The Riverside Mountains contain rocks from both the lower and upper plates of a large detachment fault and metamorphic core complex system as a result of the Miocene-age extension. The lower plate consists of a stack of metamorphosed units, which comprise Jurassic metavolcanics, the metamorphosed Grand Canyon sequence, the metamorphosed McCoy Mountains Formation and related Mesozoic rocks, and the Precambrian basement. The upper plate of the detachment fault consists of a small sedimentary basin containing Tertiary-age syntectonic (deposited during tectonic activity) volcanic units, conglomerates, and other sedimentary rocks.