Massive andesitic and basaltic flows; minor trachyte, trachyandesite, and dacite; some interbedded tuffaceous sedimentary rocks; and, locally, several hundred feet of volcanic breccia at or near base (Box and others, 1993) are primarily exposed in the Bethel and Russian Mission quadrangles of southwest Alaska. An estimated 25 percent of the unit is interbedded graywacke, siltstone, impure limestone, and pebble conglomerate (Hoare and Coonrad, 1959a, b). The rock unit forms a belt in low rolling hills on the west flank of the Kuskokwim Mountains from the Kwethluk River northward (Hoare and Coonrad, 1959a). Marine pelecypods of Bajocian age (Middle Jurassic) occur in sedimentary strata of this unit (Hoare and Coonrad, 1959a; Box and others, 1993). Unit provisionally also includes an isolated outcrop in the north-central Dillingham quadrangle that is predominantly diabase, which may intrude rocks provisionally assigned to map unit K^s, thus suggesting that its age may be Triassic or younger. Volcanic rock outcrops to the northeast within the Dillingham quadrangle were inferred to be Triassic by Wallace and others (1989); in neither case does true age control exist