The northern Quitman Mountains lie within the Sierra Madre Oriental structural province, where it projects northward from Mexico into Hudspeth County, Texas. The Permian Briggs formation with a maximum exposed thickness of about 200 feet unconformably underlies the Jurassic Malone formation, which, although absent in places, attains a maximum thickness of 100 feet. The Lower Cretaceous represented by 10,000 feet of sedimentary rocks, consists of the following formations, the maximum exposed thicknesses of which are given in parentheses: Torcer (300 feet), Yucca (5500 feet), Bluff (about 1800 feet), Cox (700 feet), Finlay (100 feet), Espy (1280 feet), and Etholen (500 feet). Unconsolidated basin deposits of late Tertiary and Quaternary age fill the valleys.
The sedimentary rocks are intensely folded and faulted in the northern Quitman area, although no one fold can be followed very far. Three thrusts faults—the Devil Ridge, Red Hills, and Quitman—have a combined net slip of approximately 8 miles. Normal faults generally have a net slip less than 200 feet, though one exceeds 1000 feet. The main orogenic disturbance is probably late Cretaceous.
Comagmatic and mildly alkalic volcanic and intrusive rocks are probably early Tertiary. Lavas and pyroclastic rocks, for which the name Square Peak volcanic series is here proposed, aggregate approximately 3500 feet and constitute a single unit in the center of the mountains. Flows range from rhyolite and trachyte to basalt; rhyolites are most abundant. Later basining, probably due to magmatic subsidence below the volcanics, has dropped the central portion of the volcanic rocks approximately 4500 feet. Centripetal dips of flows along the outer contacts are as high as 90°; average inward dips range from 20° to 40°.
The Quitman pluton, a discontinuous elliptical ring of intrusives around the volcanic rocks, is interpreted as a ring-dike with an accessory stock at the northern end. Earliest intrusion in the area was a diorite; the main intrusion averages quartz monzonite. Plugs and dikes of aplite, granite porphyry, rhyolite porphyry, and quartz latite porphyry are youngest. Scattered mineral deposits are found in the irregular contact-metamorphic aureole around the pluton.