One can more confidently trace modern Vashona groups to the flowering of late iron age civilisation in the region from the tenth century AD, which was accompanied by a significant increase in human and livestock populations. This is prominently evidenced by the survival of its stone walled ruins, madzimbabwe, whose settlements were distributed by the thirteenth century as far as the Mozambique coast on the east and Sowa pan on the west. The largest and best known of these is the ruins at Great Zimbabwe, which between 1250-1450 is estimated to have at any given time housed 11-1800 people. A recent survey by Catrien van Waarden identifies the existence throughout north-eastern Botswana of at least thirteen madzimbabwe sites in the Great Zimbabwe style. One of these sites, the Toranju ruins near Sowa, has been dated from the late twelfth century.