This plain has been formed chiefly by the deposition of sediments from Santa Clara River and Calleguas Creek. This plain contained a series of marshes, salt flats, sloughs, and lagoons prior to the expansion of agriculture. The Santa Clara River is one of the largest river systems along the coast of Southern California and only one of two remaining river systems in the region that remain in their natural states. The Oxnard Plain faces the Santa Barbara Channel portion of the Southern California Bight, extending from the abrupt transition of the steep rocky shore at Point Mugu in the Santa Monica Mountains on the south to the Ventura River on the north. Prominent on the southeastern horizon are Conejo Mountain and Boney Peak.