A combination of urban and suburban development, the West Side 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, and includes the International Wildlife Museum, Sentinel Peak, and the Marriott Starr Pass Resort & Spa, located in the wealthy enclave known as Starr Pass. Moving past the Tucson Mountains, travelers find themselves in the area commonly referred to as "west of" Tucson or "Old West Tucson". A large undulating plain extending south into the Altar Valley, rural residential development predominates, but here you will also find major attractions including Saguaro National Park West, the Arizona-Sonora Desert Museum, and the Old Tucson Studios movie set/theme park.