The San Gabriel Mountains are the somewhat dissected remnants of an elevated fault block. At their southeastern end they are wedged between the Mojave desert mass and the depressed northwestern portion of the Perris fault block. The southeastern San Gabriel range is made up of pre-Upper Cretaceous metamorphic rocks and late Jurassic or early Cretaceous plutonic rocks.
An east-west zone through the middle of the area is characterized by extensive mylonitization. Chemical analyses and other data indicate that the mylonites were derived chiefly from a dark quartz diorite, and to a lesser extent from a paler quartz monzonite, without much mixing of the two source materials. The mylonite zones were intruded by the quartz monzonite and probably by the quartz diorite. Possibly the main mass of mylonite was formed by the crushing of a barely consolidated upper portion of a quartz diorite pluton, as one or more tectonic episodes during the general period of plutonic intrusion.