The Tertiary Buck Hill volcanic series, about 4000 feet thick, covers most of the Tascotal Mesa quadrangle in Presidio and Brewster counties, Texas. The regional dip in the northern part of the quadrangle is 2°–4° SW., and differential erosion has developed three prominent levels. Green Valley in the northeastern part is underlain by Pruett and Duff beds. Rising approximately800 feet above the level of Green Valley is Bandera Mesa capped by the resistant Mitchell Mesa tuff flow. Tascotal Mesa rises 800 feet above Bandera Mesa and is capped by flows of the Rawls basalt that form a dip slope to the western boundary of the quadrangle.
The southern third of the quadrangle, set off from the northern part by east-west faults, is characterized by rugged topography developed chiefly on the Rawls basalt flows. In the southeast corner Cretaceous strata that dip to the north off the Solitario uplift form hogbacks and cuestas.
The Solitario uplift, initiated in late Cretaceous time, continued during the deposition of the Tertiary volcanic series which thins and wedges against the dome. Uplift probably was not at a uniform rate; a period of marked uplift accompanied or followed shortly the intrusion of riebeckite soda rhyolite which forms numerous hills in the southeast part of the area.
The intrusives contributed abundant large boulders to the conglomerate of the upper beds of the Tascotal formation that lap on the slopes of the rhyolite hills. Boulders of Caballos novaculite and of black chert suggestive of the Maravillas formation indicate an appreciable contribution from the folded Paleozoic rocks that were uncovered by erosion of the Solitario complex.
Late Tertiary orogenic movements resulted in compressive forces acting on the structurally downwarped Marfa basin in which the Buck Hill volcanic series accumulated. The volcanic series was folded in a broad anticlinal structure, and the southwestward-dipping beds in the northern part of the quadrangle are the west limb of this large structure. Uplift was accompanied or followed by normal faulting.
The igneous rocks of the Tascotal Mesa quadrangle are alkalic. Analcime and soda-rich pyroxene and amphibole are characteristic. Both extrusive and intrusive rocks show a silica range from 46 to 73 per cent.
Quaternary deposits are related to the physiographic history of the region; gravel-capped pediments in Green Valley are now being dissected.