The terrace deposits of the lower reaches of the Orange River on the Namibian bank are richly fossiliferous at two localities, Auchas - of Early Miocene age - and Arrisdrift - basal Middle Miocene age. This monograph is the result of 10 years of fieldwork, laboratory preparation and study of all the fossil groups encountered, starting with plants and ending with mammals, with lower vertebrates in between. The geological context of the deposits has been provided in a previous memoir of the Geological Survey of Namibia (Pickford and Senut, 2000). This paper provides additional geological and palaeobiogeographical background for understanding the context of the fossils described in this monograph.