Dunes

From the back of a post-card: "Great Sand Dunes, National Park, CO. 1920 Sand dunes towering 750 feet (214m) above the valley floor and covering over 50 square miles (130 sq. km) are swallowed by the huge scalae of the San Luis Valley and the 12,000 to 14,000 ft (3700 to 4300 m) Sangre de Cristo mountains. Prevailing southwesterly wind pile the sand at the base of three low passes in the mountains. Strong northeast winds occassionally roar through the mountain passes, reversing the usual direction of sand flow and piling the dunes back upon themselves building "chinese walls". In 2004, Congress made this area a National Park and so protected (since 1920 the dunes were a National Monument)."