The Salkantay route to Machu Picchu is four days of walking. The second morning takes you over a pass at about 4,600 metres, below the south face of the mountain. We left camp in the dark and reached the top at sunrise, moving slowly because the air up there gives you very little.
The pass is a cairn field with prayer flags and a lot of wind. Salkantay was right there, close enough that the glacier noise carried. I did not stay long. My hands had stopped working the dials well.
The path down the far side drops into cloud forest within a couple of hours. By the afternoon it was warm and green and the pass felt like a different trip.