We know, however, with certainty that a bridge was standing in the reign of King Ethelred from the references made to it, and we may fairly assume that this must have been the Roman bridge, at least so far as its main construction was concerned; and, like other Roman bridges standing at that time in Gaul and in other parts of England, it would have consisted merely of piers of masonry, with a wooden roadway passing from one to the other. It was still standing, of sufficient strength for resistance, when the Danes under Canute sailed up the river, who, being unable to force a passage, tradition says--and antiquaries have imagined they could discover traces of it--cut a ship canal through the Surrey marshes from Bermondsey to Battersea, and passed their fleet through that way. [Illustration: FIG. 1--THE UNDERCROFT OF ST. THOMAS OF CANTERBURY ON THE BRIDGE.] The history of the bridge only opens with the beginning of the twelfth century. According to tradition, the convent of St. Mary Overie, Southwark, had been originally endowed with the profits of a ferry across the river, and had in consequence undertaken the duty of maintaining the bridge in a proper state of repair when a bridge was built. This convent was refounded in 1106 as a priory of Austin Canons; and it is not a little remarkable, having regard to the duties it had undertaken, that of its founders, who were two Norman Knights, one was William de Pont de l'Arche. At the Norman town, where stood his castle and from which he took his name, was a bridge of twenty-two openings, erected, it was said, by Charles the Bald, but most likely a Roman work, across the Seine at the highest point reached by the tide. It is a further curious coincidence that this same William appears as a witness to a deed executed by Henry I., excusing the manor of Alciston, in Sussex, from paying any dues for the repairs of London Bridge.