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.