The nature and composition of desert varnish found on aboriginal rock engravings are discussed. Several theories are proposed as to its origin. Age determination of the varnish is suggested through several different experiments and on-site observations. With the age of the varnish determined, the age of associated rock engravings could also be established. For other studies of rock art, see also AATA 8-1454, 13-1125, 17-670, 18-1600, 19-608, 21-2068, and 22-61. -- AATA