Water has always been one of the principal obstacles to the improvement of transportation by land. In early times travelers were limited to wading through shallow streams or stepping across a series of large rocks in the streambed. In some cases a fallen tree would conveniently span a stream, creating a beam bridge so that people could cross from one side to the other. Occasionally, large rocks could fall across the water in such a way that a natural arch was formed. In warmer climates intertwined vines formed natural suspension chains on which one could swing across a river or gorge. Thus beam, arch, and chain bridges all probably developed from natural bridges. Each of these forms of construction has been traced back several thousand years, though which type developed first is a matter of speculation. Arch bridges have been recorded as far back as 4000 B.C., and it is likely that crude forms of beam bridges were developed even earlier in which logs or slabs of stone were used to connect raised areas on either side of the water. A historic example of a form of beam bridge is that constructed on floating supports, or pontoons, by Xerxes, the king of Persia, across the Hellespont in 480 B.C. The first organized building of bridges on a large scale took place in association with military campaigns. During the expansion of their empire, the Romans built a large number of aqueducts, which were bridges of stone and timber designed to support roads and to carry water. The Romans were excellent builders.
They were able to direct large labor forces supervised by technical experts. They developed an extensive network of roads in Europe; their bridges were about 4.5 meters wide to allow two vehicles or two units of armed men to pass each other. There remain several fine examples of Roman bridge building on the European continent, perhaps the most famous of which is the Porit du Gard aqueduct in the south of France. Towering about 47 meters above the river, it has an overall length of 275 meters.