<P> Scientists estimate that Lake Alamosa disappeared about 440,000 years ago, but the dunes themselves apparently originate from sand deposits from later, smaller lakes . A relatively new dating process, optically stimulated luminescence (OSL), is still in development . This method takes core samples of sand from deep within a dune, and attempts to measure how long quartz grains have been buried in the dark . If the deepest sand deposits can be accurately dated, the age of the dunes could be determined . Samples of sand from deep in the dunes have returned OSL dates varying between a few hundred years to tens of thousands of years old . The oldest dated deposits found so far would have accumulated in the late Pleistocene epoch, during the middle years of the current ice age's third stage . </P> <P> The dunes contain dark areas which are deposits of magnetite, a mineral that has eroded out of the Sangre de Cristo Range . Magnetite is both attracted to a magnet and can be magnetized to become a magnet itself; it is the most magnetic mineral in nature . Magnetite is an oxide of iron which is heavier than the surrounding particles of sand . When overlying sand is removed by wind, magnetite deposits stay in place and are visible as dark patches in the dunefield . </P> <P> Great Sand Dunes National Park and Preserve is located in Saguache and Alamosa Counties, Colorado at approximately 37.75 ° north latitude and 105.5 ° west longitude . The national park is located in the San Luis Valley while the national preserve is located to the east in an adjacent section of the Sangre de Cristo Range of the Rocky Mountains . Elevations range from 7,515 ft (2,291 m) in the valley west of the dunes, to 13,604 ft (4,146 m) at the summit of Tijeras Peak in the northern part of the preserve . </P> <P> The dunes cover an area of about 30 sq mi (78 km) while the surrounding relatively flat sand sheet which feeds the large dunes is actually the largest component of the entire dunes system, containing about 90% of all the sand in the park . The forested and often snowcapped mountains exceeding 13,000 ft (4,000 m) to the east are the most prominent feature, towering over the high dunes . Other features include snow - fed creeks originating high in the mountains, and several alpine lakes . Two spring - fed creeks in the sand sheet along with a few small lakes in the valley's sabkha section southwest of the dunes create a wetland that nurtures wildlife . </P>

Where is the great sand dunes national park located