The Windriver Range did not appear to have been worn down significantly nor were the rivers, riverbeds and valleys necessarily sandy implicating creation of sand.
How fast does the Wind/Bighorn flow every May during snowmelt? I think you'll find that sand in great abundance down along the Yellowstone, Missouri, and Mississippi Rivers and out around Venice, Louisiana, Buz. Just the big rocks are all that stays in the riverbed up in the mountains. And the Windriver Range looks "young" because, compared to the Wichitas or the Adirondaks, it
is young. Like one-fifth to a tenth as old.