Puente del Inca is a natural stone bridge across the Las Cuevas River, and it is one of the strangest things on the road. Mineral springs have stained the arch a rusty orange and yellow, and the ruins of an old spa hotel, swept away partly by an avalanche decades ago, cling to the bank below. Hot sulphurous water still seeps out of the rock, and the whole scene looks like a painting that has been left out in the rain. Local stalls sell crystals, souvenirs and snacks beside the bridge.
The altitude here is about 2,700 metres, so you will feel it if you walk quickly. Mendoza is more than a hundred kilometres behind, and there is no real town at this spot, only a cluster of stalls, a small cafe and a police checkpoint. Fuel is not reliable here, so arrive with a full tank, and treat the food as a snack rather than a meal.
Up the valley, the great mass of Aconcagua dominates the skyline, at 6,961 metres the highest peak outside Asia. A short side road leads to Horcones Lake and the park entrance, where you can see the south face of the mountain on a clear day, a wall of rock and ice that has killed many climbers. The name Inca refers to a legend that the bridge was made for the Inca ruler, although the real history is more earthly, with mule trains and miners using it for centuries.
The riding is excellent. The road climbs in broad curves along the river, with towering rock walls and tunnels, and the air is crisp and dry. The surface is generally good but patched by winter damage, and in the shade there can be ice well into spring. Trucks on this road are slow and heavy, so keep your distance. The best time is early morning in summer, when traffic is light and the sun hits the peaks. In winter, avalanches and snow can close the road for days.