 Along the southern extent of the central plateau some of the thicker, hard, erosion-resistant dolerite sills form large parts of the upper edge of the escarpment (see illustration on the right), but where the sills are thinner or absent then, like the portion of the Drakensberg between KwaZulu-Natal – Free State, the escarpment is composed of softer rocks, aged between 250–300 million years old.This means that in these regions the scarp has a more rounded appearance, or has eroded away to such an extent that the scarp may no longer be evident (for instance along the route taken by the N1 highway between Beaufort West, below the escarpment, and just beyond Three Sisters on the way to Richmond or Victoria West, on the plateau; and also where the Orange River has eroded a broad valley through the escarpment in the west before it flows into the Atlantic Ocean).Along most of its extent, however, it forms a 400–800 m high mountain-like ridge that roads into the interior have to negotiate, sometimes up steep winding passes, such as the Burke's, Vanrhyns, Bloukrans, Gannaga, Ouberg, Verlatekloof, Teekloof, Molteno, Goliatskraal, Daggaboersnek, Katberg, Nico Malan, and Barkly passes.