BUREAU OF ECONOMIC GEOLOGY THE UNIVERSITY OF TEXAS AUSTIN 12, TEXAS

John T.Lonsdale,Director

GEOLOGeologic Quadrangle Map No. 19

FallProng Quadrangle Kimble,Gillespie, andMason Counties, Texas

By Virgil E. Barnes

May 1956 Geology of the Fall Prong Quadrangle, Kimble, Gillespie, and Mason Counties

VIRGIL E. BARNES

GeneralSetting Granuleintraformationalconglomerate Angular granules are common in the Fredericksburg Group proximatedby following the black band scribed in the text of the Wendel quad- and Barnes,1944; Barnes, Romberg,and common near the base of the Edwards composed of one-fourth-inch aphanitic lower part of the Hensell sand; upward Included within the Fredericksburg mentioned above. rangle. Anderson,1954a, 1954b, 1955).The area limestone, butnone weremapped within Fall Prong quadrangle is in the mar- granules in a fine-grained matrix is the Hensell is finer grained, containing group of the Fall Prong quadrangle is The Comanche Peak limestone is No collectionswere madewithin of lowgravity valuesin thenortheastern the Fall Prong quadrangle. ginal portion Plateau of the Edwards common. Glauconite is mostly rather much silt and clay. In the western part about 300 feet of Edwards limestone, describedbelow in the sections measured the quadrangle, and no attempt was part of the quadrangle is associatedwith Sandstones in the probably near thesoutheasterncorner of theLlano Peak scarce but ina few beds is rather abun- of the quadrangle the upper part of the about 32 feet of Comanche lime- along Little Devils River just west of made to find silicified , although a minimum whichincludes the northern are present beneathmuch of the quad- region. Almost three-quarters of the stone, clay. dant. The beds are mostly thin with a Hensell is very calcareous. The Hensell and about 2 feet of Walnut the quadrangle and along Fall Prong. some probably exist. Some of the lime- part of the Threadgill and Squaw Creek rangle butmostly at considerabledepth. quadrangle is Edwards Plateauand the grada- few as much as 6 inches thick. sand varies widely incolor with redsand The boundaries of the units are The Comanche Peak limestone is fossi- stonebedsof theEdwardsare essentially quadrangles. Town Mountain granite The Hickory sandstone contains water rest is valleys of the Llano basin cutting quadrangle Thomp- No fossils were collectedfromthe San grays predominant. The red material tional, and in this liferous especially in its basal portion, a cemented foraminiferal sand, and crops out north of the Squaw Creek in many places in central Texas and back into theplateau. Sabamember withinthe quadrangle,but tends to be more abundantin the lower son's (1935) observationthat theseunits and indications of fossils are common in fragmental fossil material is abundant quadrangle. probably is water bearing within the The geology of the Fall Prong quad- several collections have been made from part of theHensell, but even here there should have about the rank of members some of theupper beds.Fossils fromone throughout the Edwards limestone. Diorite was encounteredin the Rown- Fall Prongquadrangle.The depth to the However, rangle is shown on a planimetric map; it in the James River section a short is considerablematerialof gray, yellow- seems logical. insteadof intro- collection, locality 19— 17A, a little more tree No. 1Kott well in the Spring Creek portion of the Hickory sandstone that is Quarternary Deposits the only topographicmap availableis the distance to thenorth. ish-gray, and greenish-gray color. ducing a new name, Fredericksburg than a mile southeast of the Gillespie quadrangle.Itis unlikely that the maxi- water bearing is about1,200 feet in the reconnaissance 30-minuteKerrvillequad- The Hensell is solittleinduratedthat could easily be dropped from group to County corner, have been identified by High gravel The areas mapped as mum is caused entirely by diorite;since northwestern part of the quadrangle; rangle. between Ellenburger Group rank, especially since a poorly superimposedmaximum Elevationsranging 1,723 Tanyard Formation it readily breaks down and forms gentle formational the Dr. Ralph Imlay as follows: high gravel are mostly in the western defined southward the Hickory sandstone is Geological Survey the and 2,245 feet were determined during Threadgill member -About 40 to 50 slopes except immediately beneath the U. S. excludes and northern parts of the quadrangle. exists to the west of the well,itis likely deeperif thedip seenat thesurface con- traversing for control, but the lowest feetof white-weathering,mostly aphanit- Glen Rose and Comanche Peak lime- Kiamichi clay from the Fredericksburg Monopleura pinguiscula White The deposits are chiefly colluvial and that a large diorite mass has intruded tinues. However, the structure of the (Wilmarth, 1938, p. 776) texana (Roemer) ? elevation was not reached. However, it ic belonging stones. Some is group r Toucasia appear to remnants a more the Packsaddle schist. is complicated, and it limestone to the lower part of the Hensell cultivated Arctica sp. be of much Paleozoic rocks is estimated that the relief within the of the Threadgill member crops out near but not as much asis cultivated farther Walnut clay The Walnut clay is Pecten (Neithea) duplicicostaRoemer extensivedepositthat once coveredmuch is impossible to predict their depth or quadrangleis about 545 feet,ranging be- the northwestern corner of the quad- east. about 2 feet thick in the section along of the surface beneath the Comanche MineralResources even their presence beneath areas of tween about 1,700 2,245 and feet. rangle. The limestone is light gray in A section along the west side of Little Fall Prong, and in the western part of Edwards limestone The Edwards Peak scarp. In the vicinity of streams outcrop. The known resources of the The quadrangle is entirely within the part mottled by darker shades of gray Devils River about 2,000 feet west of the quadrangle in the vicinity of Little limestone is estimated to be about 345 some of the high gravel deposits are mineral The quality of the rest of the sands River it is representedby an ac- probably mostly quadrangle are limited to nonmetallic Llano River drainagebasin andis mostly andyellowish gray. Thebeds are mostly the western edge of the Fall Prong Devils feetthickbetweenthefalls on FallProng streamdeposits and are in theCambrian as aquifers is unknown, tributary to James River via LittleDevils between 2 and 10 inches in thickness. quadrangle and about 9,500 feet south- cumulationof Exogyra at thebase of the and Herman triangulation station. The reworkedfrom the colluvium. Nolineof substances and water. Outside of the but the Welge sandstone and possibly soil, is mostly for range River, important branches of which are Some fine- to very fine-grained beds southwest of the mouth of Fall Prong is Comanche Peak limestone. In thenorth- lower boundary, as explained above, is demarcation exists in border-line cases which used some of thesands in theSan Saba lime- quadrangle it land, the most important nonmetallic Fall Prong and White Oak Creek. Salt especially in thelower part contain pin- described below. The section is ex- eastern part of the rests gradational and is chosen at the baseof betweendepositsthat are truly colluvium stone probably contain some water. The sand, in resources are construction materials. Branch in the northwesterncorner flows point specks of glauconite.One dolomite ceptionally well exposedin a bluff but directly on the Hensell and the a thin-bedded zone which is thought to and those that are depositedby stream Lion Mountain and Welge sandstones rest quadrangle it rests Glen deposits are directly to James River,anda small area bed in the lower part is about 2 feet in part is inaccessible. of the on correlate with a similar zone in central action. For this reason the ConstructionMaterials are about 600 feet above the Hickory in the part thick, Rose beds. The Walnut clay is shown on Gillespie mapped under onedesignation. southeastern of the quad- fine grained,paleyellowish brown, Glen Rose limestone member-The County. Building stone Some limestone beds sandstone,andthe sandstonesin theSan rangle into Threadgill the map by a solid colorline. drains Creek and and weathers to light brown. GlenRose limestone is 33 feet thick in The Edwards limestone in the Fall The high gravel is composed chiefly in theEdwardslimestone, the San Saba Saba limestone are another 400 to 600 reaches Llano River via Beaver Creek. Gastropods such as Ophileta, Gascon- the Little Devils River section, 23 feet The Walnut clay, described in the Prong quadrangle is composed of a of pebbles, cobbles,and finer materials limestone, and the Threadgill member feetnearerthe surface. The Fall Prong quadrangle is wellup adia, and Lytospira are numerous on thick in the northwesternmost outcrop sections given below, is composed of variety of rock types including lime- including some caliche.Much ofthe ma- are of suitable thickness to be used for on thesouthwesternsideof theLlanoup- bedding surfaces,butno fossilcollections in the quadrangle, 15 feet thick a mile clay grading upwardinto an impurenod- stone, dolomite, and chert. The lime- terial is limestone, chert, and dolomite ledge-stone in building. The limestone SelectedReferences lift and is covered by Cretaceous rocks were made. east, disappears ular limestone. The clay is very cal- stone and dolomite varyin composition, from the Edwards, limestone from the in is in general Barnes, V. E. (1944) Gypsum in the to the and one-halfmile the Edwards softer and Texas: crop out over entire quad- careous, silty, sandy, fossiliferous, and texture, ; Comanche Peak, and siliceous material Edwards limestone of central which the farther east. Southward along Little thicknessof beds,andhardness easier to workthan the limestonein the Texas Pub. 4301, Jan. 1, 1943, mostly yellowish gray. It is too thin to Hensell Univ. rangle except in the northwestern part MesozoicRocks Devils River but still west of the quad- this variation is very clearly shown on from the sand. other units. The present trend toward pp. 35-46. influence noticeably the vegetation and where Cambrian and rocks Cretaceous System (Lower Cretaceous) rangle the GlenRose limestone thickens, aerialphotographs by banding, because Alluvium-Deposits of alluvium are the use of somber stone should open a (1948) Ouachita facies in appear. One fault formed during the and in the subsurface in the southern culture of the area.No fossil collection of the segregation of the vegetation into mostly situated along Salt Branch, Little market for some of the greenish and centralTexas:Univ.Texas,Bur.Econ. Ouachita orogeny (Barnes, 1948) is ex- Shingle Hills Formation part of the quadrangle it may be as was made from the Walnut within the bands of marked difference in density. DevilsRiver,White Oak Creek,andFall brownish-gray stone in the Cambrian. Geol.,Rept.Inv. 2, 12 pp. posed. The Cretaceous rocks are essen- Hensell sand member (Barnes, 1948) much as 60 feet thick. The variation in quadrangle. The outcrop of the Edwards limestone Prong. Narrow belts and patches of al- The Threadgill stone is lighter colored ,Dawson,R.F.,andParkin- — son, (1947) Building stones of tially horizontalandmay dip asmuch as The Cretaceous rocksin theFall Prong thickness is caused by the lateralgrada- Comanche Peak limestone The Co- has an average density of vegetation luvium follow many of the lesser drain- but tends to be yellowish gray. Some G. A. central Texas: Univ. Texas Pub. 6 feet per milesoutheastwardin most of quadrangle are essentially horizontal, tion of the limestone and dolomitebeds manchePeaklimestone is 31.5 feet thick greater than that of the Glen Rose and ages in the area but are insignificant beds in the Edwards limestone are light 4246, Dec. 8, 1942, 198 pp. the quadrangle. Broader discussions of except in the northwestern part of the of the GlenRose to terrigenousmaterials in the Fall Prong section and 32 feet the Hensell. The hard limestone beds andhave notbeen mapped.The alluvium gray to white and are very attractive. , Romberg, Frederick, and

the stratigraphic, structural, economic, quadrangle where they dip perhaps as of the Hensell. thick in the LittleDevils River section; inthe sequence weatherslowly andhave is composed mostly of sand and silt at Road material -Caliche derived from Anderson, W. A. (1954a) Correlation and geophysical problems of the region much as 6 feet per mile southeastward. Rose the thickness elsewhere withinthe quad- only a thin soil covering and are conse- the surface and of coarser materials be- the weathering of Edwards limestone of gravity and magnetic observations The contact between the Glen geology Blanco Gil- arein cited publi- rangle probably little from these quently nearly void vegetation. mostly with the of and references below.This The thickness of the Hensell sand can limestone and the Hensell sand is arbi- varies of The neath. has been used for base-course lespie counties, Texas: Proceedings, cation on the Fall Prong quadrangle is be estimated ifthe elevationof thelowest trarily placedat thebottom of thelowest figures. The Comanche Peak limestone softer bedsdevelop a more adequate soil material in the construction of Ranch 19th InternationalGeologic Congress, one of a series of similarpublications, an point at its base and theelevation of the bench-forming limestone or dolomite. grades downwardinto the Walnut clay and are thickly covered with growth, SubsurfaceGeology Road 385.Suchmaterialis of some value Algiers, 1952, Sec. 9, Contributionsof 9, index to which is shown on the opposite base of the overlyingunit areknown.The In following the contact, as carbonate and upward into theEdwardslimestone. most of which is a scrub oak identified for surfacing secondary roads but has geophysics to geology, vol. pp. 151— The San Sabamemberof the Wilberns 162. page. The reader is referredto theindex lowest elevation on the base of the Hen- beds come or go, the contact is lowered The contact withthe Edwardslimestone by Cuyler (1931) as "Quercusfusiformis formation and the Threadgill member little value for base-course material arbitrarily placed a Sargent (mountain oak)." map to locate other quadrangles men- sell is estimated to be 1,723 feet in the or raised to the baseof the next one. In is at the base of scrub of the Tanyard formationin the north- since it causes freeze damage. Ranch (1954b) Geology and geophysics of in present part quadrangle. very thin-bedded limestone which is Above the abrupt slope of the Co- 385 less a year tioned the text. northwestern of the The mapping, however, it is almost impos- western part of the quadrangle extend Road had beenbuilt than Blanco and Gillespie counties,Texas: base of the Glen Rose in the same area sible to decideat whichpoint thecarbon- thought to correlatewith a similar zone manche Peak limestone the Edwards when a freeze destroyed large portions San Angelo Geological Society Guide- into the subsurface beneath Cretaceous book, trip— is 1,901 feet, giving an outcrop ate cease exist, so containing some chert in central Gil- limestone flattens out into gently slop- Better is Cambrian field Llano Geologic thickness beds to the basal rocks. No well data were obtainedwithin of its surface. material avail- pp. 78-90. Formations feet lespie County. ing surfaces except where oversteepened in in area, March 19-20, of 178 for the Hensell. Additional boundary of the Glen Rose is notmapped the quadrangle, and the nearest wells able somezones the Edwardslime- probably present by undercutting along major It stone and in units PaleozoicRocks beds are in the sub- as a series of steps but is gradually The Comanche Peak limestone con- streams. are rather distant.The Rowntree No. 1 some of the Cambrian (1955) Map showing correlation of System (Lower Ordovician) is consequently almost impossible to and just the north Ordovician surface in the southern part of the quad- rounded off from the baseof one to the tains much argillaceous material es- Kott well in the Spring Creek quad- Ordovician to of the geologic, gravity, and magnetic ob- rangle because the surface on which the base of the next. pecially in its portion. lower measure and describe a section of the quadrangle. Some of the Paleozoicrocks servations, Blanco and Gillespie coun- basal The rangle entered the Honeycut formation ties, Texas: Univ. Texas, Bur. Wilberns Formation (Upper Part) Hensellrests slopesingeneral southward. be Edwards limestone except in its lower are hard enough tobe used forgranules. Econ. part Most of the contact was traced on part of the Comanche Peak tends to of the Ordovician,and the OwenNo. 1 Geol. Only the Lower Ordovician of the portion where action is strongest. The Hensell sand varies widely in aerial photographs between observed nodular, is mostly fossiliferous, and is stream Tatsch well in the Dry Branch quad- Sand and gravel Some sand and Bridge, Josiah, Barnes, V. E., and Wilberns formation is known to crop out The Edwards surface is mostly rocky color and composition throughout the points withthe aid of a stereoscope, and in part burrowed.The upper part of the rangle entered rocks of Pennsylvanian gravel is present in alluvial deposits Cloud, P. E.,Jr. (1947) Stratigraphy within the quadrangle. Just north of the from the presence of hard limestone of the Upper Cambrian, uplift, area,being influenced to some extentby since the terrace of any one carbonate Comanche Peak is mostly well bedded age.It is likely, therefore,that all units along LittleDevils River andFall Prong. Llano quadrangle alongJames River 57 feet of beds and chert. The chert has a fairly Texas:Bull. Geol.Soc. Amer., vol. 58, the type of rock being transgressed. bed gradually merges with the general and in the Little Devils River area is of the Ordovician are present within the The materialis usable for local building rock belonging to the Riley formation general distribution, but some outcrop pp. 109-124. Within the quadrangle it rests on por- slope as it dies out, itis easy to raise or mostly dolomite. quadrangle; considering the faulting only. Much of the outcropping Paleozoic Cloud, E., Jr., Barnes, and 619 feet belonging to the Wilberns bands are free of it. Some of the chert P. and V. E. tions of the San Saba member of the lower the contact to the next one. The The Comanche Peak limestone is which has taken place in the pre-Cre- rock and some zones in the Edwards (1948) The Ellenburger group of formation have been measured; the in the Edwards limestoneis of a quality Wilberns formation andofthe Threadgill Glen Rose— Hensell contact is poorly softer than the overlying Edwards lime- taceous rocks of the Llano uplift, it is limestoneare of valuefor crushed rock. central Texas:Univ.TexasPub. 4621, thickness of the various units is as suitablefor themanufacture artifacts 1, 1946, 473 pp. member of the Tanyard formation. Inthe exposed within the Threadgill quad- stone and harder than the underlying of possible that several units of the Cam- June follows: and because it was used extensively by Cuyler, R. (1931) Vegetation as an subsurface within the quadrangle it is rangle, being in steep portion Walnut, Rose, andHensell; conse- perhaps some the Carbon- H. Thickness situated a Glen brian and of Water geologic : likely the on units the aborigines is referred to as flint. indicator of formations Bull. (feet) that Hensell rests all of the topography where much colluvial quently, it has erodedinto a steep slope iferous are also present. Amer. Assoc. Petr. Geol., vol. 15, pp. Wilberns formation 619 oftheLower Ordovician andseveralunits materialis creeping downwardfrom the which is characteristic of its outcrop Within the Fall Prong quadrangle The information about the Pre- A ground-water survey of Gillespie 67-78. Sabamember 284 the is the San of Cambrian. Comanche Peak and Edwards lime- throughout the quadrangle. The Co- there little evidence of presence cambrianrocksupon whichthePaleozoic County was made by Shield (1937). Palmer, A.R. (1954) The faunas of the PointPeak shalemember 188 gypsum in much The upper boundary of the Hensell stones. The terraces in the Glen Rose are manche Peaklimestone has a distinctive of such as is seen of lie limited to gravity There Only one well,110 feet deep, was inven- Riley formation in central Texas: MorganCreek limestonemember 127 rocks is data. Paleontology, vol. 28, pp. 709- sand is rather sharp in thenortheastern not as well developed within the quad- vegetation cover especially on north northern Gillespie County. Banding is gravity toriedin the Fall Prong quadrangle. The Jour. Welge sandstonemember 20 is a ridge, possibly part of a -786. — part of the quadrangle whereitis incon- rangle as they are in Gillespie slopes clearly on undisturbed throughout the Edwards water level in 1936 stood 67 feet below Riley formation eastern which shows aerial closedhigh,running north-souththrough Rock Committee (1948) clay, within the quadrangle, as seen on the surface, probably ColorChart LionMountainsandstonemember 57 tact withthe Walnut but westward County; therefore, the stereoscope is of photographs as a black band. A narrow- the quadrangle. The apex of the ridge the and the water is Rock-color chart: Natl. Research boundary clear in many aerialphotographs; if gypsum originally the The solids SanSabamember—The upperpart of the is not cut less valuein tracing thelower boundary. leaf oak identifiedby Cuyler (1931) as is quite sharp where it crosses thenorth- from Hensell sand. total Council, Washington, D. C. places the Hensell is in contact is Sargent (Texas oak)" had beenpresent its removal wouldhave contained are 309 parts per million.No the San Saba member crops out in the where The Glen Rose limestone terminated "Quercus texana central border of the quadrangle. This Romberg, Frederick, andBarnes, V. E. Rose beds. The Glen Rose part of the dominant the allowed the overlying beds to collapse, informationon ground water is recorded (1944) gravity obser- northwestern part of the quadrangle. It with Glen eastward in the northern is the tree on Comanche ridge appears to be superimposed on the Correlation of limestone is absent in the northeastern map;thepointchosen forits termination Peak limestone. destroying the continuity of the banding. for Kimble County, and none was sought vations with the geology of the is in part innormal stratigraphicposition northwest flank of the large high in the Smoothingiron mass, Llano part the quadrangle of lateral purely arbitrary. The portion while mapping the Fall Prong quad- granite beneath the Ellenburger and in part in of because is In mapping the Comanche Peak lime- lower of the Edwards Harper quadrangle to the southeast and County, Texas: Geophysics, vol.9, pp. gradation the limestone of is section rangle. fault contact with it. The rocks are of the Glen The Glen Rose limestonein Gillespit stone, points at which its boundaries limestone describedbelow ina appears to die out as it crosses the 79-93. Rose terrigenous the measured along Prong. section mostly fine- to coarse-grained, yellowish- to sediments of County consists of alternating beds oi crossroads were placedon aerialphoto- Fall The southeastcorner of theFall Prong quad- The Hensell sand varies in its quality Shield,Elgean (1937) Recordsofwells, originally by (1944) analyses graylimestone;one18-inch, dark yellow- Hensellsand. limestone, dolomite, clay, and sand or graphs. In addition points of contact described Barnes rangle. In both the northeastern and as an aquifer. In some areas in the drillers' logs, and water in is part here, using Gillespie County, Texas: State Board ish-orange dolomite bed; and near the The Hensell sand is in general very better stated, beds having various pro- were mapped at many places between in redescribed the northwesternparts of the quadrangle the northern and western parts of the quad- terminology the of Water Engineers, 51 pp. top a small amount of aphanitic, light- poorly sorted and ranges from coarse portions of these materials and in ad- roads and on alloutliers. On portions of color of Rock Color value of gravity is less, indicating that rangle it will be an easily available (1948) distinguishing Thompson, S. A. (1935) Fredericks- gray limestone. material such as boulders, cobbles, dition silt. Inthe Fall Prong quadrangle the aerial photographs for Gillespie Chart and dolo- gravity minima will be centered outside source of ground water, and at greater mite burg group of Lower Cretaceous with In the James River section the first pebbles,andgranules through thevarious the Glen Rose consists mostly of dolo- County having stereoscopiccoverageand and limestone. theborder of the quadrangle.Inthe area depth in the rest of the quadrangle some special reference to north-central significant sandstone in the San Saba is sand sizes to silt andclay. No conglomer- mite and silty clay. Allof the Glen Rose on all photographs for Kimble County Another section wholly within the Ed- of outcroppingPrecambrian rocks of the water may be present. In the southern Texas: Bull. Amer. Assoc. Petr. Geol., vol. 19, pp. 1508-1537. about 36 feet beneath the top of the ate was mapped within the quadrangle, limestone is described in the Little the boundaries were traced under the wardslimestone is measured along White Llano uplift, large gravity maxima are and eastern parts of the quadrangle Wilmarth, M. G. (1938) Lexicon of member andmay be the sameone that is but outcrops of it arepresentalong Little Devils River section, and 9 feet of it is stereoscope; where stereoscopic cover- Oak Creek partly just within the Fall associated with Packsaddle schist and^ somewater might be found near thebase geologic names of the United States, distance in Prong section age waslacking forGillespie County, the Prong quadrangle and partly in the large gravity minima are associated of the Edwards in in fault contact with Ellenburger rocks Devils River a short from the described theFall given limestone.Southward Part 1:U. S. Geol. Survey Bull. 896, near the road. western edge of the quadrangle. below. boundaries can stillbe very closely ap- Wendel quadrangle. The section is de- withTown Mountain granite (Romberg Kerr and Gillespie counties springs are 1244 pp.

StratigraphicSections

LittleDevils River Section ThicknessinFeet Feet above The amount of insoluble residue, for portions of the section, after removal of ThicknessinFeet Feet above Description Interval Cumulative base carbonateminerals with dilutehydrochloric acidis as follows: Description Interval Cumulative base ThicknessinFeet Feetabove — — - Description Interval Cumulative base 10. Dolomite microgranular, yellowish gray to, 1 59 113 -114 eet abovebase Percentresulue 6. Dolomite microgranular, grayish yellow, 5 45 33 38 grayish yellow with a pattern of dark yellowishi massive,porous,lip of falls at 36 feetinsection. Fredericksburg group: 46 feet measured orange, veryargillaceous,slightly silty. 2- 7 57.8 — - Edwards—limestone:14 feet measured — 7- 12 50.2 7. Dolomite microgranular, dark yellowish gray 5 50 28 33 1. Dolomite microgranular; light olive gray; 8 8 164 -172 11. Clay calcareous, slightly silty, light greenishi 16 75 97 -113 12- 17 41.0 of several shades, porous, in part calcitic,

slightly argillaceous; petroliferous; porous; gray. 17- 22 54.8 massive at base to burrowedand honeycombed vuggy, some vugs as as 4 inches in size; 22- 27 73.5 at top. much 12. Dolomite— microgranular, grayish yellow, ar- 4 79 93 - 97 — - thick,blocky, rest interval 27- 32 85.5 8. Limestone yellowish gray, very argillaceous, 12 62 16 28 bottombed 2 feet of gillaceous,slightly silty andsandy. massive with an irregular nodular surface. 32- 37 72.9 sandy, silty,massive; burrowed with little con- Molds of gastropodsandpelecypods inupper Hensellsand member:— 93 feet measured - 37- 42 71.8 trast in lower part but with marked contrast part. — 13. Siltstone and clay siltstone,poorly indurated, 18 97 75 93 42- 47 59.5 in upper part where burrowsare dolomiticand 2. Limestone very fine grained, mottledbetween 3 11 161 -164 light greenish gray from 75 to 77.5, 79.5 to 81, 47- 52 70.4 betweengrayish orangeand yellowish orange; grayish orange and dark yellowish orange, 82.5 to 83.5, and 86 to 88.5 feet; remainder 52- 57 46.0 weathers differentially. bedding indistinct except in upper 3 inches, clay,light greenish gray. 57- 58 79.6 Oysters commonin lower foot. — - 9. Limestone— yellowish argillaceous, - intraformational— conglomerate in upper part. 14. Clay light greenish gray except for one thin 6 103 69 75 93- 97 13.1 gray, very 4 66 12 16 3. Dolomite microgranular,light grayish orange, 2 13 159 -161 palered streak near bottom. 111 45.3 somewhat sandy and silty, soft, burrowed, scarce, mottled similar to above, somewhat argilla- — - 113-114 27.1 fossils one calcite geode 2 inches in 15. Siltstone(?) light greenish gray, bedding 2 105 67 69 124-126 39.1 diameter. ceous, poorly bedded, porous, weathers into wavy,hackly cross break. forms. — - 126-131 11.3 Walnutclay: 2 feet thick rounded 16. Clay calcareous, silty, in part sandy, mostly 10 115 57 67 — - at top in irregular lobate 131-138 13.5 10. Clay andlimestone verycalcareous,light gray 2 68 10 12 Chalcedonic chert light greenish gray, some pale red, a masses up to 2 feet in length, tones of brown 2-inch 138-143 7.7 clay grading upward into very argillaceous, and grayish— brown. indurated—bednear middle,otherwiserecessive. - 143-148 5.0 light gray limestone, both sandy, silty, bur- 4. Limestone white, beds % to 1 inch thick, 1 14 158 -159 17. Siltstone calcareous, yellowish gray to light 1 116 56 57 148-153 5.1 rowed. splitsreadily along bedding. greenish gray, indurated, resistant forming a 153-158 6.1 Exogyra and other fossils. Comanche Peak limestone and Walnut clay: ledge.— - 158-163 4.9 ShingleHillsformation:10 feet measured 32 feet thick 18. Clay calcareous and silty; mostly pale red 23 139 33 56 163-168 2.2 Glen Rose—limestone member: 10 feet measured 5. Dolomite— microgranular, very light grayish 6 20 152 -158 from 33 to 46, 48 to 51, and 53 to 56 feet; 168-172 1.7 11. Limestone light yellowish gray, top somewhat 4 72 6-10 orange mottled between grayish orange and remainder light greenish gray. darker yellowishgray,argillaceous,silty, sandy, dark yellowish orange, somewhat argillaceous, — - burrowed, indurated, slightly irregular bed- 19. Clay calcareous, silty, sandy, light greenish 4 143 29 33 ding; nodular fromabundant poorly preserved distinctly bedded,bedsvery thin to 1footthick. zone vertical,cylindrical, Fall Prong Section — gray; a of white con- fossils burrows; cracks in top 2 6. Dolomite microgranular,light grayish orange, 5 25 147 -152 nearmiddle up 2inches diameter and feet filled cretions to in ThicknessinFeet Feet by Walnut clay; calcite geodesup to 4 inches mottled between grayish orange and dark and 6 inches long, some are compound, some above yellowish orange, weathers medium gray, potato-shaped. Description Interval Cumulative base in diameter— common. are 12. Clay yellowish gray, silty, calcareous, con- 3 75 3-6 slightly argillaceous, porous, essentially one 20. Clay — calcareous, silty, pale red, 12 155 17-29 bed. mottled Fredericksburg group: 68 feet measured tains pyrite nodules which have oxidized on 7. — argillaceous, grayish yellow 38 134 -147 yellowish gray near middle; breaks with a Edwardslimestone: 36 measured exposure. Limestone 13 blockyfracture,slumpslike clay. — feet - (not mottled, slightly sandy, massive, burrowed* - 1. Limestone extremely fine grained except for 5 5 73 78 13. Clay— white exposedin 1951). 3 78 0-3 throughout, dolomitic, 1 21. Clay— verycalcareous, sandy, light gray,reces- 5 160 12 17 scattered microgranular dolomite rhombs, one burrows average inch The amount of insoluble residue after removal of carbonate mineralswith dilute in diameter. sive, somewhatconcretionary. massive bed, slightly fossiliferous. — gray hydrochloric acidis as follows: Numerous gastropod casts and caprinids, a 22. Sandstone very fine grained, light gray, top 3 9 - 12 A few olive chertnodules. 163 Dolomite — microgranular, yellowish gray 22 27 51 - 73 eet aboveIbase Percent res\ ue few Exogyra. foot somewhat concretionary, indurated, forms 2. to — light olive gray, calcitic near middle, porous 4- 6 90.4 8. Limestone argillaceous, sandy, nodular, 8 46 126 -134 one bed which thickens southward at the ex- with in part of two beds 38.7 weathersrecessive,yellowish gray mottled. pense of interval 23 and incorporates interval pores molds fossils; 6-10 Very fossiliferous,mostly Exogyra. 24. withbreak at 65 feet. 10-15 26.7 ShingleHillsformation: 126 feet measured — Chert, olive— gray to light olive gray nodules. - 15-20 17.5 GlenRoselimestone member:33 feet thick 23. Clay very silty, calcareous, pink at base to 2 165 7-9 3. Limestone microgranular to extremely fine 4 31 47 51 20-25 13.2 From58 to 120feet thebluff was too steep to climb, light greenish gray at top, breaks with blocky grained, yellowish gray, some yellowish orange 25-30 12.5 and the rock is described as seen from a distance. fracture,recessive, pinches out to south. specks,soft,in partnodular. 30-35 8.0 — — 4. — microgranular, yellowish gray, 5 36 42 - 47 9. Clay verycalcareous, silty, sandy,light green- 12 58 114 -126 24. light gray,slightly sandy, irregularly 2 167 5-7 Dolomite 35-42 8.9 Siltstone 6 very thinly bedded, rest ish gray in lowerpart, inupper part yellowish, bedded, pinches outlaterally. bottom inches 42-47 7.9 darker colored,mottled,burrowed ( ?). — medium bedded. 47-50 9.5 * 25. Sandstone silty, argillaceous, calcareous, 5 172 0-5 Chert at 45 and 46 feet moderatebrown,thin 50-55 5.3 "Burrowed" refers to the passage of ananimal through thesediment producinga texture different from coarsely mottled red and light gray, weathers inconspicuous plates; at 47 feet nodules. 55-60 3.8 that in the surrounding material. Openings mostly did not exist, aince the soft sediment closed behind the knobby as if unevenly cemented, upper 2 feet Comanche— Peaklimestone:30 feet thick 60-65 15.5 as it progressed. Openings that did exist were soon filled. The "honeycombed"rock so common at recessive; sand mostly quartz, some chert, 5. Dolomite microgranular, yellowish gray, some 4 40 38 - 42 65-73 3.3 thesurface is mostly a differentially weathered,burrowed rock. angular to slightly rounded,rough. yellowish orange specks,distinctly bedded. 73-78 11.8 GEOLOGIC MAP OF THE FALL PRONG QUADRANGLE, KIMBLE, GILLESPIE, AND MASON COUNTIES, TEXAS 1956