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

Stonewall quadrangle is south of the Llano region and is in the marginal portion of the Edwards Plateau where much of the plateau surface has been destroyed by erosion. Along the southern edge, lobes of the Edwards Plateau are present, which coalesce southward to form a continuous narrow east-west plateau belt part of which is just south of the Stonewall quadrangle. Most of the quadrangle is within the gently undulating, broad Pedernales River basin.

The geology of the Stonewall quadrangle is shown on a planimetric map, and the only topographic map available is the reconnaissance 30-minute Fredericksburg quadrangle. Elevations ranging between 1,423 and 1,969 feet were determined during traversing for control, but neither the highest nor the lowest elevation was reached. However, it is estimated that the relief within the quadrangle is about 600 feet, ranging between about 1,400 and 2,000 feet in elevation.

The quadrangle is largely within the Pedernales River drainage basin and is mostly drained by Three Mile Creek, South Grape Creek, and its tributary Dry Creek. Cave Creek enters the Pedernales River within the quadrangle, and other important branches drain a considerable area in the vicinity of the river. A small area in the southeastern corner of the quadrangle is drained by branches of the Blanco River.

The Stonewall quadrangle is high on the southern side of the Llano uplift, and rocks of Cambrian and Ordovician age outcrop in the northern part as inliers surrounded by Cretaceous rocks. Faulting related to the Ouachita orogeny (Barnes, 1948) is present. The Cretaceous rocks are essentially horizontal, dipping eastward about 10 feet per mile.

Broader discussions of the stratigraphic, structural, economic, and geophysical problems of the region cannot be given in the space available. References cited below deal with some of these problems. This publication on the Stonewall quadrangle is one of a series of similar publications, an index to which is shown on the opposite page. The reader is referred to this index map to locate other quadrangles mentioned in the present text.