The West Side has areas of both urban and suburban development. It is generally defined as the area west of I-10. Western Tucson encompasses the banks of the Santa Cruz River and the foothills of the Tucson Mountains. Area attractions include the International Wildlife Museum and Sentinel Peak. The Marriott Starr Pass Resort & Spa serves travelers and residents. As travelers pass the Tucson Mountains, they enter the area commonly referred to as west of Tucson or Old West Tucson. In this large, undulating plain extending south into the Altar Valley, rural residential development predominates. Attractions include Saguaro National Park West, the Arizona-Sonora Desert Museum, and movie set/theme park developed at the Old Tucson Studios.