Geologic map of the Stonewall quadrangle, Gillespie and Kendall Counties, Texas

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Barnes, Virgil E. (Virgil Everett), 1903-1998

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University of Texas at Austin. Bureau of Economic Geology


Stonewall quadrangle includes part of the southern margin of the Llano region and part of the eastern margin of the Edwards Plateau where most of the plateau suriace has been destroyed by erosion. An east-northeastward-trending lobe of the Edwards Plateau crosses the quadrangle near its southeastern corner. The area to the north is in the broad gently rolling Pedernales River basin, and that to the southeast is in a more rugged marginal part of the Blanco River drainage area. The geology of the Stonewall quadrangle is shown on a U.S. Geological Survey 7.5-minute topographic quadrangle map (contour interval 20 feet, scale 1:24,000) which covers the same area as the Stonewall quadrangle planimetric map, scale 1:31,680 (Barnes, 1952i). The relief in the quadrangle is about 600 feet; elevations range from about 1,395 feet where Pedernales River leaves the quadrangle to about 1,995 feet at the highest point on the Edwards Plateau. The quadrangle is mostly drained by Pedernales River and its tributaries- Blumenthal, Cave, Roundhead, Threemile, and South Grape Creeks, Beckman, Gellerman, Salt, and Arnelger Branches, and other unnamed drainages. In the southeasternmost part the drainage is southward to Blanco River. Stonewall quadrangle is high on the southern side of the Llano uplift. Cambrian and Ordovician rocks occur in patchy outcrops in the vicinity of Pedernales River and northward. Cretaceous rocks and some Quaternary surficial deposits occupy the remainder. The one fault within the quadrangle is related to the subsurface Ouachita structural belt (Flawn et al., 1961, pp. 65-81) and trends northeast-southwest. The Paleozoic rocks mostly dip gently southward to southwestward at angles up to about 8 degrees, and the relatively flat-lying Cretaceous rocks dip eastward about 10 feet per mile. This publication on the Stonewall quadrangle is the fourth of a series of Central Texas geologic quadrangle maps which are being compiled as topographic bases become available. An index map for geologic maps already published on planimetric and topographic bases and others planned for publication on the new topographic bases is shown with the geologic map. During the period 1939- 1947, the writer, assisted by Louis Dixon, mapped geologically and made a gravity survey of the Stonewall quadrangle.


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