segment of the southern Rocky Mountains, extending southward parallel to the Rio Grande for 150 miles (241 km), through Socorro, Sierra, and Doña Ana counties in southern New Mexico, U.S. Spanish missionaries named the mountains for Saint Andrew, the disciple of Jesus. Salinas Peak (9,040 feet 【2,755 metres】) is the highest point in the range. The mountains are dry and barren and include the White Sands Missile Range, where the world's first atomic bomb was exploded on July 16, 1945, at “Trinity Site.” The White Sands National Monument, encompassing a vast dune field made of decomposed gypsum (calcium sulfate) from the San Andres Mountains, lies just to the east.