Highly deformed and metamorphosed Proterozoic(?), Paleozoic, and Mesozoic rocks occur in the Big Maria Mountains of southeastern California. Proterozoic(?) rocks are gneisses, metagranites, and schists. Cambrian to Permian metasedimentary rocks are calcite marbles and quartzites, with lesser amounts of calc-silicate rock, dolomitic marbles, and pelitic schists. Paleozoic rocks are generally equivalent to those of the Grand Canyon region. Pre-Late Jurassic rocks are both metasedimentary (metaarenite, metatuff, metaconglomerate, and quartzite) and metaigneous (orthogneiss and amphibolite). Mineral assemblages in Proterozoic(?) rocks indicate at least two episodes of amphibolite-grade (staurolite zone) metamorphism. Three types of metamorphism were recognized in Paleozoic and Mesozoic rocks: primary, cataclastic, and retrograde. Mineral assemblages formed during primary metamorphism are intermediate between those of regional metamorphism (garnet zone green-schist-staurolite zone amphibolite facies) and contact metamorphism (upper hornblende hornfels facies). Feldspar geothermometry and the occurrence of kyanite in Mesozoic metasediments suggests temperatures of about 535°C and pressures of about 5 kbar during primary metamorphism. The pressures seem to be too high due to unknown factors affecting the formation of kyanite, since 5 kbar is (a) normally too high for wollastonite to form in contact [sic] metamorphosed rocks, and (b) indicative of burial to 19 km and crustal thickness is only 21 to 23 km in southeastern California and southwestern Arizona. Primary and cataclastic metamorphism were syntectonic and retrograde metamorphism was posttectonic. K-Ar dating indicates that the primary metamorphic event took place around 59 m.y. B.P. and was contemporaneous with Laramide intrusive activity throughout western and southern Arizona.