A high-altitude construction puzzle
At nearly 13,000 feet on the Bolivian Altiplano, the ruins of Pumapunku show some of the most extraordinarily precise stonework of the ancient world. Part of the larger Tiwanaku archaeological site, Pumapunku was constructed by a pre-Incan culture that flourished between 300 and 1000 AD. The site is a scatter of enormous, worked blocks of red sandstone and grey andesite. The largest single stone is a red sandstone slab measuring 7.8 meters long and 5.2 meters wide, with an estimated weight of 131 metric tons.
The logistical challenges of this construction were immense. Geochemical analysis shows the red sandstone blocks were moved from a quarry about 10 kilometers away, up a steep incline. The much harder andesite, a volcanic rock used for the most detailed carvings, came from the Copacabana Peninsula, a trip of about 90 kilometers that would have required crossing the waters of Lake Titicaca. The Tiwanaku culture accomplished this without the use of the wheel or a known writing system.
Recreating pre-Incan tools
The extreme precision of the stonework at Pumapunku has led to speculation, but archaeological experiments offer viable explanations. The builders used two main types of stone: a softer red sandstone for large structural platforms and a much harder grey andesite for detailed work. This hard volcanic rock registers just below quartz on the Mohs scale of hardness.
Archaeologists and experimental stonemasons have demonstrated that these cuts were achievable with the materials on hand. The process was laborious but not impossible. Initial shaping was likely done with heavy stone hammers. For the perfectly flat planes and sharp angles, workers would have used a technique of grinding. By rubbing a flatter stone with an abrasive slurry of sand and water, a motivated team could slowly grind the andesite blocks down to achieve surfaces with remarkable planarity.
The crisp interior right angles and drill holes that seem to defy primitive tools can also be explained. Experiments have shown that straight grooves can be cut by sawing with stone tools and abrasives. Perfectly circular holes could be created using hand-powered or bow drills, employing materials like hollow cane or bone in combination with sand to bore through the hard rock. The famous "H-blocks," with their multiple angles and interlocking shapes, were likely made with a system of templates and repeated measurements, allowing for standardized, if not identical, components that fit together with exacting precision. To hold these massive structures together, the Tiwanaku used a unique copper-arsenic-nickel bronze alloy to create I-shaped cramps, which were cast in place within sockets carved into the blocks.