Named; name derived from series of tunnels dug during 1950's for experimental explosions on Nevada Test Site. Study area is within southwest Nevada volcanic field, Nye Co, NV, Great Basin province. Divided informally into members 3 and 4 which are equivalent to units 3 and 4 and tunnel beds 3 and 4 of now abandoned Oak Spring and Indian Trail Formations (Hanson and others, 1963, USGS Prof. Paper 382-A; Gibbons and others, 1963, USGS Map GQ-215). Type section extends about 5000 ft east-northeast from drillhole UE-12n14, across east face of Rainier Mesa. Reference section is designated from 1152-1667 ft in drillhole UE-12n14. Overlies Tub Spring Tuff (revised); underlies Grouse Canyon Tuff (rank raised) and older than comendite of Split Ridge both of Belted Range Group (rank raised). Nomenclature chart (table 1). Assigned Miocene age based on 40Ar/39Ar dates for underlying and overlying units.

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