How does Terry Pratchett use imagery in Pyramids: The Book of Going Forth?

Imagery:

"For as the world tumbles lazily, it is revealed as the Discworld-flat, circular, and carried through space on the back of four elephants who stand on the back of Great A'tuin, the only turtle ever to feature on the Hertzspring Russell Diagram, a turtle ten thousand miles long, dusted with the frost of dead comets, meteor-pocked, albedoeyed."