Two normal fault systems are present in Nevada Test Site and Nellis Air Force Range. The earlier system consists of two sets that strike northeast and northwest, and the later system, a single set, strikes north. In the southern Kawich and Belted Ranges the Belted Range Tuff, dated at 13 to 14 m.y. and faulted only by the north-trending faults, laps onto older volcanic rocks that are broken by both fault systems. The older faults probably started to form during or shortly after the extrusion of the oldest welded tuff dated at 26.5 m.y. (age dates by R. W. Kistler, written commun., 1964), as is evidenced by essentially the same fault densities and displacements in pre-Tertiary rocks as in Tertiary rocks. The north-trending faults first began to appear after the deposition of the Fraction Breccia, dated at 17.8 m.y., and the present mountain ranges were well defined prior to extrusion of the Thirsty Canyon Tuff dated at 7 m.y. In the southern part of the Bombing and Gunnery Range near the Las Vegas Valley shear zone, north-trending faults that are inferred to be the same age as those to the north change strike gradually from north to northeast as the valley is approached from the north. This rotation, interpreted as being caused by drag during later right-lateral movement rather than a change in the stress field, suggests that much of the displacement along the shear zone has taken place within the last 17 m.y.