Gillespie Tuff. Pink, welded, quartz latite tuff. Formation generally thick, uniform in composition and texture throughout, well indurated, and commonly exposed in bold columnar-jointed cliffs. Underlies Center Peak Latite (new); overlies Walnut Wells Monzonite and Animas Monzonite (new). Age is Tertiary.

Source: US geologic names lexicon (USGS Bull. 1350, p. 289).

Pg. 50-52, pl. 1. Gillespie Tuff. Quartz latite composition. Thickness more than 1,500 feet. Rests with apparent conformity upon Cedar Hill Andesite (new) from Cowboy Spring area northward; to south lies with apparent conformity upon Bluff Creek Formation (new). In southern part of quadrangle, formation overlain unconformably by unit of well-bedded sandstone and breccia about 30 feet thick. Above this unit is Center Peak Latite. In northern part of quadrangle, unconformably underlies Park Tuff. Age is Tertiary.
Named from exposures on Gillespie Mountain, in sec. 33, T. 30 S., R. 18 W., Walnut Wells quadrangle, Hidalgo Co., southwestern NM. Most prominent and widespread volcanic unit in Walnut Wells quadrangle. Extends into northern Mexico.

Source: Publication; US geologic names lexicon (USGS Bull. 1350, p. 289).

Age modified from Tertiary to Oligocene on basis of a K-Ar biotite date of 32.1 +/-0.7 Ma and of 32.9 +/-0.7 Ma. The latter date obtained from an unpublished thesis. Underlies a major part of the wilderness area which lies in the Pedregosa basin. Geologic map.

Source: GNU records (USGS DDS-6; Denver GNULEX).