Sacsayhuamán: Stones Cut Like a Lock Against a Moving Earth

Sacsayhuamán stands on the hills north of Cusco, Peru. The Inca built most of the complex in the 15th century, starting under Pachakuteq and continuing through the reigns of Topa Inka Yupanqui, Huayna Qhapaq, and Huáscar Inka. Three stepped walls carry large polygonal blocks laid dry, with no thick mortar bed filling the joints. Every block carries its own pattern of projections and recesses, dressed to match the stone beside it.

Cusco lies in a part of the Andes where earthquakes are routine. UNESCO lists earthquakes as a threat to the city's heritage, especially after the 1950 earthquake. The Inca raised three stone walls on a hillside that still shakes today, and the walls still stand. So did the builders set out to assemble a structure that rides out shaking, or did that durability arrive by luck as a side effect of careful stone fitting? The story passed around most often gives the sharper answer. The stones work like a lock. When the ground moves, the wall shifts a little, then the faces of the stones grip one another and hold the wall in place. The site and the fitting method point to builders solving their own problem, because a shaking hillside needs a wall that gives a little and returns instead of a rigid mass that stands until it cracks.