<P> The idea that the dunes could be destroyed by gold mining or concrete - making alarmed residents of Alamosa and Monte Vista . By the 1920s, the dunes had become a source of pride for local people, and a potential source of tourist dollars for local businesses . Members of the P.E.O. Sisterhood sponsored a bill to Congress asking for national monument status for Great Sand Dunes . Widely supported by local people, the bill was signed into law in 1932 by President Herbert Hoover . Similar support in the late 1990s resulted in the monument's expansion into a national park and preserve in 2000 - 2004 . </P> <P> The park contains the tallest sand dunes in North America, rising to a maximum height of 750 feet (229 m) from the floor of the San Luis Valley on the western base of the Sangre de Cristo Range . The dunes cover an area of about 30 sq mi (78 km) and are estimated to contain over 5 billion cubic meters of sand . </P> <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>

Where did the great sand dunes come from
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