Tertiary tuff beds and related sediments with intercalated lava flows, aggregating over 2000 feet, are named the Buck Hill volcanic series for outcrops in the Buck Hill quadrangle, Brewster County, Texas. The volcanic series rests unconformably on the Upper Cretaceous Boquillas formation. The Lower Cretaceous is represented by the Georgetown limestone, the Grayson clay, and the Buda limestone.
The lower 900–1000 feet of the Buck Hill volcanic series is named the Pruett formation, and the upper 1000–1400 feet, the Duff tuff. Lava flows up to 325 feet thick, designated the Cottonwood Spring basalt, separate these formations. A limestone pebble conglomerate and arkosic sandstone at the base of the Pruett mark the Cretaceous-Tertiary unconformity. The Pruett tuff commonly is calcareous and grades to beds of fossiliferous freshwater limestone. A massive layer of breccia-conglomerate occurs near the top of the formation, approximately 60 feet below the base of the Cottonwood Spring basalt. Lava flows intercalated with the Pruett sediments, from oldest to youngest, are the Crossen trachyte, the Sheep Canyon basalt, and the Potato Hill andesite. The Duff formation is chiefly silicic tuff with minor breccia and a few thick beds of stream conglomerate. It is overlain by the Mitchell Mesa rhyolite, the youngest rock of the volcanic series in the quadrangle.
In the southern part of the quadrangle Straddlebug Mountain and Buck Hill are small intrusives of syenite; similar syenitic intrusives cut the Cretaceous, the Pruett tuff, and the Sheep Canyon basalt in the Elephant Mountain area. The igneous rocks are alkalic and chemically resemble the analyzed rocks from the Terlingua-Solitario region to the south.
The main structural features are west and northwest-trending normal faults. Two major fault zones divide the quadrangle into three blocks with a middle down-dropped segment. Differential hardness of the rocks of the Buck Hill volcanic series controls the topography.