What are the motifs in The Last Continent: A Discworld Novel by Terry Pratchett?

Time travel is a recurring idea in the story. In Terry Pratchett's novel, The Last Continent, everything is predicated on the reality of Time Travel. Protagonist Rincewind the Wizzard finds himself at the end of the preceding novel, Interesting Times, marooned by magic on a parched, barren continent, where the natives promptly knock him unconscious. Some months later, he is still alive on Fourecks, thanks to the care of its Creator. The unfinished land is suffering severe drought, which is drying up the underground reservoirs, threatening all life. Rincewind is shown drawings on rock walls that look 40,000 years old, but which have not existed a month b