nathan;
The best that I can remember after over 55 years is that both films were done around The Left Mitten Butte and to the West is a large sand and salt flat just North of Gouldings Lodge and Trading Post. I fly right over it when I land at Gouldings Air Strip.
In the Horse Soldier Charge in Fort Apache and the Paymasters Stagecoach chase at the first of Yellow Ribbon with Ben Johnson, I think what you are talking about in the background is North and East of Gouldings Trading Post and are called "Big Chief Mesa," "Sentinel Mesa" and "Pyramid Rock." They are much larger than they look in the films.
Just South and East is where Duke is with the Supply Wagons in Fort Apache and watching the Charge and you can clearly see the Left Mitten Butte in the Background.
In this type of film where you have a chase scene like in these two Films you must have a smooth road for the Camera Truck and the wind must be just right so you do not see the dust from the Camera Truck. In those days the roads were still dirt.
I will be there for a Shoot next week and I will try to get more Info. for you.
Chilibill