From the NY Times:
CAMBRIDGE, Mass. â€” Conquistadors from Spain came, they saw and they were astonished. They had never seen anything in Europe like the bridges of Peru. Chroniclers wrote that the Spanish soldiers stood in awe and fear before the spans of braided fiber cables suspended across deep gorges in the Andes, narrow walkways sagging and swaying and looking so frail.
Yet the suspension bridges were familiar and vital links in the vast empire of the Inca, as they had been to Andean cultures for hundreds of years before the arrival of the Spanish in 1532. The people had not developed the stone arch or wheeled vehicles, but they were accomplished in the use of natural fibers for textiles, boats, sling weapons â€” even keeping inventories by a prewriting system of knots.
So bridges made of fiber ropes, some as thick as a manâ€™s torso, were the technological solution to the problem of road building in rugged terrain. By some estimates, at least 200 such suspension bridges spanned river gorges in the 16th century. One of the last of these, over the Apurimac River, inspired Thornton Wilderâ€™s novel “The Bridge of San Luis Rey.”