San Juan Mountains, a range of the Rocky Mountains in southwestern Colorado and northern New Mexico. It extends southeastward for about 150 miles (240 km) from Uncompahgre Plateau to the junction of the Rio Chama and the Rio Grande. The mountains form part of the Continental Divide. Several peaks exceed 14,000 feet (4,267 m) in height; Uncompahgre, at 14,309 feet (4,361 m), is the highest. Near the source of the Rio Grande is Silverton, one of the best preserved of Colorado's old silver-mining towns.