We began our trek on a cloudless morning, in the small dirt plaza of a sleepy Andean village, at an elevation of just over 13,000 feet. Even at such a high altitude, the thin mountain air felt crisp and invigorating. Ahead of us, a woman wearing a scarlet pollera, the traditional handwoven woolen skirt ofPeru, shooed an unruly herd of alpacas down the winding track through fields of tawny stubble. A two-hour hike brought us to a vantage point on the southern flank of the Sacred Valley, from where we gazed across at the snowcapped peaks of the Urubamba mountain range. Crenellated glaciers descended from the 19,394-foot Veronica, their huge seracs and crevasses looking at this distance like ridges and troughs on a colossal sheet of crumpled paper. A thousand feet below, the extraordinary circular terraces that the Incas had carved into the hillside centuries earlier appeared like giant corrugated seashells.