There is one other structure in Babylon that deserves mention, and that is the bridge over the Euphrates, since its remains are those of the earliest permanent bridge of which we have any record in antiquity. It will be noted from the ground-plan of E-temen-anki[178] that the procession-street leads past the corner of the Peribolos to a great gate-way in the river-wall, guarding the head of the bridge which crossed the Euphrates on stone piers. The river at this point appears to have been one hundred and twenty-three metres in breadth. The piers are built in the shape of boats with their bows pointing up-stream, and their form was no doubt suggested by the earlier bridge-of-boats which they displaced. The roadway, as in boat-bridges in Mesopotamia at the present day, was laid across the boatpiers, and must have been very much narrower than the length of the piers themselves. The bridge, which is mentioned by Herodotus[179] and Diodorus,[180] was the work of Nabopolassar, as we learn from the East India House Inscription, in which Nebuchadnezzar states that his father "had built piers of burnt brick for the crossing of the Euphrates."[181] The stone used in its construction, which is referred to by Herodotus, was no doubt laid above the brick-piers, as a foundation for the flat wooden structure of the bridge itself. The later river-wall was the work of Nabonidus and it marks an extension of the bank westwards, which was rendered possible by the building of Nebuchadnezzar's fortification in the bed of the river to the west of the Southern Citadel.[182] The old line of the left bank is marked by the ruins of earlier river-walls, traces of which have been uncovered below the north-west angle of the Peribolos.[183] It was doubtless to protect the Peribolos and E-sagila from flood that the bank was extended in this way. The buildings that have hitherto been described all date from the later Assyrian and Neo-Babylonian periods, and during their first years of work at Babylon the excavators found nothing that could be assigned to the earlier epochs in the history of the capital. It was assumed that the destruction of Babylon by Sennacherib had been so thorough that very little of the earlier city had survived. But later on it was realized that the remains of the older Babylon lay largely below the present water-level. The continual deposit of silt in the bed of the river has raised the level at which water is reached when digging on the site of the city, and it is clear that at the time of the First Dynasty the general level of the town was considerably lower than in later periods. During recent years a comparatively small body of water has flowed along the Euphrates bed, so that it has been possible on the Merkes Mound to uncover one quarter in the ancient city. There trenches have been cut to a depth of twelve metres, when water-level was reached and further progress was rendered impossible, although the remains of buildings continued still lower.