The Tascotal Mesa quadrangle is in southeastern Presidio and southwestern Brewster counties, Trans Pecos Texas. Bandera Mesa and Tascotal Mesa are reached most easily from Marfa, 50 miles north, by the Marfa-Lajitas road which traverses the quadrangle from north to south.

The rim rock of the Vieja Rim, the quartz pantellerite of Lord, is named the Bracks Rhyolite.

Tertiary rocks, including sandstone, conglomerate, shale, pyroclastics, tuff, and lava, are preserved in Big Bend National Park and in a much larger area to the west and northwest. Some of the rocks have distinctive characteristics that enable recognition by their lithology.

Mountain-bounded basins or bolsons are the most important aquifer systems throughout most of southwestern and western United States and northern Mexico.

Hot springs and wells in West Texas and adjacent Mexico are manifestations of active convective geothermal systems concentrated in a zone along the Rio Grande between the Quitman Mountains and Big Bend National Park.

The Chinati Mountains caldera, which lies in Trans-Pecos Texas in the southern Basin and Range Province, was formed by eruption of the Mitchell Mesa Rhyolite.

The 37-Ma-old Infiernito caldera of the northern Chinati Mountains is a major eruptive center of the mid-Tertiary volcanic field of Trans-Pecos Texas. Volcanism in the Chinati Mountains began with eruption of a sequence of rhyolitic to trachytic lavas, the precollapse lavas.

The Solitario is a circular, 14- to 16-krn-diameter, mid-Tertiary laccolithic dome and caldera in southern Trans-Pecos Texas; it is among the world's largest laccoliths.

The Bofecillos Mountains and Big Bend Ranch State Park contain a complex sequence of volcanic, volcaniclastic, and intrusive rocks that erupted from sources both within and outside the area.