Gillespie County
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Conglomerate (Building Stone), Gillespie County, Texas
Specimen Location: Outcrops along the west valley wall of Threadgill Creek upstream from Lange's Mill, Gillespie County, Texas. Rock Description: Lower Cretaceous basal conglomerate with pebbles of limestone from the Threadgill member, the Wilberns formation, and the Cap Mountain limestone member; also some hard quartzite pebbles of Welge and Hickory sandstone and very rarely a small pebble of granite and some chert. Specimen dimensions: 13 x 10 x 4.5 cm Weight: 760 grams UT Collection ID: TMM-2232 Virgil Barnes described, "the most desirable conglomerate yet found lies above the Ordovician Threadgill member of the Tanyard formation and the Wilberns formation in the Lange's Mill area" on p. 158 of "Building Stones of Central Texas", Univ. of TX publication No. 4246, 1942. I believe that this is a sample that Virgil collected from that location. In the permanent collection of the Jackson School of Geosciences, University of Texas at Austin.
Flint, Gillespie County, Texas
Location: Gap Creek County: Gillespie County, Texas Chemical formula: SiO2 (microcrystalline quartz) Specimen size: 5 x 4 x 2.5 inches Specimen weight: 562 grams Collection reference: TMM 1586 In the collection of the Jackson School of Geosciences at the University of Texas at Austin. Brown flint prehistoric arrowhead points (Nolan type) have been recorded from Gillespie County.
Gypsum, Nodular ('chicken wire'), Gillespie County, Texas
Mineral: Gypsum Location: Temple-Inland Quarry (north of Fredericksburg) County: Gillespie County, Texas Chemical Formula: CaSO4· 2H2O Crystal System: Monoclinic Specimen Size: need to measure Specimen Weight: need to weigh Specimen in the private collection of Professor J. Richard Kyle, Jackson School of Geosciences, the University of Texas at Austin. Other interesting facts: Occurence in Cretaceous Kirschberg Evaporite Member of Edwards Formation
Serpentinite, Gillespie County, Texas
Serpentinite, a building or decorative stone composed of one or more serpentine minerals (in this case Antigorite) Locality: Coal Creek Serpentine Prospect, north Gillespie County, Texas Mineralogy: > 90% antigorite, ~4% opal, 1-2% magnetite, lesser amounts of chromite, pyrite, calcite Crystal System: Antigorite is monoclinic Specimen size: 4 in x 2.5 in x 1 in thick Specimen weight: 280 grams In the BEG hand sample collection. Additional Information: Virgil Barnes, et al. assessed the PreCambrian Serpentinite deposits around the Llano Uplift in 1950 and provided the mineralogy above (BEG Pub. #5020). They mapped the Coal Creek serpentine mass at 3.7 miles long and ranging from 0.3 to about 1.4 miles wide. It's been reported that serpentinite from this area has been used for terrazzo chips.