The day crosses one of the great invisible borders of South America: the line where Cuyo ends and Patagonia begins. It is a long drive through volcanic steppe, mountain passes, and thinly settled valleys — a day shaped more by the road itself than by anything at either end of it.
Ruta 40 Sur: Entering the Volcanic South
The departure from Malargüe at 7:30 am is an early start on a route that earns it. The road south is the Ruta Nacional 40, one of the longest national routes in the world, running the full length of Argentina from the Bolivian border to the Strait of Magellan. This section of it — known to travellers as the Ruta 40 Sur — is among its most remote.
The first stop comes quickly: the Cañón del Río Grande en rocas volcánicas, a short detour to the spot known as La Pasarela. Here the Río Grande — the most voluminous river in Mendoza province, fed by Andean glaciers near the Chilean border — has cut a narrow canyon through basaltic rock. A natural crossing gives access to easy walking paths on both banks; the contrast of dark volcanic walls and fast, pale water is immediate and striking. The river drains south toward the Río Colorado, the natural boundary between Mendoza and Neuquén, and the volcanic geology here is a prelude to what dominates the landscape for the next hundred kilometres: the Payunia, a volcanic field that rivals Kamchatka for density of cones per square kilometre, with some 800 counted craters across 15,900 km² of dark lava flows and scorched pampa.
Back on the road south, the route passes through the gravel stretches between La Pasarela and Bardas Blancas — perhaps 80 kilometres of ripio varying in condition — before rejoining sealed road at Bardas Blancas and crossing the Río Grande on a narrow bridge. The river crossing marks the practical transition: to the left is the access to the Caverna de las Brujas, a limestone cave system about 8 kilometres off the route, worth the detour if time and conditions allow. Continuing south and east, the route climbs the sinuous Cuesta del Chihuido — reaching nearly 2,000 metres (6,560 ft) before descending again — then crosses the Río Barrancas into Neuquén Province. The Río Barrancas and Río Grande merge a few kilometres downstream to form the Río Colorado. On one side: Cuyo. On the other: Patagonia.