I observed a large fireball roll across a mountain ridge about two miles from my home in north central Fremont County in June 2013. It made no sound, and later inspection by a neighbor revealed no burnt trees or other vegetation. The event lasted only a few seconds. I observed it from an altitude of 9000 feet, and estimated it to occur at an elevation of about 9700 feet. The fireball seemed to roll across the ridge effortlessly, and caused no visible fire, which seemed impossible, considering its estimated diameter of about 20 feet. I have not observed a repeat of this event.