The Dead Horse Mountains stretch along the horizon to your left. Remote, rugged, and dry, they remain the least explored part of the park. The Dead Horse are part of a larger range, the Sierra del Carmen, which extend into Mexico. The limestone in these mountains formed from sediments deposited by the last sea to cover this area. Millions of years later, the birth of the Rocky Mountains raised this land. Intense pressure uplifted and tilted great blocks of earth including the Sierra del Carmen and Dead Horse Mountain ranges. The Rio Grande cuts across this uplifted block, carving Boquillas Canyon, one of the park's three great canyons, with walls 1,200 feet high.