It might have been a previously unknown part of the valley that they just happened to find sometime prior to 7 but after 6. The valley is huge so there are probably many parts of it that we have not seen.
How it formed, it would be hard to come up with the full details. But I think it is a natural formation; they probably just happened upon it and thought they could use it for meetings due to the large place, and with the unique structure it was easily identifiable, so it would have been hard to come to the wrong place.