The Simien Mountains are an escarpment in the northern Ethiopian highlands where the plateau breaks off into pinnacles and thousand-metre drops, and the monkeys that graze along the top of it sleep over the edge. Geladas are the only primate on Earth that lives by eating grass. That keeps them out on open meadow all day, which is where leopard and hyena hunt. The cliff is the trade-off: safety at night, paid for with a climb down at dusk and back up at dawn. They move in bands hundreds strong. If you sit still on the grass a herd will feed right around you, grunting and chattering as it shuffles past, some of them within arm's reach. The males carry lion-like manes, and the bare red patch on the chest is what gives the animal its other name, the bleeding-heart monkey. It is about as close as anyone gets to a wild primate anywhere.
The classic walk follows the same line the geladas do. It links the Sankaber, Geech and Chennek camps over several days along the very lip of the escarpment, across Afro-alpine meadow and 4,000-metre passes, with the drop always at your shoulder. Nights up here fall below freezing. Bearded vultures bank and hang along the cliff on the updrafts, and now and then one drops a bone onto the rocks below to crack it for the marrow. Walia ibex work the ledges underneath them, and they live nowhere else on Earth: down to a few hundred animals thirty years ago, and guarded back up since. None of this is empty country. The flat ridges you cross are the last of the lava surface of a vast volcano, worn back into towers and ravines, and Amhara farmers still work barley terraces and run zebu over them. At dusk the herd goes over the edge, down to ledges where nothing can follow. In the morning it comes back up onto the grass your tent is pitched on.