Complex sentence: Several volcanoes existed on the floor of Lake Cahuilla and are now emergent at the southeastern margin of the Salton Sea, including the Cerro Prieto and the Salton Buttes.
Split into simpler sentences: Several volcanoes existed on the floor of Lake Cahuilla, now emergent at the southeastern margin of the Salton Sea. These include the Cerro Prieto and the Salton Buttes.