A wide dry valley, forests of cactus and, hidden among them, one of the oldest dams in the Americas. Purron is the name of a huge earthen embankment in the Tehuacan-Cuicatlan valley of southern Puebla, built perhaps two and a half thousand years ago, and over the centuries it grew to be one of the largest dams of ancient Mesoamerica, holding water for fields in a land that gets little rain. It does not look like much from the road: a long, low ridge of earth and stone, covered in scrub. But it is a testament to the farmers who lived here, in a valley where agriculture was already old when the Egyptians were building pyramids, and where early maize was grown thousands of years ago.
The valley is a biosphere reserve and a World Heritage property, famous for its forests of columnar cactus. Giant tetechos and cardones stand in thick groves like candelabra, with agave, yucca and spiny shrubs between them, and the whole landscape glows green and gold in the late afternoon. Birds such as the cactus wren and a little parrot called the Mexican parakeet use the cactus, and the rocky hills are full of fossils from the time when the area lay beneath a shallow sea. San Juan Raya, the other stop on the ride, is known for its remarkable fossil beds, with sea snails in the rock.
The roads in this valley are not wild, but they have their moods. Highway 135 runs along the valley floor between Tehuacan and Oaxaca, a fast, busy route with trucks, while the side roads to the villages turn to gravel and rough track, where a careful pace and good tyres are needed. Heat is the main challenge. Between April and September it can be fierce, so start at dawn and carry plenty of water. The best months are November to March, with clear skies and warm days, and cold, bright nights.
Coxcatlan, about eight kilometres away, is the nearest village of any size, but for fuel and a proper meal, Tehuacan, the bigger town, is the place to fill up. The town is also known for its mineral water, bottled and sold across Mexico, and for the little salt-works of nearby Zapotitlan, which have produced salt for centuries. Local food is rich in corn and chile, with goat and mole common on the menu. Respect the cactus: it grows slowly, and many species are protected.