Simi Valley is a synclinal valley in Southern California in the United States. It is an enclosed or hidden valley surrounded by mountains and hills. It is connected to the San Fernando Valley to the east by the Santa Susana Pass & 118 freeway, and in the west the narrows of the Arroyo Simi and 118 freeway connect to Moorpark and Ventura, California. The relatively flat bottom of the valley contains soils formed from shales, sandstones and conglomerates eroded from the surrounding hills of the Santa Susana Mountains to the north, which separate Simi Valley from the Santa Clara River Valley, and the Simi Hills.