Onyx travertine deposits have long been known to occur in the Trough Namib in the Sperrgebiet, Namibia, but only recently have they been mapped in the Plain Namib. Previous interpretations concluded that the travertines of the Sperrgebiet accumulated in lagoonal settings close to the sea, but the ones in the Plain Namib, some of which lie at altitudes of over 500 above msl, indicate that their genesis had little to do with proximity to the sea, but that they formed where cold lime-charged groundwater emerged at springs, precipitating some of their dissolved lime as they warmed up when they rose towards the land surface and the remainder of the lime at the surface, often as lobes with sloping margins. Most of the Kaukausib and Tsirub travertines are associated with sand, grit and conglomerates, some of which are fossiliferous, thereby providing biochronological controls on the period of accumulation. Based on fossil mammals found in the intercalated calcified sands, some, if not all of the travertine occurrences in the Plain Namib are Plio-Pleistocene. The only known exception is an Eocene occurrence at Eoridge, well south of the Kaukausib Valley. Keywords: Onyx travertine, Calcified sand, Calcified grit, Calcrete, Fossil mammals, Termite hives, Pliocene.