atlantic coast, and they sometimes contain diately below Mr. Hensel's house at Travis Peak and hard limestones above described produce the stone, the Comanche Peak limestone. These two lignite and fossil bones. The sandston§ contains post-office, in the western part of Travis County. bench-and-terrace topography of the slopes of formations frequently occur near the summits of Austin. buttes in the area of the Edwards Plateau and do that of a hen's egg to a foot or more in diameter. and where the surface stratum is of homogeneous Limestone ledges...... not occur in the Coastal Plain, except in one small They also vary greatly in color. On fresh frac­ texture, it weathers into innumerable little ridges, Limestone ledges containing requienius. The outcrop along the eastern foot of Mount Barker, ture some are almost jet black; others are light crests, and drainage lines, illustrating in miniature three layers above described form a slope to the top of the hill (or bluff) above the face proper too small to be shown on the map. blue, gray, or opalescent; still others are delicate the processes of erosion and mountain carving. of the bluff...... 1 0 The yellow clays of the Walnut formation most pink in color. There is some, but not conclusive, These minutely eroded limestone surfaces are tech­ 40 Ledge of hard limestone, 10 inches above basal commonly occupy a terrace or bench resting upon evidence that each particular kind of flint occupies nically known as " karrenf elder," and are formed sheet flint. The upper part of the ledge con­ tains rather small nodular flints ...... 15 - 0 a hard stratum of the Glen Rose formation, with a definite horizon. by the solvent effect of rain water upon the sun- 39 Limestone weathering out and giving rise to a a nipple-like hill of the Edwards limestone occur­ In most cases the Edwards limestone may also heated limestone surfaces. The crevices in these good deal of red clay, apparently representing readily be distinguished by the peculiar aberrant level areas of Edwards limestone country are the zone of calcitized fossils found in the high ring above. This bench forms a conspicuous bluff above McGill's Ford...... 6 6 feature of the landscape of the Edwards Plateau. mollusks of the genera Monopleuva, Requienia, usually grass covered, with occasional patches of Massive, thick ledges of limestone; detail not Oomanclie Peak limestone. This is a persistent and Radiolites bivalve fossils which have cor- scrub oak. The surface is very rocky, the karren- exposed ...... 23 8 Soft, white, arenaceous limestone...... 2 2 bed of white, chalky limestone, presenting a shat­ nucopiate form, suggesting a resemblance in shape felder protruding in jagged points through the Soft, arenaceous limestone...... 3 10 tered, reticulated appearance on weath- Persistent to the horns of cows, goats, and sheep. rich but scanty soils. Sometimes residual flints Ledge of limestone, rather soft, emitting odor of The formation is stratified into a succession of occur in such immense quantities over these sur­ petroleum ...... 2 10 ering. It is partly characterized by an Chalky limestone, forming little caves, composed abundant fossil fauna containing a large massive beds accompanied by very few flaggy and faces that one is apt to mistake them for a water- of a good many small ledges; discoidal flints at number of Exogyra texana, which is especially marly layers. Some of the strata are harder than rolled gravel formation. top ...... 4 0 Hard limestone, emitting odor of petroleum under abundant in its basal portion. It is from 40 to others and project beyond the softer layers in the The base of the Edwards limestone is found blows of hammer. Texture of limestone rather 50 feet thick, thinning toward the Rio Grande. profile of the hills as overhanging shelves; others capping many of the high buttes of the Edwards mealy. Nodular flints; occasional discoidal are soft and erode very rapidly. In the Austin Plateau, as shown on the geologic map, while the flints in top...... 2 1 Although it is insignificant as regards thickness, Two thin ledges of limestone; layer of sheet flint and may be considered the base of the Edwards quadrangle the Edwards limestone is almost upper and greater portion is exposed along the in top...... 1 2 limestone, it is one of the most persistent paleon- inseparable from the underlying Comanche Peak, Bear Creek country, east of the Balcones scarp. Ledge of thick, massive limestone...... 5 0 Hard, yelloAvish limestone...... 2 0 tologic horizons of the Texas Cretaceous sections. since both are composed chiefly of carbonate of This is well displayed in the banks on the south Hard, thick, massive ledge of siliceous limestone, In the Austin quadrangle this formation occurs lime. The Comanche Peak strata are less con­ side of the Colorado in the western part of Austin, ringing under blows of hammer. At the base solidated, and, as they are somewhat argillaceous, between McDonald's brickyard and the city dam. there is a layer, about 9 inches thick, of opales­ only within the area of the Edwards Plateau, cent, pinkish or brownish flint. Apparently usually as # chalky slope immediately below the possess a more marly texture than the Edwards Owing to faulting, this section is somewhat com­ the limestone is being converted into flint by cap rock of Edwards limestone and above the limestone, which is usually a firm, white, ringing plicated and is not continuously exposed at any replacement, and the process has not yet been completed...... 6 0 bench occupied by the Walnut formation. A limestone of great hardness and durability; so single locality. The details as made out at three Soft, chalky limestone, dissolving and forming typical exposure of this formation may be seen that the Edwards weathers into cliffs, while the localities at Austin are shown in the accompany­ small caves...... 3 0 on the top of Mount Barker, northwest of Austin, Comanche Peak is wrought into lower slopes; but ing sections and figures. It will be noticed that Soft, chalky limestone with very large (may be 1 foot long), irregularly shaped blue flints at top. 2 3 and its section is shown on Columnar Section in most cases reliance must be placed on paleonto- in the lower portion of the Bee Creek section, White, chalky limestone, apparently siliceous; sheet 2. logic determinations to distinguish the two forma­ which is still above the base of the whole of the zone of flint near top. The flints blue, discoidal, and tending to form sheet...... 6 8 Edwards limestone. The Comanche Peak lime­ tions. formation, arenaceous marls and limestones are Massive ledge of hard, bluish limestone...... 7 0 stone is the base of a thick group of limestones " Neither is the Edwards limestone always rather numerous. These play an important part Very hard limestone...... 0 6 consisting of the Comanche Peak and Edwards sharply defined from the overlying Georgetown, in the artesian conditions from the Colorado south- A layer of enormous blue flints, in some places over 1 foot thick...... 1 0 formations. The upper formation contains a large except by paleontologic criteria. It is true that westward. Thick, massive ledge of limestone, rather soft, number of flint nodules with vast quantities of the Georgetown limestone is somewhat more arena­ The following sections represent the entire yellow in color, and slightly arenaceous ...... 5 5 Ledge of hard, yellowish limestone with a zone of JRudistes and aberrant Chamidce, and was called ceous, but the differences are so slight that their thickness of the Edwards limestone exposed on flints tending to form a sheet at base...... 1 4 by Shurnard the Caprina limestone. detection requires the trained eye of the geologist. the downthrown side of the fault in the bluffs of Soft, white, slightly arenaceous limestone, com-, This formation is the most conspicuous and The Edwards limestone, being more purely the Colorado between Austin and the river level posed of thin ledges; upper 2 feet, middle 4 feet, lower 1 foot...... 7 0 extensive in the Texan-Mexican region. It is calcareous than any other of the Comanche series, at the mouth of Bee Creek. (See Columnar Sec­ Soft, yellowish or whitish limestone with Inyer of composed mostly of limestone strata, but there probably corresponds to the deepest tion sheet 2.) flattish, bluish flints forming a sheet at top. Evidence of This is really three ledges; upper ledge, with are some marly layers. It shows slight variation submergence of the Comanche epoch. deep sub­ The base of the beds is concealed in the Coastal mergence. flints at top, 2 feet; middle containing concre­ in color, composition, texture, and mode of weath­ It is true that in.the Glen Rose forma­ Plain, lying probably less than 100 feet below No. tions of calcite in lower part, 4 feet; lower ledge ering. In general the beds are whitish, although tion occasional thin beds of chalk occur, and that 1 of section C, but can be seen on the upthrown exposed at low water, 1 foot...... layers of buff, cream, yellow, or dull gray are some of these are composed almost entirely of side of the fault, capping the remnants of the Total, Deep Eddy section...... 121 5 frequent. These colors depend much upon weath­ foraminifera, but such chalks usually contain a plateau. /Section of bluff at mouth of Bee Creek. ering. In composition many of the beds are as considerable percentage of clay, recognized as an Ft. In. /Section of bluff on Barton Creek about 1 mile above Barton Limestone slope, detail not exposed...... 11 0 nearly pure carbonate of lime as can be offshore deposit. Spring', Trams County. Beds of pure Layer of enormous blue flints...... 1 0 found in nature, but some have small carbonate of Topographically the Edwards limestone is one 22. Arenaceous limestone...... 5 lime. Georgetown limestone : * Ft. in. 5 admixtures of silica, epsomite, chloride of the most important formations in Texas. In 5. Grayish limestone, irregular fracture, with 21. Hard, yellowish limestone with sheet flint at base. 1 4 of sodium, and perhaps other salts as yet unde­ fact, it is the determining factor in the topography Alectryonia carinata and Gryphcea wash- 20. Yellowish, rather hard limestone, somewhat sili­ itaensis...... 1 ceous; thin band of chalky limestone at top; termined. Occasional bands of soft brownish- of the whole of the Edwards Plateau. Its hard­ 4. Yellow or reddish calcareous shale .."...... 4 calcite concretions near base...... 6 0 yellow stone are intercalated with the limestone. ness being superior to that of the overlying and 3. Alternating layers of hard and soft limestone 19. Sheet flint at top (sheet flint at top of lowest ledge of Deep Eddy Bluff); three ledges of limestone: These bands are popularly called " magnesian," and underlying beds, its consequent resistance to ero­ with Alectryonia carinata, Gryphcea wash- itaensis, JSxogyra americana, etc ...... 18 upper, 1 foot; middle, 2 feet 6 inches; lower are composed largely of exceedingly fine-grained sion has preserved it as the capstone of the 2. Hard, grayish limestone ...... 33 (containing calcite concretions), 3 feet...... 6 1. Soft, chalky limestone, with a saline taste. ... 13 18. Sandy limestone, with two zones of nodular flints siliceous material, like tripoli. As these beds innumerable buttes and mesas of this portion of near middle; sheet flint at base; mass of requi­ often contain flints, the silex may be of organic the State and of the extensive Edwards Plateau Total thickness of Georgetown limestone ... 69 4 enias just above the sheet flint...... 10 origin. Clay is absent except as a minor consti­ and. Grand Prairie regions. Not only are most of Edwards limestone : Soft, yellow, calcareous sandstone, a part of the preceding ledge, about...... 3 tuent in the few marly layers. Iron is sparingly the buttes and mesas capped by it, but these are 49. Nodular limestone full of requienias (first Yellow, cherty limestone, about...... 0 present as pyrites, and is revealed by the red accompanied by scarps overlooking the lower- req uienia bed) ...... 3 0 Three or four ledges of rather soft, whitish or yel­ 48. Nodular limestone; nodules as large as one's lowish limestone; the upper ledge containing a color of the clay that weathers out of a few beds. lying valley prairies which follow the stream. head...... 2 0 great mass of requienias, the others fewer,...... 8 Exceedingly fine siliceous particles occur, espe­ The walls of the canyons which many of the 47. Hard, chalky limestone ...... 3 0 Solid, white limestone, granular, not very hard; 46. Thinly laminated limestone (the so-called cially southward from Comanche County but streams have cut are also composed largely of contains a great many requienias near top ..... 6 11 "lithographic flags ").....'...... 8 9 Yellow, arenaceous limestone...... '...... 4 0 no sand grain, pebble, bowlder, lignite, or other the Edwards limestone, especially the portions of 45. White, sublaminated, chalky limestone. The Blotched, arenaceous limestone...... 3 undoubted piece of land-derived debris has ever those crossing the Bear Creek belt. The cliffs lower part of Nos. 45 and 46 contain many Soft, yellow, arenaceous limestone...... 2 fossils Mxogyra texana, Pholadomya Hard, yellowish, granular limestone, with shell been found. of the Colorado between Austin and Bee Creek knowltoni, etc ...... 8 5 fragments, gray on fresh exposure...... 1 The limestones vary in degree of induration form a conspicuous exposure of this limestone. 44. Nodular limestone, no requienias ...... 1 Q Soft, yellow, arenaceous limestone or calcareous 43. f Nodular limestone with many requieuias from hard, ringing, durable strata to soft, pul­ To its hardness is also largely due the topography sandstone...... 2 (second requienia bed)...... 3 9 Ledge of nonindurated, granular limestone, with verulent chalk that crumbles in the fingers and of the limestone mountains of Mexico. e. Laminated limestone...... 1 0 indurated blotches, which are structureless and resembles very much the prepared article of com­ It shows many types of weathering. Some of d. A series of hard limestone ledges (eight in flinty looking...... 1 0 number), separated by the thinly laminated Ledge of white, rather soft limestone, with many merce. Some of the beds are coarsely crystalline, the strata make bold cliffs nearly 50 feet in layers. There are some flints, about as very irregularly shaped flints in a zone about with calcitized fossils, and are susceptible of high height, the faces of which, although apparently of large as a man's fist, Radiolites and Chon- the middle of the ledge. The flints are mostly drodonta munsoni...... 45 8 polish. The beds also vary in texture. Some of homogeneous texture, weather into small open small, bluish in color, and do not show con­ c. Flaggy layer with discoidal flints ...... 2 4 centric banding; about...... 3 them are porous and pervious, while others are caverns. This weathering sometimes brings out (29.) b. Hard limestone, forming a shelf along this Ledge of white, rather soft limestone; no flints; a close grained and impervious. Some are homo­ a thinly laminated structure associated with white portion of Barton Creek and its bottom at few fragmentary fossils...... 2 the bridge below, eroded into deep pot A soft, arenaceous ledge. The lower 1 foot 10 geneous throughout; others have hard and soft efflorescence. The bottoms of caverns of this holes. The lower 2 feet of this layer con- CJ ^} / inches is a subledge. In the upper part (near spots, the latter dissolving by the percolation of character are filled with a layer of w~hite, pulver­ - tains very large -blue flints, often 1 foot top) are concretionary bodies that in form resem­ across. Some of them are oval, others underground water and constituting what is popu­ ulent earth. The residual products of other ble flints, but are not flints in texture. These flattened out and very irregular in outline. bodies are hard, apparently siliceous, and con­ larly termed " honeycombed " rocks. The harder massive ledges weathering into caverns are ver­ The upper part of bed contains small flints . . 12 8 tain white blotches, some of which appear to be spots in some cases seem to be in process of indu­ milion-colored clays, in which are beautiful fossils a. Limestone ledges, with some flattened flints. of foraminiferal origin...... 5 All of the flints in this section belong to the Hard limestone, whitish Or bluish, without flint; ration, suggesting a step in the formation of flints. composed entirely of crystallized calcite. blue variety...... 11 0 not fossiliferous...... 4 The holes in the honeycombed layers often repre­ The hard limestones weather into vertical, Base of a is Barton Creek bed. Arenaceous limestone, has a tendency to lamina­ sent what were once spots containing soluble salts square-cut bluffs, while the soft and more homo­ Total thickness of strata in bluff ...... 171 11 tion, but in the ledge the laminated character is not always evident. The upper part of the of iron and other accessory minerals. geneous beds of marly or chalky tex­ Series of Section of Deep Eddy Bluff, south of the Colorado River, ledge by solution becomes porous. The rock alternate The formation can usually be distinguished by ture form slopes. Where these hard cliffs and west of Austin. has a considerably absorbent power for water, the immense quantity of flint nodules embedded and soft beds occur in alternation there slopes. 43. Nodular limestone with requienias at top (the and has a dark (wet) appearance, due to con­ second requienia bed of the Barton Creek sec­ tained water...... 5 9 in and between the limestones and is a corresponding alternation of scarps and slopes Thick ledge of white limestone, not very hard, Great quan­ tion). scattered over the surface everywhere. tities of in the topographic profile. oxidizing yellow from contained iron. Contains flints. * The Georgetown limestone is assumed to be 70 feet thick at Austin. Future a large number of irregularly shaped flint These are of many shapes ; some are Where the Edwards formation makes extensive study may modify this estimate. The uppermost layers, characterized by nodules. These may be as much as 1 foot long, fusiform, like elongated roots ; others are knotty, stretches of level country, such as that of the Kingena wacoensis, are here missing. but usually are rather small 3 or 4 inches in t The beds in the Barton Creek section below 43 can not be correlated layer length. They are bluish in color and have a like warty potatoes ; others are parts of extensive Bear Creek country between Manchaca and Oak for layer with the Deep Eddy Bluff section; therefore numbers are not used in the description of the former section for beds below the one numbered 43 concentrically grained structure, resembling the sheetsjar very flat lenses. They vary in size from Hill and that upon the summit of the plateau, except the one lettered 6. which is equivalent to 29 of the section below. graining of pine wood-. Their long axes are not Ft. In. intercalated in the seams adjacent to the shell white on exposure. Both the shales and the flags eter of Pilot Knob the Austin chalk has been always parallel to the bedding planes of the limestone, an important exception to the usual horizons. In places the shells are cemented into are distinguished from the other Cretaceous altered in places to a firm, white, crystalline rock, position of the flints relative to the stratification thin layers of indurated argillaceous limestone, rocks, especially those immediately above and almost a marble. On Onion Creek north of Pilot of the limestone ...... 5 9 making persistent bands in the middle of the clay below them, by their laminated character. The Knob beds of volcanic tuff are interstratified with 1. Ledge of yellowish or whitish limestone, without flints; in a thin layer, about 6 inches thick, at beds. clay shales are often accompanied by the chalk. (See fig. 8.) the top of this ledge there is an enormous num­ Above the zone of Exogyra arietina the clay mixtures of arenaceous material which ber of Requienia texana...... 4 0 MONTANA DIVISION. is somewhat barren of fossils, except near its sum­ make a thin laminated siliceous flag-o teethbones. and Total, Bee Creek section...... 104 8 mit, where it becomes slightly arenaceous and con­ stone, and nearly always contain a great Taylor marl. The Austin chalk is overlain by The thickness of the Ed wards limestone exposed along tains impure limestone slabs bearing other fossils, abundance of fish teeth and bones. (See figs. 38 a deposit of calcareous clays, locally known as bluffs of the Colorado between Austin and Bee some of which also occur in the upper layers of to 41.) From this fact the formation has been "joint clays," estimated to be about 540 feet thick Creek is as follows: Ft. In the Georgetown limestone. Among these fossils called the fish beds. The strata also contain a few in the Colorado River section. When fresh these Bluff on Barton Creek, beds 49 to 43...... 29 11 is a gryphseate oyster, Q-ryplma mucronata Gabb. remains of mollusks, principally of inoceramus and beds are fine-grained, tough, unctuous, blue clays. Deep Eddy Bluff, beds 42 to 24 ...... 99 8 Bluff at mouth of Bee Creek, beds 23 to 1 ...... 104 8 Buda limestone. This formation (the Shoal oysters. Bones of a large saurian were also found They are apparently unlaminated until exposed Grand total...... 234 3 Creek limestone of previous writings) overlies the in this formation near the Deaf and Dumb Asylum. to weathering, when their laminated character is Del Rio clay, and though very thin, being only 45 The formation is rarely over 50 feet thick any­ developed. Their accessory constituent is lime. Total thickness, including the 70 feet or more of the lower part of the formation preserved on the sum­ feet thick, is a very conspicuous and easily dis­ where in the vicinity of Austin. Geologically it On atmospheric exposure their color, owing to mits of the plateau, about...... 300 tinguished formation. It occurs in ledges, which, represents the uppermost horizon of the Eagle oxidation of the contained iron, changes to a dull when exposed in vertical bluffs, always have a Ford shales, so extensively developed in northern yellow. Because of the rapid disintegration the WASHITA DIVISION. rough, nodular appearance, and are not evenly Texas. The abrupt contact below with the Buda character of the unaltered clay is seldom seen, The Washita division, which has its greatest bedded. When they form surface exposures the limestone represents an unconformity or a time except when fresh material is brought up by the development in Texas to-the northward, toward country is usually extremely rough and the lime­ break in the sedimentation of the Cretaceous well digger or exposed in freshly cut ravines or Red River, is represented in the Austin quadras stone is 'full of cavities. period, the only distinct one traceable in the creeks. At the Blue Bluffs of the Colorado, 6 gle by three formations Georgetown limestone, Its outcrop oxidizes to a slightly darker color Texas region. miles east of Austin (fig. T), there is a good, fresh, Del Rio clay, and Buda limestone. than the limestones already described. On frac­ The formation usually occurs in low bluffs vertical exposure. Georgetown limestone. This formation consists ture it is light yellow, with blotches or spots of capped by the Austin chalk, or as flat areas upon At the top, as seen at various places in Travis of a group of impure white limestones, regularly pale pink, as if it had been subjected to fire. In the summits of the escarpments of the Buda lime­ County east of Austin, the clays grade into the banded, and alternating with bands of marly clay. places it is very hard, but in general is of varying stone. It outcrops along a very narrow belt in marls of the Webberville formation. Before exposure they are dull blue in color, but texture, usually lumpy; in some spots it is efflores­ spots north and south through the quadrangle, as Their middle portion apparently con- andmarl ' when weathered they are white or yellowish. The cent and decays into a soft, pulverulent material shown on the map. A typical outcrop may be tains. no well- 1 -preserved J J!iossils, "I 1but A lower portion of the section as exposed at Austin having a slightly saline taste. The minute red and seen in Shoal Creek just above the Pecan street impressions are abundant in places. In the base contains thicker and more massive beds than the pink blotches are peculiar to this lime- Red s otted bridge, Austin, and in the banks of the creek of the beds Exogyra ponder osa, a large, heavy upper. At the base there is a hard, gray, massive stone and have given to it the local llmestone- thence to its mouth. (See Columnar Section oyster, is abundant. limestone, about 27 feet thick, which is succeeded name of "burnt limestone." Microscopic study sheet 2.) Here it rests upon the Buda limestone. The marls constitute the greater part of the by chalky, argillaceous limestones that become has revealed the fact that the rock is made up It also underlies nearly all the Sixth Ward in the topographic subprovince denominated the Taylor nodular upon weathering. The thickness of the largely of foraminifera, filled and coated with a northwest portion of the city. It occurs south of Prairie. The hills are usually gently rounded, formation is from 65 to TO feet. Intervening mineral which in all probability is glauconite. the river along the banks of Bouldin Creek, and but frequently along the streams low bluffs are between the top of the undoubted Edwards lime­ Exteriorly the limestone presents no appearance is best seen where the railroad tAvice crosses that formed by the clays slipping down. They erode stone and the base of the Georgetown limestone indicating that it contains foraminiferal remains, creek near the Granbury residence in South with great ease, enormous gullies being formed is a bed of very soft, granular, chalky limestone but, so far as examined, it is more largely com­ Austin, where it produces a small bluff about 25 by heavy rains in a very short time. After about 18 feet thick, having a saline taste, which posed of them than any other rock of the whole feet in height. It also outcrops between Man­ rains the roads throughout this country are inde­ may be classed with either the Edwards or the series. In one thin section Itotalia, Textularia, chaca and Buda in the bluffs of Onion Creek, and scribably bad, first from the readiness with which Georgetown. Grlobigerina, and fragments of three or four other near Watters station, in the northern part of the deep gullies are formed, and secondly because The Georgetown formation is paleontologically genera of foraminifera have been recognized. quadrangle. of the soft and extremely sticky character of the characterized by Epiaster elegans, Ammonites In the Austin section the Buda limestone rests Austin chalk. Surmounting the Eagle Ford mud. (Schlcenbachia) leonensis, Gryphma washitaensis, without apparent gradation upon the Del Rio shales, without break in sedimentation, is a white, Webberville formation. This, the highest for­ Exogyra americana, and Kingena wacoensis. clay, indicating a rapid physical change in sedi­ chalky limestone of coarse texture, occurring in mation of the upper Cretaceous within the Austin These fossils occur throughout in definite zones mentation. It is abruptly overlain by the thin beds of various thickness, separated in places quadrangle, underlies the eastern margin of the and associations, and some of the strata are com­ laminated bituminous clays of the Eagle Ford for­ by crumbling marly beds. This chalk is glaring Taylor Prairie in the quadrangle. The marls of posed almost entirely of them. Above the more mation. The outcrop of this formation is propor­ white in color as seen in the streets of the Taylor formation grade upward massive lower layers is an agglomerate of Grry- tionately very limited, being better displayed at Austin and along the San Antonio almost imperceptibly into the clay gfauconitic pJiCBa washitaensis. Associated with this is found Austin than at any other locality. It can be road southward. Occasionally there marls of the Webberville formation, an oyster, Alectryonia carinata, a familiar Euro­ found in the bluffs of the Manchaca belt near the are slight blotches of yellow from the the latter containing greenish grains of glauconite pean form, which occurs only at this horizon in streams, such as Bear Creek at Manchaca, and oxidation of the little balls of iron pyrite which and, in the upper part, impure limestones and the Austin section. At the top is a stratum of Onion Creek at Buda. Characteristic exposures it contains. Before weathering, and especially blacker clays. These clays do not oxidize into a massive limestone less than 3 feet thick, consisting are seen in the steep scarps of Shoal Creek in the when impregnated with ground water, this chalk clear yellow buff, as does the Taylor formation, of a homogeneous calcareous matrix thickly city of Austin, shown on Columnar Section sheet has a bluish tint. The rock usually weathers in but are of a greenish-yellow color, caused by the studded with ^Kingena wacoensis. 2, and in the bluffs on the south side of the river, large conchoidal flakes. Under the microscope presence of the glauconite. The thickness of this These beds do not occupy very extensive areas, where Bouldin Creek enters the valley, at the the chalk exhibits a few calcite crystals, particles formation can not be determined except by an but occur from place to place in the Bear Creek crossing of the International Railroad and the of amorphous calcite, and a great number of the estimation of the regional dip. belt intervening between the International and Oatmanville road. It forms precipitous cliffs with shells of foraminifera and other minute organisms. The beds contain fossils characteristic of the Great Northern Railroad and the eastern margin toppling projections, owing to its jointed structure. In composition it varies from 85 to 94 per cent of upper division of the upper Cretaceous (the Rip- of the Edwards limestone. They are best shown calcium carbonate, the residue consisting of mag­ ley group of the Atlantic coast, the Montana GULF SEEIES. in the cut of the International and Great Northern nesia, silica, and a small percentage of f erric oxide. group of the Rocky Mountain region, and the Railroad at the west end of Pecan street in the Strata of the Gulf series underlie all that por­ Although sometimes so much indurated that it Corsicana and Eagle Pass formations of northern city of Austin (see Columnar Section sheet 2) and tion of the Austin quadrangle in the Coastal can be used for building purposes, it is generally and southern Texas). The following forms from on the slopes of the hills near Taylor's limekiln. Plain east of the Manchaca belt, and even include too soft and crumbling for such use. It is princi­ Webberville have been identified by Mr. T. W. Del Rio clay. This is a peculiar greenish-blue a part of that belt. pally used as road material, but is a poor one, the Stanton : Anomia conradi, Leda protexta, Corrida laminated clay which weathers dull brown or The series presents four recognizable forma­ road builders preferring the far more durable crassiplica, Drillia ? distans, Splienodisciis lenticu­ yellow and forms a very black soil. It is from 80 tions in the Austin quadrangle Eagle Ford, materials of the lower Cretaceous. The fossils of lar is. Three fossil oysters, Exogyra costata, to 100 feet thick at Austin, where it has its typi­ Austin, Taylor, and Webberville. The Wood­ the Austin chalk are very abundant, consisting of GrypJiwa, vesicularis, and Alectryonia larva, are cal occurrence in Shoal Creek, shown on Colum­ bine (Dakota) formation, which marks the base a great number and variety of species, especially common and characteristic fossils. nar Section sheet 2, and at Fish Pond bluff near of the upper Cretaceous in Texas and elsewhere inoceramus, oysters, and ammonites. Its thickness Although probably not the very highest layers the mouth of Barton Creek. It outcrops imme­ north of the Brazos, is absent in the Austin is difficult to determine, but averages about 500 of the upper Cretaceous system, the Webberville diately beneath the Buda limestone and rests region. feet in other parts of the State. The Manor well beds are the highest Cretaceous exposures seen upon the Georgetown limestone, the uppermost These formations are the result of one continu­ shows the thickness in the Colorado section to be along the Colorado River > for below Webberville. band of which is characterized by the occurrence ous sedimentation and pass one into the other about 410 feet. they are overlain by the basal division of the of Kingena wacoensis. It is an especially import­ without sharp lines of demarcation. They are The Austin chalk is the most conspicuous of Eocene Tertiary. Good outcrops of the forma­ ant feature of the geologic column,. composed mostly of indurated strata marls, the formations of the county, outcropping along a tion are exposed at only a few localities, although » , . , Laminated marly clays, and glauconitic clays with one wide belt extending through the central portion, the residual soil occupies an extensive area. Low, makingo a break in the monotonous ciaymg great contain- sequence of limestone beds, and pos- o?SnPecuifar conspicuous chalk formation (Austin chalk) near and constitutes the principal formation upon vertical bluffs of the formation may be seen just sesses lithologic and paleontologic char- oyster their base. Owing to the soft nature of these which the city of Austin is built. Here fine out­ north of the east base -near Onion Creek and near acters which render it easily recognizable. It can formations the surface occupied by them is usually crops are exposed in the street cuttings and along the water line of the Colorado between the mouth always be identified by means of a peculiar fossil, flat, or gently rolling, and devoid of scarps. The the bluffs of Waller Creek. Fairview Park, in of Onion Creek and at Webberville. (See Colum­ Exogyra arietina, a little oyster shown on Illus­ mantle of detritus is also deeper than in the South Austin, presents excellent exposures of the nar Section sheet 2.) tration sheet 2, figs. 35 and 36. This occurs in the area occupied by the formations of the Comanche Austin . chalk, the white roadways being cut The topographic expression and the general greatest abundance, weathering out by the thou­ series. through this material, while the drainage has fur­ characters of .the soil are about the same as in the sand in a state of perfect preservation. Attached nished abundant bluffs and canyons. Along the case of the Taylor marl. The country is open to these shells, especially on the umbonal region, COLORADO DIVISION. south side of the margin of the valley the Austin prairie except where mesquite trees, which mayv are small cubes of iron pyrites. Upon decompo­ Eagle Fordformation. -This formation consists chalk forms a picturesque bluff from 2 miles below be abundant, occur, and the soils are black and sition this coats the shells with thin layers of of laminated bituminous clays, shales, and impure the Deaf and Dumb Asylum, finally disappearing sticky. The lands erode easily and after heavy brown iron oxide, and converts the lime into limestone flags, usually blue or black when to the eastward at the Montopolis bridge. (See rains gullies are often produced similar to those numerous crystals of fibrous selenite, which are unweathered, but becoming light yellow and Columnar Section sheet 2.) Around the perim­ described in the discussion of the Taylor marl. Austin. 6

EOCENE PERIOD. The Uvalde gravel also caps the round hills the river bluff north of Del Valle. In the eastern raphy between Pilot Knob and Buda, as well as from Onion Creek southward toward the south part of the county the sediments become finer and the distribution of the formation, suggests that at There is but one formation in the Austin quad­ border of the quadrangle and covers the eastern begin to spread over a large area of upland. one period in its history this area was Old lake bed. rangle which may be positively referred to the half of the Mustang ridges. Denudation has Capitol terrace. Below the Asylum level there a small lake, caused by the obstruc­ Eocene period. This is the Lytton formation. resulted in the almost total destruction of the are traces of several other terrace planes bor­ tion of the Pilot Knob protrusion, through the Lyttonformation. This formation, named from Uvalde Plateau north of the river. The most dering the Colorado River, but the next in northern end of which the stream later cut an Lytton Springs, situated in the southeast corner conspicuous occurrence of this gravel north of the importance is that upon which the capitol and outletting canyon, thereby lowering its level. of the quadrangle, consists of laminated clay, Colorado is at the large butte known as Bald the residence portion of Austin are built. Tillot- IGNEOUS ROCKS. clay and sand, and sandstone, the latter often Knob, just east of Manor, which stands about 750 son University is also situated upon it. This showing cross bedding. The color before oxida­ feet above the river and is the highest point in the terrace is very similar in material to the Asylum Basalt. Eight miles east of south of Austin is tion is bluish, but when subjected to weathering northeast quarter of Travis County. The summit terrace, being composed chiefly of the debris of a peculiar hill called Pilot Knob. It is composed becomes white, yellow, or reddish, depending of this hill is composed of a thick mass of the flint the Paleozoic granites of the Llano region. It of black volcanic rock, which, according to the upon the amount of ferruginous matter contained, gravels, and it is readily seen that it is but a rem­ can be traced eastward down the river. East of determination of Prof. J. F. Kemp, is a nepheline the degree of oxidation, and the extent of leach­ nant of a now almost destroyed area, and is the Austin the terrace gradually merges into the wide species of the family of basalts known as limburg- ing. Only a few fossils were found in the area continuation of the St. Elmo Plateau of the south bottom of the Colorado. South of the river this ite. (Pilot Knob, a marine Cretaceous volcano, covered by this formation within the Austin side of the river. The flints of the old plateau terrace forms the cap of the high bluff or valley by Robert T. Hill, with notes on its petrography quadrangle. They were Venericardia alticostata now veneer the newer surfaces. wall of the Colorado bottom at the State Asylum by J. F. Kemp: Am. Geologist, November, 1890.) Conrad, Turritella, and Calyptrcea. The fauna The innumerable flints which so largely compose for the Deaf and Dumb. At Pilot Knob this material constitutes the belongs to the basal division of the Eocene and the Uvalde formation were all derived from the /Second bottoms. The Colorado has carved still remains of the neck or core of an corresponds to that of the Midway formation of Edwards limestone of the Edwards other and lower valleys below the level of the ancient volcano occupying an area of anTxtillct of Arkansas and Alabama. Plateau west of the Balcones fault line, ?{foosteposlt Asylum and Capitol terraces and above the pres­ about 3 square miles,.-, irom» the! center volcano. The relations existing between the Lytton and and it is evident that this great deposit ent flood plains and east of the mouths of Barton of which rise three stubs or hills of the black Webberville formations have not been posi­ of flint gravel represents the destruction of the and Shoal creeks, which are filled with rich columnar basalt, which are encircled by an area tively determined because actual contacts were Edwards formation, which once existed over the alluvial soil. Although standing high above the composed largely of tuffs and debris. Pilot not found within the quadrangle. The faunal central province in Bur net, Llano, and other coun­ present waters of the river and never overflowed Knob was evidently the center or focus of an break is very complete and the abrupt Faunal break ties. When it is considered that there is not one by it, these lower benches are known as the "Bot­ ancient volcanic outbreak, and within a radius of manner in which the Eocene abuts cSceeo£se foot of flint to 100 feet of limestone in the toms," from the very evident fact that in late 6 miles of this locality there are many minor against the Cretaceous suggests an ero- and Eocene' Edwards formation, it can be imagined how great geologic time they were the flood plains of the accompanying phenomena, such as small dikes sion unconformity, such as has been shown to must have been the denudation which resulted river. These really consist of five or more dis­ and beds of tuff. exist along the Frio River in western Texas, in in the deposition of the flints in the Uvalde tinct levels of sedimentation, but the differences of The only outcrop occurring north of the Colo­ southern Arkansas, in Mississippi, and in Alabama. formation. elevation are so slight that they can not be rado River is about If miles east of the Austin The area occupied by the Lytton formation lies Although the Uvalde formation now occurs as delineated on the map. The second bottom is post-office, at the point where the Austin and in the southeastern portion of the quadrangle and high as 300 feet above the adjacent bottom of the widely cultivated and its agricultural conditions Northwestern Railroad turns northward out of is bounded on the northwest by a line extending Colorado, it is probable that it was not originally are very different from those of the upland Cre­ the Colorado Valley. This is a small neck of approximately from near Elysium to a point 1|- a continuous sheet across the whole eastern half taceous lands. nepheline-basalt, less than 100 square feet in area. miles northwest of Lytton Springs. The thick­ of the county, but may have been deposited in an Flood-plain deposits. The flood plains or true Due south of Austin, on the opposite side of the ness of the formation within the quadrangle is estuary or group of estuaries and have been rep­ bottoms of the stream are rather extensive, and river, in a ravine which makes down to the river about 300 feet. resented coastward by finer and finer sediments. in the region of the Coastal Plain apparently from the east side of Fairview Park, there is

The topography consists of low hills usually hav­ The exact acreCD of the Uvalde formation can not large in proportion to the small stream of water another small outcrop of the basaltic rock. One- ing rather gentle slopes, but occasionally, where be stated; but that it was laid down after the which normally occupies them. That of the quarter of a mile from where the International there is a capping of sandstone, the hillsides are Balcones faulting began is indicated by the exten­ Colorado from Austin eastward consists, first, of and Great Northern Railroad crosses Williamson rather precipitous. The soil is .usually sandy, sion of the material up the stream valleys of the a wide sand bed, occupied by the stream at low Creek, and about three-fourths of a mile west of but contains argillaceous and ferruginous material, Edwards Plateau, and in every condition it seems water and during times of ordinary rises, bordered St. Elmo, a railway cutting exposes a peculiar varying in amount from place to place. The sur­ to correspond with the Lafayette formation, which by a higher bench, from 5 to 15 feet above the mass of rotten volcanic tuff capped with Uvalde face is clothed with a growth of hardwood timber, marks the close of the Pliocene and the initiation river, of alluvial land, which is overflowed only in gravel and filling a dike or fissure in the Austin in large part post oak. of the Pleistocene. (See Eighteenth Ann. Kept. times of extraordinary flood. The stream itself chalk. U. S. Geol. Survey, Pt. II, p. 260.) flow~s in places over its own debris and in other Two miles south of the above locality, on the SURFICIAL ROCKS. places is cutting into the bed rock. San Antonio road, about a mile north of St. PLEISTOCENE PERIOD. The Colorado and other streams in the Austin The accompanying table shows the locality, Edwards College, there are other outcrops of quadrangle in past times have veneered the Cre­ Although the Uvalde terrace has been so nearly height, and composition of some of the terraces similar material. Along the canyon of Onion taceous and Eocene formations in places with vast destroyed that its former continuity can be recog­ recognized on both sides of the Colorado River. Creek north of Pilot Knob there are beautiful sheets of old alluvium. These formations may be nized only with difficulty, the later terraces which The altitudes given represent as nearly as possible exposures of tuffs interstratified with the Austin divided into several categories, the oldest being mark the slopes of the valley of the Colorado are the original deposition surface of the formation. chalk, containing many casts of fossils made up of hypsometrically the highest. very plainly recognizable. There are many of these, all of which may be classified into three Table showing relation of terrace phenomena on the sides of the Colorado Valley near Austin. NEOCENE PERIOD. distinct categories Asylum terrace, Capitol South side of river. North side of river. Uvalde formation. East of the great Balcones terrace, and second bottoms. Composi­ Altitude Composi­ fault the surface, at an altitude of from 640 to Asylum terrace. An old high-level terrace, Location. tion. above sea. tion. Location. 750 feet, is covered with a thick deposit of flint usually a mile or two back from the river, occurs bowlders, usually mixed'with a marly matrix of as the summit of a bench which marks the interior Feet. Hills 3 miles southeast of Manchaca...... 1 calcareous material derived from the nearby for­ margin of the so-called Colorado bottoms. This 775 1 725 mations. The flints are often large and all show terrace in the city of Austin is located north of the 2 675 signs of having been rolled. The .original extent State University and forms the sandy post-oak flat TWrpn TTill 4 675 ' 1 of this old gravel, as well as the configuration on which the State Lunatic Asylum is situated, St. Elmo divide ...... 1 650-675 which existed at the time it was deposited, can about 150 feet above the river. It is traceable 635 . 2 Lunatic Asylum, Austin. not be determined with exactness, because only for many miles toward the east. Lytton Springs...... 1 2 625 patches of the gravel remain. So abundant are 615 2 University of Texas. The material of this terrace is formed almost 1 550 these remnantal areas, however, that the original exclusively of granitic debris derived from the 525 1, 2 State capitol; Tillottson College; terrace extent of the deposits in this quadrangle may be granite mass of Burnet County, in the Terrace east of mouth of Walnut Creek. approximately restored. The Uvalde. alluvial shape either of fine quartz sand, the f^ftfc0* 1 500 2, 3 Fourth street terrace. deposit radiates out from the canyons of the Col­ 480 2, 3 Railroad station and city bridge bluff. plastic feldspathic material oxidizing Del Valle Bluff, lower part...... 2 475 orado, Barton Creek, and Onion Creek into a fan- into red clay, or of gravel consisting of large 4 430 2, 3 shaped area, with its broader edge toward the angular pieces of quartz. Pieces of the Paleozoic 4 422 2, 3 Stream way, Austin. eastern margin of the quadrangle. It is repre­ limestone of the Burnet-Llano region and some sented in the hilly district west of the great fault flint from the Edwards limestone also occur. The 1 = Rolled flints, white limestone, and other debris of Cretaceous formations of the Edwards Plateau. 2 Largely quartz and feldspar from granite of the Burnet region, with mixture of Paleozoic limestone. by narrow terraces along the borders of the material of this old terrace is so recent looking 3 - Red Beds debris. valleys of the river. The greatest body of the that, were it not for its high position above the 4 = Mixture of the above materials. Uvalde formation lies between the Colorado and river and the progressive series of lower terraces Onion Creek marl. Another Pleistocene for­ this material, which fact has suggested the Creta­ Williamson Creek, where it constitutes a distinct which have been cut out of it, it would be deemed mation having considerable extent in the Austin ceous age of at least a part o;f the old Pilot Knob plateau. Here the gravel sheet is so mixed with recent. quadrangle is the alluvial deposit which borders eruptions. Up to date no occurrence of this the calcareous matrix that it often makes a rich, The Asylvmi terrace is characterized by a dense the banks of Onion Creek. For this formation material has been found west of the Balcones black soil, which has usually been mistaken for growth of black-jack and post oak. West of we shall use the name Onion Creek marl, although fault zone. the residium of the underlying Cretaceous forma­ Shoal Creek, toward the Balcones fault, small it contains in places some gravel. Both the marl GEOLOGIC STRUCTURE. tion in situ. As this alluvial gravel sheet is not patches of it are preserved upon the dissected sur­ and the gravel are the calcareous debris of the readily removed, it persists in places as a capping faces. Remnants in Taylor's quarry and along Cretaceous area through which this stream flows. The geologic structure or arrangement of the of hills or divides, while the weathering of its the plateau between Shoal Creek and the Inter­ It consists of wide belts of a yellow, calcareous rocks in the Austin quadrangle is that of a mono­ edges above the softer underlying formation national Railroad clearly show that the Asylum marl, sometimes 40 feet deep, which extends up cline broken by a zone of faulting, with intrusions produces sharp slopes around its margin along terrace once extended up to the fault line. the valley of Onion Creek from its mouth below of igneous rocks on the downthrown side of the the lateral drainage of the Colorado and Onion South of the Colorado the Asylum level has been Del Valle to west of Buda, where it emerges from fault. The general inclination of the strata is Creek from 2 miles east of Oatmanville to near almost destroyed, but patches of it are preserved the highland. The Onion Creek marl, however, is from north of west to south of east, in conformity the town of Del Valle, on the south side of this here and there, especially on the hill 'south of principally developed above where that creek cuts with the inclination of the whole of the Regional river. (See fig. 5.) Barton Spring on the Oatmanville road and along its canyon through Pilot Knob, and the topog- Coastward Slope between the Rocky Mountains 7 and the Gulf. The rate of dip is different, how­ The Cretaceous rocks of the Comanche and The history of the Coastal Plain since the devel­ contain oil in commercial quantity, as has been ever, in the two greater provinces represented in Gulf series, which constitute the larger part of opment of the Balcones fault system in Eocene proved by experiments at Watters. the quadrangle, the Edwards Plateau and the the geologic formations, are old sea muds, com­ time has been complicated. Our knowledge of Three miles northwest of Austin a little very Coastal Plain. This change of inclination accom­ posed in part of the debris of preexisting lands this history is only sufficient to suggest the broad fine-grade paraffine oil was found in a dug well, panies the faulting of the Balcones zone, which and in part of calcareous material extracted from and inviting field which the subject offers to the as reported to the writer by Dr. Edgar Everhardt. passes northeast and southwest through the quad­ the sea waters by the animals which inhabited future student. Since Eocene time the plain as a Unfortunately no great quantity could be pro­ rangle and which will presently be described. To them and deposited as their fossil remains. The whole has been uplifted some 700 feet above the cured. Therefore the indications are that oil does the west of the main Balcones fault the dip of the latter, at least, is the source of much or most of sea along its interior margin and the sea has not exist in commercial quantities in the Austin strata is almost parallel to that of the regional the lime material, which is the predominant com­ receded from the Balcones scarp to its present quadrangle. slope, or less than 20 feet to the mile, and hence ponent of the Cretaceous rocks. These rocks position. STEUCTUEAL MATERIAL. the rocks in that region lie in sheets nearly parallel were laid down in the margins of the Gulf of The portions of the Colorado and other streams to the surface. East of the main Balcones fault Mexico as it invaded and conquered a land which within the Edwards Plateau likewise present an Building stone, brick clays, lime, cement, and line the strata have a greater inclination, and they had existed in Jurassic time and progressed west­ interesting Pleistocene history. In the building sand abound. become deeply embedded to the east by increased ward into what is now the Rocky Mountain valley of the Colorado (see figs. 4 and 9) S Building stones. These occur in the lower dip in that direction. The regularity of the dip region of the United States. The preexisting there.u, are erosion surfacesf whichi i. seem to/ region- Cretaceous rocks, where there are many layers of in this region is broken by numerous minor faults land consisted of a complex of Paleo­ indicate a correspondence with the terrace deposits white and yellow limestone suitable for various (see figs. 3 and 6), which fact, together with the zoic and Archean rocks, such as are ceoushinad of the later epochs in the Coastal Plain. Onion uses. In the upper part of the Edwards forma­ lack of good exposures along which the dips can now being reexposed by erosion in the and Barton creeks have terraces through the high­ tion there are certain flaggy layers which have be measured, renders it impossible to ascertain Burnet quadrangle. From what is known of the lands which are evidently of great age and seem to been used for paving and curbing in Austin with certainty the rate of dip. history of the Cretaceous formations lying to indicate that those streams are more ancient than and which make a durable stone for fences and From the records of the Manor well the Austin the northward, it is probable that this period the other minor drainage. Up both these streams, house walls. These have been extensively quarried chalk, which outcrops 5|- miles to the westward, of subsidence was broken in the middle of Creta­ as well as the Colorado, there is a distinct bench in the western part of Austin and the material at Sprinkle, is reached at a depth of 590 feet ceous time by an episode of elevation, and that covered with flint debris, which seems to indicate enters into the structure of many buildings in below the surface and 690 feet below the altitude hence the epoch consisted of two subsidences that these streams also took part in bringing down that city. of Sprinkle. Hence it is apparent that the strata instead of one. the vast supply of gravel which formed the oldest Another limestone in close proximity to this is of the Coastal Plain in this portion of the Austin The old Cretaceous sea encroached upon the or Uvalde terrace. known as the Austin marble. This is composed quadrangle become embedded through fault dis­ preexisting Jurassic land until the former land The Austin quadrangle furnishes many data of of an agglomerate of calcified fossils, Austln placement at the rate of about 130 feet to the mile. surface had sunk to a depth equaling u Hft |n the value for the understanding of the later history principally Jtudistes and Requienia. marble- The Balcones fault zone consists of a series of the thickness of the lower Cretaceous cret1/ceofushe of the Coastal Plain and the successive uplifts The stone in the quarry has a Avhite, chalky color steep faults running north of east and south of formations, or about 1800 feet. The period' which it underwent before the final reclamation on fresh fracture, but is sometimes varied by cream- west through the quadrangle and principally Dakota formation, which represents the littoral of of all the great eastern and southern provinces of yellow and pale-pink colors. It is susceptible of traceable along the western margin of the Coastal the second subsidence, is missing in the Austin the Texas region from the Gulf of Mexico. These a high polish and makes beautiful marble for Plain. This zone of faults, as a whole, results in and other quadrangles south of the Brazos, and it data are recorded in the drainage of the old ter­ interior purposes, having a creamy tint, like the the droppingJ-i o down of the western mar- Zone_ of,, fault- is probable that in this region of the State the races and in vast deposits of upland gravel. marbles of Portugal, Spain, and North Africa, gin of the Coastal Plain along the foot [ffiS the sea bottom did not completely emerge during The testimony of the Uvalde formation is harmo­ which produce harmonious effects in decoration. of the Balcones scarp. Although the scarp' the mid-Cretaceous episode, as was the case to the nious with and corroborative of that of the topog­ This material is not commercially utilized for zone is composed of a number of minor faults northward. The change of level, however, is raphy of central Texas as to the former extent marble, although the stone is largely used for along its western margin, there is a major fault indicated in the Austin quadrangle not only by of the lower Cretaceous formations over the cen­ rubble work, as may be seen in the government- which is very conspicuous and easily traceable the absence of the Woodbine and lower part of tral region and the vast denudation that must have building at Austin. In the lower part of the from Watters southward via Spice wood Springs, the Eagle Ford formations, but by the abrupt taken place. There is no doubt that at the time Edwards formation and in the Glen Rose forma­ Amboy, the east foot of Mount Bonnel, Bee Creek, change of sediments taking place as noted in the of its deposition the agents of erosion tion there are layers of a slightly arenaceous, open- and Oak Hill. This fault results in a downthrow contacts between the Buda limestone and the base were very active, over whati , is now cen- TheColoradoineancie ancient textured, buff-colored limestone which is of the River. to the eastward, so that the Edwards limestone, of the Eagle Ford clays, the former being an off­ tral Texas and that the Colorado then class known as Caen stone and of the kind much which caps the Edwards Plateau to the westward, shore marine foraminiferal limestone and the emptied into the sea close to the present line of used throughout the world for carving. Some is found some 500 feet lower on the eastern side latter a shallow-water clay. the Balcones fault. The Uvalde formation also layers of the Austin chalk, notably that taken from of the river in the stretch between the eastern foot Toward the close of the Cretaceous, regional tells the story that at the time of its deposition the quarry near Amboy station, are also used for of Mount Bonnel and Austin. There are a num­ subsidence was again succeeded by elevation and the Colorado had not cut down into the Paleozoic building purposes, but this stone is not very dur­ ber of minor faults, which produce little or no the oceanic waters again receded from the Rocky and granitic beds of the Central Denuded Region, able. effect upon the topography, but which are visible Mountain region toward the present coast line of but was still flowing over a Cretaceous floor, for Many of the Cretaceous limestones are soft in stratigraphic sections. This minor faulting is the Gulf. This is indicated in the shallow-water very little if any of the debris of these earlier for­ in the quarry, where they may be cut with a well shown in Shoal, Barton, Williamson, and character of the Taylor marl and the Webberville mations, which makes so large a part of the later saw or axe, but grow hard on exposure to the Onion creeks. The details, however, are very formation, succeeding the Austin chalk. There alluvium of the Colorado, is represented in the atmosphere. complicated, as is shown in fig. 3, which is a care­ were emergence and erosion at the end of the Uvalde terraces. Brick days. Excellent brick clays are found fully prepared representation of a section along Cretaceous. Subsidence was renewed during the Concerning the history of the volcanic phe­ in the alluvial terraces of the Colorado and are the south bank of the Colorado. Eocene. nomena little can be said. There is evidence, as extensively used locally and shipped to various

EDWARDS PLATEAU BALCONES FAULT ZONE B LACK PRAI Rl E

KCP* Kgr Cedar Hill BeeCreek Smith Creek Boulc/in Creek Kdr Kpd Kef

Sea level Fig. 3. Section along the south side of the Colorado River, passing through Austin, showing the details of the Balcones fault zone, and the increase of dip toward the east. Horizontal and vertical scales, 1 inch = 3000 feet. It is impossible to tell how far eastward this As the ocean was withdrawing over the vast elsewhere shown, that during the Austin epoch of parts of the State. The quality of bricks produced zone of faulting continues, owing to the fact that area from the Rocky Mountains to the eastern the upper Cretaceous, Pilot Knob was a marine is excellent. Both white brick of the Milwaukee individual faults can not be traced in the uncon- margin of the Austin quadrangle, erosion and volcano; this is indicated by the apparent con­ type and red brick are made. The manufacture solidated material of soft marl and sandy clay for­ uplift were producing changes in the configura­ temporaneity of the deposition of its volcanic of tile, drainage pipe, and other articles of this mations in the eastern half of the quadrangle. tion of the Central and Plains provinces of the debris and that of the Austin chalk. kind has not been attempted. Along the western margin of the Coastal Plain, Texas region. The sea border during Eocene Lime. Lime making is an extensive industry, in the White Rock, Manchaca, and Bear Creek time was approximately along the east­ ECONOMIC GEOLOGY. which is carried on at Austin and McNeill, the countries, where the rocks are more consolidated, ern half of the Austin quadrangle, and product being shipped as far east as New Orleans , . . . , , I1, i of faulting. the faults are plainly visible. during this epoch the older rivers, such The Austin quadrangle abounds in many of and west to the Pacific coast. This lime, com­ The volcanic necks, such as Pilot Knob and the as the Colorado, were depositing great loads of those mineral resources which add to wealth and mercially known as Austin lime, is exceedingly small plugs in east and south Austin, are forced land debris on the margin of the sea, laying down commercial development, especially structural white and pure and is made from the Edwards through the subhorizontal sedimentary rocks. a load of sediments whose weight may have caused material such as building stone, cement, and limestone. The underground extent of these intrusions has the development of the great fractures now known lime and road metal, and contains a diversity Cement material. The Cretaceous rocks of not been determined, but it is probably small. as the Balcones fault zone, and initiated the pres­ of agricultural soils. Metallic minerals are lack­ Travis County abound in materials for making The interpretation of the structure has an ent configuration of the Coastal Plain. This, ing, and mineral fuels in-commercial quantities both natural and Portland cement. The flaggy important bearing upon the determination of the however, is entirely hypothetical. are apparently absent. arenaceous limestone of the Eagle Ford formation, depths of underground reservoirs and constitutes Since that epoch the Edwards Plateau has been when burned in a kiln, produces a natural cement OIL. an interesting problem for future students of the maintained as a great physiographic feature in the of fair quality, like that which for many years local geology. Texas region, but its present summit level, occu­ The Webberville formation, which underlies the was manufactured at San Antonio. Materials for pied by the hard Edwards limestone, is only a eastern edge of the area, is oil bearing, but the making artificial Portland cement are unusually GEOLOGIC HISTORY. lower remnant of its former surface, at one time quantity and commercial value of the oil have not abundant, and from them a product equal to the The rocks and configuration of the Austin most probably largely covered by the upper been determined. This formation, which is the best foreign cements now so extensively imported quadrangle illustrate the geologic history of Cretaceous formations, which have since been same as that which supplies the Corsicana oil, is into Texas can be manufactured. This material detailed portions of the Coastal Plain and Plateau removed. not sufficiently embedded to justify the expectation includes limestones in a marly or chalky condition, of the Great Plains, but until more detailed study The history of the Rocky Mountain region and of oil in the Austin quadrangle, but it presents a clays, and sands, all of which may be procured, in is made of the larger provinces as a whole the the Plains Province, to the latter of which the favorable field for exploitation on the western close proximity to one another and to lines of history can not be fully understood. Hence the portion of the Edwards Plateau within the Austin margin of the adjacent Bastrop quadrangle. transportation. Hitherto the chief obstacle to the remarks upon the history recorded within this quadrangle belongs, has been a long 'era of erosion The Eagle Ford shales often yield globules or development of a cement industry has been the quadrangle can be considered only as a partial and elevation, during which canyon cutting has other small quantities of oil. but as these shales cost of fuel and the difficulty of obtaining sites contribution to a larger subject. been more or less intermittently progressing. are very thin there is no probability that they and trackage. Austin. 8

Sand. Quartz sand of excellent quality for material of the Colorado and other bottoms, while present too remote from fuel or transportation to and that, theoretically, the water from'these will building purposes abounds in the alluvial valley large flint gravel of excellent quality for crushing give them value. rise to the surface-at points below 600 feet in of .the Colorado. into durable and sharp road metal abounds on altitude. SOILS. the margins of the uplands within the Black Owing to the structural conditions previously FLINT. Prairie. One of the chief natural products of Travis described, these reservoirs are embedded at differ­ Flint is found in commerial quantities in the The nepheline-basalt of Pilot Knob makes one County is the superb group of agricultural soils ent depths in the regions of the Edwards Plateau Edwards limestone west of Austin and in the old of the hardest and most durable road metals to be which result from the surficial weathering of the and the Coastal Plain. In the Edwards Plateau gravel deposits of the Coastal Plain. Elsewhere found, but it is hardly probable that the necessities rocks. Each formation described in this folio the basement Trinity reservoir, which is the best material of this kind is extensively used in the of the country would warrant the present cost of weathers into a peculiar soil which has its own source of supply, is embedded from 300 feet above arts, for sanding sandpaper, glass making, ball crushing and utilization. qualities of production, as is attested by the dif- sea level along the western border of the quad­ grinding, mixing with potters' clays, and for other ference of natural growth upon them. The rela­ rangle to about sea level at Mount Bonnel. In purposes, and is imported into the United States CABINET SPECIMENS AND MINERAL CURIOS. tive merits of these soils can not be discussed here, the region of the Coastal Plain the basement from France and England. No use has yet been A small industry has been carried on in mining nor has scientific investigation advanced sufficiently reservoir is embedded about 1800 feet beneath made of the Texas material. the beautiful crystals of calcite, aragonite, and to give a final presentation of them. In general, Austin, or 1200 feet below sea level, and probably strontianite which occur in the recesses of the however, the deep and rich soils are found in the 3000 feet, or 2500 feet below sea level, in the ROAD METAL. bluffs of Mount Bonnel. Epsomite is also found area of outcrop of the Gulf series of rocks, while longitude of Manor. Road metal of unusually excellent quality and in this locality in quantities suggestive of value. they are shallower in the region of the Edwards There are several reservoirs above the basement of many varieties may be found in the quadran­ Some beds may possibly be capable of supplying Plateau and in the Bear Creek and Manchaca belts. Trinity sands which may be struck at shallower gle. Limestones and siliceous hydraulic marls strontianite for use in the arts. The abundant depths, but these are impregnated with mineral for making light macadamized country roads and fossils, if properly collected, also have commercial ARTESIAN WATERS. matter. drives are abundant: The Buda limestone and value for educational uses. The underground waters of the Austin quad­ Shallow flowing wells at depths of less than certain layers of the Edwards and Glen Rose rangle are carefully discussed in more extended 1000 feet could probably be obtained along the limestones are especially suitable for this purpose. GLASS SAND. reports. (See Eighteenth Ann. Rept. U. S. Geol. banks of Lake McDonald between Volente and In the experiments in road building thus far Some of the sands of the Travis Peak formation Survey, Pt. II; and Twenty-first Ann. Rept., Pt. Mount Bonnel, the depth decreasing to the west­ made in the quadrangle soft and unsuitable layers are of sufficient purity to suggest their possible VII.) It may be stated, however, in a general ward. have too often been used. Gravel of any size may use for glass-making purposes. Their outcrop in way, that all portions of the quadrangle are be obtained in great quantities in the alluvial the western part of the Austin quadrangle is at underlain by the reservoirs of the Trinity group May, 1900.