The Mind Masters

From the back cover:

The first summons of the new season is called. Your name is drawn---you join the expedition. Your mission is to investigate rumors of a stockpile of ancient artifacts. Following a sketchy map, you cross the great river Twainway the day after leaving your settlement. Two days later, in unexplored wilderness, you encouner hostile nomad tribes. After finding traces of a lost expedition, you know your'e on the trail of something big. And then you find it, hidden down in a valley, forgotten and untouched since the Devastation. You were searching for a great store of artifacts. Here it is. The whole valley is an artifact!"