The Rainbow Serpent is agreed to be one of the most powerful, all consuming and widespread Ancestral Beings of Aboriginal Australia. It pervades not only oral history but also ceremony, performance and visual art, and is associated with great acts of creation and destruction. Although Rainbow Serpent imagery varies, its supernatural nature is usually emphasised by its form, often through the incorporation of attributes derived from a variety of natural creatures. Rock paintings of the Being are scattered across the continent but a particularly large sample can be found in western Arnhem Land. We analyse 107 of these in detail and note variation across both time and space through the use of multivariate statistical techniques. We hypothesise about the origins of the oldest images and compare key elements of Rainbow Serpent stories to those of 'Yam Style' Rainbow Serpents, reputed to be 4000-6000 years of age. It is our contention that an archaeological context for many of the more significant aspects of Rainbow Serpent oral history can be found over several millennia.