PLATEAU DWELLERS by Steven Govorchin
The story is set on and around a small island continent in the hypothetical alternate universe. This sketch does not appear in the novel; the readers were intended to form a mental picture of the landscapes as they journey along with the protagonist. But if you like, here's the map!
Plateau Dwellers: A full-length novel about an intellectual/philosophical adventure set in a quasi-Neolithic period of a hypothetical alternate universe, available on BookBaby, Amazon, Barnes and Noble, plus many other booksellers.