The rocks exposed in this study area represent the eastern-most exposures of the Roskruge Mountains, themselves part of a long chain of low ridges and hills extending as far north as the Silverbell Mountains, and as far south as the Coyote Mountains. This part of the Roskruge Mountains contains rocks that can be grouped into three major subdivisions as follows: (1) a sequence of steeply tilted and folded sedimentary and volcanic rocks containing andesite, rhyolite, and both fluvial and eolian sandstones, (2) a thick pile of Cretaceous rhyodacitic welded ash-flow tuff—here containing only the Tuff of Sharp Peak, and (3) much younger middle Tertiary volcanic and sedimentary rocks. A small granitic pluton intrudes rocks of subdivision 1 but it is not entirely clear how it relates to subdivision 2 rocks. The younger Tertiary rocks are only slightly tilted, and overlie moderately to steeply tilted welded ash-flow tuffs. However, both of these sequences are everywhere separated from subdivision 1 rocks by the Recortado Well Fault. Therefore, the relative age of the steeply tilted rocks is uncertain, although exposures of similar rocks to the west (Ferguson et al., 2000) suggest that the sequence is older than subdivision 2 and 3 rocks. Furthermore, sequence 1 strata include quartz arenite of suspected eolian origin that is probably correlative with a regional suite of Jurassic wind-blown sand recognized in southern Arizona (Bilideau and Keith, 1986; Tosdal et al., 1989). Subdivision 1 rocks are overlain unconformably (and apparently conformably as well) by the Unit of Tunnel Well—a volcanogenic deposit of uncertain origin (map unit KJtw). It is not certain whether rocks exposed south of Tunnel Well below the Tertiary rocks on (map unit Kt) are also the Unit of Tunnel or are younger