Xerxes's pontoon bridges were constructed in 480 BC during the second Persian invasion of Greece (part of the Greco-Persian Wars) upon the order of Xerxes I of Persia for the purpose of Xerxes's army to traverse the Hellespont (the present-day Dardanelles) from Asia into Thrace, then also controlled by Persia (in the European part of modern Turkey).Bagnall, N. (2006) The Pelopennesian War. New York: Thomas Dunne Books
The bridges were described by the ancient Greek historian Herodotus in his Histories, but little other evidence confirms Herodotus's story in this respect. Most modern historians accept the building of the bridges as such, but practically all details related by Herodotus are subject to doubt and discussion.
The bridges in Herodotus's Histories
Herodotus relates in his Histories that the Second Persian invasion of Greece already intended by Darius I was carefully prepared by his son Xerxes I. A canal crossing the Athos peninsula was constructed. Among many other things he also ordered ships of war to be furnished and ropes of papyrus and of white flax to be prepared for the bridges.Herodotus 7.21 and 7.25
During the time Xerxes and his huge army were marching from Sardes to Abydos, then an important harbor on the Hellespont, two bridges were built from there to the opposite side near Sestos over a distance of seven stadia (some ), but were destroyed by a storm before the army arrived.Herodotus 7.33, 34 Xerxes was enraged and had those responsible for building the bridges beheaded. He is then said to have thrown fetters into the strait, given it three hundred whiplashes, and branded it with red-hot irons as the soldiers shouted at the water.Herodotus 7.35
New bridges were constructed by lashing penteconters and triremes together. 360 ships were used to construct the northeasterly bridge and 314 ships were used for the southwesterly bridge. Anchors were lowered at either end of the boats to keep them in place and cables, alternatively made of white flax and papyrus, were stretched from shore to shore to hold the boats together and were tightened by large winches.Herodotus 7.36; Abbot, J. (1917) Xerxes. New York: Brunswick Subscription Company The weight of the cables of flax is said to have been one talent per cubit.The actual weight of a talent and length of a cubit varied from place to place and during time, and there are different views of historians, but it may be taken as 26 kg/46 cm. Three openings were provided for the passage of small boats. Wooden planks were laid across the cables, and brushwood and soil used to cover the planks and form a dirt roadway.Strassler, R. B. (2007) The Landmark Herodotus. New York: Pantheon Books Screens were put up on either side of the bridges to prevent the horses and other animals from panicking at the sight of the sea below.
The crossing of the Hellespont took seven days and nights, the army using the northeasterly bridge and the huge crowd of attendants and baggage animals the southwesterly bridge.Herodotus 7.55, 56
After the crossing, the bridges were left behind. When a part of the Persian army later retreated to the Hellespont, they only found the debris of the bridges destroyed by another storm.Herodotus 8.117
Details After Herodotus hardly indicated the location of the pontoon bridge across the Bosphorus built some 30 years earlier by Xerxes's father Darius I, but did not provide any specific information about that bridge, the wealth of details given for the bridges across the Hellespont is astonishing and, upon cursory reading, seems to provide a clear picture. However, upon closer examination, almost every detail of the bridges is the subject of discussions, doubts and questions.Questions about the bridges are summarized by N. G. L. Hammond, in the introduction to his article. Herodotus's narrative should, perhaps, not be taken as a bridge engineer's sober technical report but rather as a vivid illustration of the grandeur of the Persian king which would make the Greek victories appear even more outstanding.
Details in the play The Persians by Aeschylus (an Athenian playwright) written in 472 BC, less than a decade after the bridge is said to have been built, tend to corroborate the idea of it. In that play, Atossa, the mother of Xerxes, learns of what a disaster her son's invasion of Greece has been. The play is Hellenocentric.
Aeschylus was an Athenian playwright who had fought in the battle of Salamis, and witnessed there the destruction of the Persian fleet.
Dardanelles The surface current to the Mediterranean flows at an average speed of 1 1/2 knots but varies according to wind directions which may also cause the water level to rise by some . There is an undercurrent in the opposite direction. Bills and points may cause eddies and shoals.The Black Sea Pilot, p. 17
At present, the narrowest part of the Dardanelles between Çanakkale and Kilitbahir () is about 1.4 km (1,530 yards) wide and has a maximum depth of .Nautical chart at GeoHack-Dardanelles, MapTech It has the strongest current and in shipping it is considered the most difficult part of the Dardanelles.The Black Sea Pilot, p. 30 Çanakkale was built during the early Ottoman Empire on the alluvial gravel fan deposited by a river, in winter often a torrent,The Black Sea Pilot, p. 30: Rhodius River descending from the nearby mountains (and recently tamed by the Atikhisar dam). 2500 years ago, this alluvial fan may not have protruded into the strait as it does today.
Abydos, the town mentioned by Herodotus, was north of Çanakkale on the Asiatic shore near Nara Burnu (formerly Nagara) (). There are shoals to the south and to the west of Nara Point, but the depth in the center of the strait is as much as . The current is running at more than 2 kn, but there are large eddies around Nara Point.The Black Sea Pilot, p. 32
Location of the two bridges The location of the bridges between Abydos and near Sestos on the opposite shore, as indicated by Herodotus, is accepted by many historians. The British Major-General Frederick Barton Maurice, on a visit to the area in 1922, considered a beach further north to be the only acceptable location for a bridge from a military point of view; but there, the distance across is more than 3 km (3,280 yards).
The presently narrowest part at Çanakkale appears to be beyond all question. Most probably, it was not as narrow then, and there was a constant danger of the untamed river becoming torrential at very short notice and washing away whatever was in its reach.
Two bridges were necessary because the narrow roads in the Chersonese required that, to avoid leaving the head of the very long column of troops without food and water, the column of troops and the supply column march in parallel.Barker, p. 41
Ships Herodotus is clear in telling us that only pentecontes and triremes, i.e. only naval vessels, were used for the bridges. Apparently, this is accepted by all historians. However, it does not make sense to use expensive naval ships (unless such ships were available in abundance since most of them were contributed by the Greek coastal city-states in Asia Minor, by Phoenicia, by Syria, and by Egypt, all of which were under Persian rule, at the time) were for a task which would be better fulfilled by simple and cheaper merchant vessels with a larger beam, deeper center of gravity and higher freeboard; on the other hand, slender ships would also make sense since they provided the least resistance to the surface current in the Hellespont (Dardanelles). The lowest oar ports of a trireme were about 30 cm above the waterline and were normally fitted with leather sleeves,Barker, p. 31 an aspect which does not really qualify them as a bridge carrier.
Ships carrying a bridge should all have the same height in order to provide a flat bridge deck and thus, one may assume that the bridge consisted only of penteconters (if not of commercial ships) and that the larger and higher triremes were only used on either side of the passageways.
Anchors Pontoon bridges across rivers are usually held in position by anchors fastened to the bow and stern of each boatHoyer, p. 403 and thus, at a first glance, Herodotus's description appears to be correct. However, the depth of the strait is not mentioned at all in his Histories. Modern historians do mention the depth,e.g. Barker, p. 30; Hammond, p. 93 in the little chart but do not discuss it anywhere as a problem for anchoring.
The length of anchor ropes must be several times the depth of water in order to prevent damage to the ship caused by a jerking anchor rope and to prevent the anchor from dragging along the seabed. Ships in the center of the strait would thus have had to use anchor ropes with a length of several hundred meters each. The total of 674 ships would not only have required 1348 heavy anchors,Hammond (p. 98) cites Robert Chapman, A treatise on ropemaking as practiced in private and public ropeyards ... (Philadelphia 1869) that an iron anchor for a ship comparable to a penteconter would have to weigh some 136 kg. The overall total weight would thus be 1348 anchors x 136 kg/anchor = 183,328 kg or 183 tonnes. but also some 300 km of anchor ropes. One may question whether at that time it would have been possible to manufacture such huge amounts within a relatively short time. Even if iron anchors existed already then,Hammond, p. 98Barker (p. 34) talks about large blocks of stone it is unlikely that the iron manufacturing was capable to produce some 183 tonnes of iron anchors. In addition, the anchorage is not safe: the long ropes cannot prevent the ships from swinging and colliding, in particular when eddies add to the confusion and long ropes get entangled. Last, but not least, it seems to be impossible to find the right points for dropping the anchors so that their long lines would hold the ships properly lined up across the strait.
If one adheres to the ships being anchored as described by Herodotus, one has to take into account that each bridge together with the space required for the anchor ropes would have occupied a strip up to 900 m wide. However, the shore at Abydos would not have been wide enough to accommodate two such bridges.
There is a further technical point: The addition of anchors and of cables reaching from shore to shore provides added holding power to the ships only in theory, i.e. if the ships' tension on the anchor ropes and on the cables is exactly equal, but in practice, it is not possible to tune them to such a degree, especially not under the influence of changing winds, currents, eddies and undercurrents. But then, the whole load has to be borne by either the ropes or the cables, without the other (slack) one contributing anything to the horizontal load bearing capacity of the installation.
Therefore, there appears no alternative but to assume that the ships were held in place by the long cables only, and that anchors were used only temporarily to hold ships in shoal waters until they were attached to the cables.
Length of the bridges The length of seven stadia or some 1,300 mThis is not the place to discuss the different types of stadia and the various views on their length as indicated by Herodotus is too short in any event.