In order to traverse the 12,000-foot peaks and deep river gorges of the Qinling and Daba Mountains which lie between Xi'an and Chengdu, the early Qin engineers built what are known as zhandao or plank roads. Square holes 26 inches deep and a foot high were chiseled into the cliff about a yard apart and into them were driven either stone slabs or wooden planks which jutted out horizontally from the cliff face to form the basic support for the road. Wooden planks when propped up from below with struts and enclosed by a railing, made a road bed stable enough to support large armies on horseback. But when slabs of stone were used as planks (right top), the roadway formed using the planks as a base was only a narrow ledge (right middle) which often projected from a sheer cliff with no railings a dizzying thirty to sixty feet above the river. It was such dangerous spots that gave this road the reputation of being harder to travel than "climbing to the blue heavens." Today, although none of the plank roads survive, many holes in the cliff face still mark the sites once spanned by planking (below middle and right), and some even contain their original stone slabs (lower right). In addition, two of the most famous of the old wooden plank roads have been rebuilt as tourist attractions. The best of these lies just north of Guangyuan along a verticle cliff of the Jialing River (below left).