How do they form

A unique example of mother nature in action. Involving volcanic, plate techtonics, erosion by wind & water, and time. The sand accumulated near the base of the mountains on the east side of the valley from the prevailing wind. The winds shift the sands from its interactions with the unique geographic features of the valley & mountains

Many long years of wind blowing sand which drops at the mountains that block the wind from going further.

Yep! And they surely are something to see! Absolutely amazing!

The sand blowns up from the valley and gatthers at the foot of the mountains and then stream s

Y vette described it perfectly. If y I u want to slide on them bring a disc or so coaster type thing with you..

Yep! Over thousands of years, the wind swept sand from nearby San Luis Valley and blew it up against the Sangre de Cristo Mountains. The mountains act like a big catcher's mitt and captures the sand. It collects at the valley bellow the mountains, forming the Great Sand Dunes!