So I took a trip to Yellowstone recently. I've always wanted to see the park and it's incredible, a treasure hunt was just an added incentive. I thought about Specimen Ridge too (Amethyst Creek just sounds like a perfect place for treasure), but I checked up Rose creek as was guessed at pretty thoroughly here:
https://www.reddit.com/r/FindingFennsGold/comments/34bgi6/think_ive_solved_a_fair_bit_of_the_puzzle/
No treasure, but I felt like I found a great hiding place. I had to walk it about 2 miles from the main road, but you can see tire tracks leading up to this location from the back of the Lamar Station. He could have driven up to within a quarter mile of the spot.
Fenced in location Fenced in location
And then up on the hill a "blaze" I found was an orange, lichen-covered boulder with a perfect little treasure-chest sized nook in it.
Nook Orange Boulder Orange Boulder Still Visible from Rose Creek
About here: https://www.google.com/maps/@44.9089222,-110.2315488,321m/data=!3m1!1e3
The pictures don't do it justice. Even in the overcast weather, it's pretty bright orange. Wrote the email I found for Forrest, no reply. After searching actually in Rose creek, I really don't think it's in or near a creek. There's just too much erosion and it would be covered or strewn about in a matter of years. Most of the waterfalls are created by downed trees and wouldn't be around long enough.
If you have a chance, follow the tire tracks there and checkout what you see. Maybe look in that weird fenced in section; I could only see it from the opposite hill and didn't really think anything of it. And once I got away from the treed in valley and back in the open on the hillside I didn't want to go back in. Don't do it without bear spray or a gun... there are large animal kills all up and down the creek.