Great Sand Dunes National Monument Favorite Tips by frank_delargy Top 5 Page for this destination

Gold in them thar dunes???

Favorite thing: The darker patches that you see on the sand dunes are made from magnetite. It is heavier than other minerals and mechanically it ends up on the surface. In geology circles , it is known that the presence of magnatite often is an indicator that gold is present.
I understand that in the early 1900's people mined the dunes for "flour gold" . Fortunately for us, they couldn't find enough gold to make it a profitable operation.

Geology

Favorite thing: To answer all the 'Why' questions in the intro, the following information is purloined from DesertUSA web site.
http://www.desertusa.com

The San Luis Valley, averaging less than 8 inches of precipitation a year, is the only true desert of the Colorado Rockies. Formed by the Rio Grande Rift , it is bordered on the west by the Tertiary volcanic San Juan Mountains, and on the east by the Precambrian granite of the Sangre de Cristo Mountains. A fault runs along the western base of the Sangre de Cristos.

The valley is filled with layer upon layer of sand, gravel, clay, lava and ash to a depth over 10,000 feet. About 12,000 years ago, when glaciers from the last Ice Age began to melt, the swollen Rio Grande River deposited glacial debris, sand and gravel across a large portion of the San Luis Valley. The Rio Grande, from its head waters in the San Juan Mountains, has been meandering through the valley, depositing sand ever since.

When prevailing southwest winds sweep across the valley, they carry sand from the sparsely vegetated surface toward the natural barrier of the Sangre de Cristos. Here, in a sheltered corner at the foot of the mountains, the wind funnels through three low passes -- Mosca, Music and Medano -- depositing sand as it loses velocity and rises.

Most of the sand consists of fine-grained, rounded quartz, pumice, ash, and lava from the San Juans. Sangre de Cristo sand, comprised of many rock types, is more coarse-grained because it does not travel as far. Sand grains from both sources range in size from 0.2 mm to 2 mm.

The oval-shaped dune field thus created -- the Great Sand Dunes-- is about 6 miles across with some dunes rising 700 feet above the valley floor. Medano Creek, running north and south of the dunes, helps regulate the size and position of the entire field.