At the Quebrada de los Cuervos the grassland stops without warning. The gorge starts at your feet and drops a hundred metres, in the Treinta y Tres department of eastern Uruguay, with a stream running as a dark ribbon between palm crowns at the bottom. The plains up here take hard winter frosts. Down in the gorge the wind never reaches and the sun only does for half the day, so the air stays damp and mild all year. What grows in it has no business being this far south: 87 kinds of tree and shrub, 22 ferns, and pindó palms rooted into bare rock.
Going down is three kilometres of rock and root with a few moves on your hands, and the surprise is that it gets warmer as you lose height. The palms that were crowns from the rim arrive at eye level and then overhead, and the sky narrows to a strip. It ends at the Yerbal Chico: clear water over stone, a small rocky beach. The climb back is the same hundred metres taken the hard way. The cuervos of the name are not crows but vultures, and nothing lifts until the rock has warmed. They sit on the ledges early and are up on the thermals from late morning, passing the rim at your eye level. The turkey vulture holds its wings in a shallow V and rocks side to side; the black vulture flies flatter and flaps in short bursts. Most of the protected area is the grassland above, much of it private land where families still graze cattle: the line was drawn around the ranches rather than through them. From mid-November the tall grass fills with capuchino seedeaters, small threatened finches that come south to breed and are gone again by March. The sierra is called the Yerbal, after the yerba mate growing wild in these gorges — in a country that drinks more mate a head than any other and grows almost none.