There are sand dunes in Yellowstone? I had no idea!
The dunes are now stabilized by vegetation, last active maybe 15,000 years ago is my guess. Likely formed from wind picking up fines in the adjacent moraine and recently glaciated area. Northern line of trees is at the base of a moraine, the patchy trees or tree free area south of that is in a series of dunes:https://www.google.com/maps/place/44%C2%B054'06.3%...
I could be wrong, but I think my assessment is at least decent.
Little doubt your assessment was spot on, as you were on spot to see exactly what was going on.
I had no idea there were any sand dunes anywhere in Yellowstone, which is why this caught my eye.
Did you feel lucky when you made it out of the Specimen Ridge area without the Specimen Ridge (wolf) Pack consuming you?
I still haven't seen any wolves in Yellowstone, Owen and Heather saw two coyotes chasing a black wolf that evening in the Lamar Valley, but I was back at camp vegging out, happy not to be doing anything at that time. I'm mostly bone anyway, a smart wolf would look elsewhere for a real meal.