Wikipedia says they date back 480 million years, during the Ordovician period. I think the atmosphere would have been pretty breathable, but plants wouldn’t conquer the land in a serious way for at least another 60 million years or so, with bugs following them. Nothing saw those mountains. If you time-traveled to the Pangean mountains, you would be the only being to ever observe them in their formative years. Eons would pass before the first plant set root at their feet. The sterile and desolate emptiness would be beyond human experience.