|Great Sand Dunes National Park and Preserve|
photographed from the International Space Station (NASA photo).
Many visitors wonder: Why is there so much sand only here, but not at other locations along the mountains?That is the San Luis Valley on the lower left and the Wet Mountain Valley to the upper right, so the top of the photo is roughly northeast.
In this view from space, part of the answer becomes clear. The Sangre de Cristo Mountains are curved here, and at the same location are low passes to funnel wind and sand from the valley floor into this pocket.
Then why doesn't sand accumulate, for instance, at the base of La Veta Pass to the south? The mountains also curve there below a low pass.. The answer is that this northeast part of the San Luis Valley is a closed basin. Streams carrying sand into this basin don't exit, so all the sand they carry is deposited here. In the past, these streams fed into huge lakes; when these lakes disappeared through natural climate change, vast quantities of sand blew and accumulated here below Mosca, Medano, and Music Passes. In other parts of the valley, and in most places in the Rocky Mountains, sands are continually washed away and carried downstream into larger and larger rivers.
A lot of visitors also pronounce the name of the creek that flows by the dunes as "Meh-DAH-no," thinking that that is the correct Spanish pronunciation, whereas in the Wet Mountain Valley, you hear something more like "MAD-uh-now" or "MAD-uh-no," usually in reference to Medano Pass, which connects the two valleys.
The latter is actually closer to the Spanish Médano — note the accent mark — which means "sand dune" and comes from an old Castillian word for mountain.