Geologic map of the Palo Alto Creek Quadrangle, Gillespie County, Texas

Palo Alto Creek 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 northern edge of the quadrangle, a portion of a finger of the plateau trends eastward, and in the west-central part, another finger of the plateau frays out into a number of isolated outliers. The rest of the quadrangle is a portion of the gently undulating broad Pedernales River basin.

The geology of the Palo Alto Creek quadrangle is shown on a planimetric map, and the only topographic map available is the reconnaissance 30-minute Fredericksburg quadrangle. Elevations ranging between 1,556 and 1,975 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 550 feet, ranging between about 1,525 and 2,075 feet in elevation.

The quadrangle is entirely within the Pedernales River drainage basin and is mostly drained by Palo Alto Creek and its tributaries. Barons Creek drains the southwestern corner of the quadrangle, and Willow Creek and the tributaries of North Grape Creek drain a small area in the northeastern corner.

The Palo Alto Creek quadrangle is high on the southern side of the Llano uplift, and rocks from pre-Cambrian to Ordovician in age outcrop as inliers surrounded by Cretaceous rocks. The faulting accompanying the Ouachita orogeny (Barnes, 1948) is not exposed, but dips in Paleozoic rocks up to 9 are present. The Cretaceous rocks are essentially horizontal, dipping eastward about 8 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 Palo Alto Creek 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.