The Simien Mountains are an escarpment in the highlands of northern Ethiopia where the plateau ends in pinnacles and drops of a thousand metres, trekked along the very rim among herds of gelada monkeys that graze the grass by day and sleep on the cliff face at night. The mountains are the worn-down remains of a vast volcano that piled up here around 30 million years ago, and erosion has cut its edges into towers, spires and gorges that fall away from the Afro-alpine meadow at your feet; Ras Dashen, the highest point in Ethiopia at 4,550 metres, stands at the far end. The classic route links the Sankaber, Geech and Chennek camps over several days, following the lip of the escarpment across meadow and 4,000-metre passes with the drop always on one side and giant lobelias standing in the grass on the other.
The geladas are the reason the walk is unlike any other. They are the only primates on Earth that live by grazing grass, and they move in bands hundreds strong; settle quietly on the meadow and a herd will feed right around you, grunting and chattering as they shuffle past, the males with their lion-like manes and the red patch of bare skin on the chest that gives them the name bleeding-heart monkey. Each evening the herds shuffle to the very lip of the escarpment and climb down to sleep on narrow ledges, hanging in the void beyond the reach of any predator. Above them lammergeiers, the bearded vultures, bank and hang along the cliff on the updrafts, now and then dropping bones onto the rocks below to crack them for the marrow, and walia ibex, found nowhere else, pick their way along the cliffs. Beyond the park's edge, Amhara farmers work barley terraces and graze zebu cattle across the plateau much as they have for centuries, and the trek passes through their villages. The Simien is a roof of Africa with the edge cut off, and the monkeys sleep on the cut.