River sediments helped to create the broad and flat terrain and river valleys between the San Joaquin Hills and the much higher Santa Ana Mountains to the east. Sulphur Creek's watershed is located in a southern area of the San Joaquin Hills where many smaller hills have formed in a wider area instead of a single, tall, narrow mountain chain. These lower hills are composed primarily of sedimentary rocks and date from the Miocene period. The oldest rocks in this area are referred to as the Vaqueros Formation, while the younger are named the Monterey Formation. This hilly area is drained by Sulphur Creek to the northwest, Salt Creek to the south, small and unnamed coastal canyons to the southwest, and small tributaries of Trabuco and Oso Creeks to the east. This area is located northwest of the San Juan Creek valley, southeast of the Aliso Creek valley, and west of the Oso Creek drainage.