<P> Creation of the San Luis Valley began when the Sangre de Cristo Range was uplifted in the rotation of a large tectonic plate . The San Juan Mountains to the west of the valley were created through extended and dramatic volcanic activity . The San Luis Valley encompasses the area between the two mountain ranges and is roughly the size of the state of Connecticut . Sediments from both mountain ranges filled the deep chasm of the valley, along with huge amounts of water from melting glaciers and rain . The presence of larger rocks along Medano Creek at the base of the dunes, elsewhere on the valley floor, and in buried deposits indicates that some of the sediment has been washed down in torrential flash floods . </P> <P> In 2002, geologists discovered lakebed deposits on hills in the southern part of the valley, confirming theories of a huge lake that once covered much of the San Luis Valley floor . The body of water was named Lake Alamosa after the largest town in the valley . Lake Alamosa suddenly receded after its extreme water pressure broke through volcanic deposits in the southern end of the valley . The water then drained through the Rio Grande, likely forming the steep Rio Grande Gorge near Taos, New Mexico . Smaller lakes still covered the valley floor, including two broad lakes in the northeastern side of the valley . Large amounts of sediment from the volcanic San Juan Mountains continued to wash down into these lakes, along with some sand from the Sangre de Cristo Range . Dramatic natural climate change later significantly reduced these lakes, leaving behind the sand sheet . Remnants of these lakes still exist in the form of sabkha wetlands . </P> <P> Sand that was left behind after the lakes receded blew with the predominant southwest winds toward a low curve in the Sangre de Cristo Range . The wind funnels toward three mountain passes--Mosca, Medano, and Music Passes--and the sand accumulates in this natural pocket . The winds blow from the valley floor toward the mountains, but during storms the winds blow back toward the valley . These opposing wind directions cause the dunes to grow vertically . Two mountain streams--Medano and Sand Creeks--also capture sand from the mountain side of the dunefield and carry it around the dunes and back to the valley floor . The creeks then disappear into the sand sheet, and the sand blows back into the dunefield . Barchan and transverse dunes form near these creeks . The combination of opposing winds, a huge supply of sand from the valley floor, and the sand recycling action of the creeks, are all part of the reason that these are the tallest dunes in North America . </P> <P> Sufficient vegetation has grown on the valley floor that there is little sand blowing into the main dunefield from the valley; however, small parabolic dunes continue to originate in the sand sheet and migrate across grasslands, joining the main dunefield . Some of these migrating dunes become covered by grasses and shrubs and stop migrating . The dunes system is fairly stable as the opposing wind directions balance each other out over time . Also, the main dunefield is moist beneath a thin layer of dry surface sand . While the top few inches of sand are blown around during windstorms, the moist sand remains largely in place . </P>

Where are the largest sand dunes in the us