Long Valley, caldera, c.10 by 20 mi (15 by 30 km), Mono co., E Calif., at the eastern edge of the Sierra Nevada. Formed by a catastrophic eruption c.730,000 years ago, Long Valley and nearby areas have been the scene of volcanic activity for more than 3 million years. The resort city of Mammoth Lakes is in the caldera in the southwest section, in an area of many small lakes. Nearby Mammoth Mt., 11,050 ft (3,368 m), on the southwest rim of the caldera, is composed of about dozen lava domes and formed from 200,000 to 50,000 years ago. Devils Postpile National Monument, outside the caldera to the west of Mammoth Mt., protects basaltic columns, some 60 ft (18 m) in height, that formed from lava that erupted 200,000 to 100,000 years ago. North of Mammoth Lakes, the Inyo and Mono crater fields stretch along a 30-mi (50-km) system of volcanic fissures from the northwestern caldera floor toward Mono Lake (87 sq mi/225 sq km). Eruptions in this area began in the caldera 300,000 years ago, moving to the north over time; the most recent eruption formed Paoha Island in Mono Lake and ended c.A.D. 1850. Since the late 1970s seismic and other geologic activity in the Long Valley area has led to concern over the possibility of a new eruption.