The study area, encompassing about 260 km2 within Madison county, is situated 19 km due east of Dillon, Montana. It lies athwart the middle third and narrowest part of the Ruby Range, and detailed mapping provides data for a geologic history and complete cross sections, basin to basin, through the uplifted block. Included are all of the Mine Gulch and parts of the Metzel Ranch, Beaverhead Rock Southeast, Belmont Park Ranch, and Red Canyon 7 1/2-minute quadrangles.
The Ruby Range is one of several non-parallel mountain systems in southwestern Montana whose exposed cores are composed of multiply folded, high-grade Precambrian (pre-Beltian) metamorphic rocks (Fig. 1). Similar exposures of complexly deformed basement rocks immediately encircling the range crop out beneath a spotty cover of sedimentary and volcanic rocks in the Black-tail, Snowerest, Greenhorn, Gravelly, Madison Tobacco Root, and Highland Ranges (Klepper, 1950; Keenmon, 1950; Berg, 1976; Hadley, 1969a, 1969b; Millholland, 1976; McThenia, 1960; Burger, 1969; Cordua, 1974; Duncan, 1976; and numerous others).