The physiography of southern Yukon is dominated by upland plateaux and plateau remnants that probably evolved in the Tertiary, culminating in a mature erosion surface about Miocene time. Variations in the elevation of this surface are thought to result from uneven uplift and faulting in the Late Miocene or Pliocene. The Tintina and Shakwak Trenches are young grabens superposed on the upland and the mountain ranges are youthful dissections of raised parts of the plateau.In the Miocene, water from central Yukon probably drained to the coast across the region occupied by the St. Elias and Coast Mountains. About the late Miocene, general uplift occurred and the St. Elias and Coast Mountains rose unevenly above interior parts of the Yukon. As a consequence, the drainage became entrenched. During deglaciation, ice in the St. Elias outlasted that in the interior and this forced derangements of the entrenched stream system. It was abandoned in favour of the less direct northwest drainage of today. The northwest drainage persists, but is unstable and in danger of capture by more vigorous streams that flow directly to the coast.