Cerro de los Siete Colores is a stack of deep time you can read from the village square. The greenish slates at its foot were laid down as seabed mud roughly 600 million years ago; the pink and red clays banding the slopes above them are only three to four million years old. In between sit white limestones from around 400 million years back and mustard-yellow sandstones full of sulphur, deposited while dinosaurs were still alive. Each colour is a different mineral — copper in the greens, iron oxides in the reds, sulphur in the yellows. Sea, lake and river sediments piled up here for hundreds of millions of years, the rise of the Andes tipped the whole pile on edge, and erosion sliced it open above the adobe roofs of Purmamarca. The hill anchors the Quebrada de Humahuaca, the 155 km valley that has funnelled traders, Inca runners and Spanish mule trains through the mountains and that UNESCO inscribed as a World Heritage Site in 2003.
Come before midday. The hill stands west of the village, so morning sun strikes it head-on and every band separates; by mid-afternoon you are squinting into the light and the colours go flat. The Paseo de los Colorados, a 3 km loop, leaves the last houses along a streambed that only runs after summer storms, passing the cemetery and cardón cacti taller than a house. Down in the plaza at 2,324 metres, weavers set out llama wool beside a chapel built in 1648 of adobe and cardón wood. That open access is recent and hard-won: a landowner fenced two routes onto the hill in December 2018, and within weeks the province declared the hill and the walk a Natural Historic Monument and expropriated the 150 hectares around them, so the paths stay free and unfenced.