Santa Rosa Island covers 84 square miles and is structurally complex due to the left-lateral, Santa Rosa Island Fault. The Miocene (Relizian-early Luisian) Beechers Bay Formation is the time stratigraphic equivalent of the Monterey Formation. It is divided into 5 informal members deposited as distinct lithofacies on a submarine fan. All the members are composed primarily of sediments rich in volcanic detritus. Members A (oldest) and E (youngest) are composed of dacite-andesite conglomerates, basalt conglomerates, and interbedded pebbly sandstones. They were deposited on the inner (upper) fan. Members B and D were deposited in the channeled portion of the upper mid-fan. Member B is a series of feldspathic dacite-andesite litharenites and interbedded siltstones. Member D is a true volcaniclastic sandstone that signifies an increase in volcanic activity at a nearby source. Member C is a rhythmically bedded sequence of feldspathic volcanic litharenites and siltstones that were deposited on a more distal part of the mid-fan. All members of the Beechers Bay Formation contain small amount of metamorphic rock fragments including blue schist clasts thought to have been derived from the Catalina schist. Paleoslope and paleocurrent indicators found in each member suggest the source area was located to the east-northeast. No suitable source is present there now. The source area is inferred to have been a volcanically active ridge located south of the present location of Santa Rosa Island, near Catalina Island. This area produced large quantities of dacitic and andesitic material which was deposited by sediment gravity flows on an adjacent submarine fan. The small quantities of metamorphic material was shed from nearby Catalina Island.