Quebrada de los Cuervos cuts more than 100 metres into a country that is otherwise almost all open grassland. The Yerbal Chico stream carved the gorge out of Precambrian rock — quartzite, gneiss and schist, some of the oldest crust on the planet — and in doing so built a trap for warmth and damp. Sheltered from the wind and frost that shape the plains above, the canyon holds a subtropical forest that has no business being this far south: 87 species of tree and shrub, 22 kinds of fern, pindó palms rooted straight into the rock, and wild yerba mate, the plant that gives the surrounding Sierras del Yerbal their name. The cuervos of the title are not crows but New World vultures — black, turkey and lesser yellow-headed — which nest on the cliffs and hang on the thermals rising off sun-warmed stone.
Two trails leave the visitor centre, and they are not the same day out. A 500-metre path ends at a rim viewpoint where the grassland simply stops: the gorge opens under your feet, the stream a dark ribbon between palm crowns. The other goes down — roughly three kilometres of rock, roots and a few hand-over-hand moves — and the surprise is that it grows warmer as you descend, the air turning still and humid as the walls close over you. It ends at the Yerbal Chico, where clear water runs over stone and fine sand past a small rocky beach. Out on the sierra above, threatened seedeaters fly in from the north each November to breed. Rangers run a visitor centre at the gate of what became, in 2008, the first place Uruguay ever entered into its national protected-areas system.