Named for exposures in southern Old Woman Mountains. Type locality is in sec.26 T5N R18E, Milligan 15' quad, San Bernardino Co, CA, including Lucky Jim mine area. Forms Old Woman pluton and underlies much of southern Old Woman Mountains. Pluton consists of large body with generally steep foliations in eastern part of range and 3 or 4, connecting, tiered, west-thinning lobes or concordant sheets in western range. Main facies consists of gray, equigranular, medium-grained hornblende-biotite granodiorite, biotite granodiorite, and less common monzogranite. Local porphyritic facies consists of light-gray, medium-grained biotite granodiorite and lesser monzogranite containing alkali feldspar phenocrysts 0.5 to 1 cm across. K-Ar age of 64.5 Ma on biotite suggests Cretaceous age for mylonitic deformation. Overall age considered Late Cretaceous based on U-Pb (zircon) age of about 72 Ma (from main facies).

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