Radial drainage centred on Brown Mountain suggests that the Monaro basalts are remains of a former shield volcano, here called the Monaro volcano. At the time of the volcanos eruption the Great Escarpment did not exist and relief was probably low about 50 Ma ago. The volcano diverted original north-flowing drainage to the west, via the Snowy River. Rivers near Jindabyne and Adaminaby (indicated by the Kiandra basalts) that originally flowed northwest were reversed by tectonic uplift (possibly 1 km vertical) of the Snowy Mountains post-22 Ma. The reversed rivers were deflected by the Monaro volcano, one to the Murrumbidgee, the other to the Snowy River. Uplift was therefore in two phases: the first, post-50 Ma; the second, post-22 Ma.