Inside Machu Picchu

Excavation and soil analyses done by Kenneth Wright in the 1990s showed that the terraces were built in layers, with a bottom layer of larger stones covered by loose gravel. On top of the gravel was a layer of mixed sand and gravel packed together, with rich topsoil covering all of that. It was shown that the topsoil was probably moved from the valley floor to the terraces because it was much better than the soil higher up the mountain.