At some time or other, the idea occurred to an unknown person of engineering genius in advance of his time, that it might not be impossible to construct a causeway across this marsh; and that such a causeway would be extremely useful and convenient for those who used the Thorney Fords. Perhaps the causeway was his own invention; perhaps the work was the first causeway ever constructed in this country; perhaps the inventor began on the smallest possible scale, with a very narrow way across the marsh to the nearest dry ground, which was, of course, somewhere beyond Kennington; perhaps the work, colossal for the time, carried the merchants and their caravans across the whole extent of the marsh--five miles and more--to the rising ground of Deptford or Greenwich, the nearest point to Dover. The causeway was not unlike those which now run across the Hackney Marshes; that is to say, it was raised so high as to be above the highest spring tide, about six feet above the level of the marsh. It was constructed by driving piles into the mud at regular intervals, forming a wall of timber within the piles, and filling up the space with gravel and shingle, brought from Chelsea--'Isle of Shingle'--or from the nearest high ground, where is now Clapham Common. The breadth of the causeway, I take it, was about ten or twelve feet. The construction of the work rendered the passage across the marsh perfectly easy, and greatly facilitated that part of the trade of the island which lay in the midland and on the north. When was this causeway, the first step in road-making, constructed? Perhaps it was a Roman work. I think, however, that it is older than the Roman occupation; and for these reasons. When London was first visited by the Romans it was already a flourishing city with a '_copia negotiatorum_;' in other words, it had already succeeded in attracting the greater part of the trade which formerly passed through Thorney. Had the Romans built the causeway, they would have constructed it along a line drawn from one of the two old ferries to Deptford. The causeway, therefore, must have existed when the Romans arrived upon the scene, together with, as we shall see immediately, the second causeway connecting the ferry with the first causeway. I dare say the Romans strengthened the work: turned it from a gravelled way, soft in bad weather, into one of their hard, firm Roman roads; faced it with stone, and made it durable. If South London were to be stripped of all its houses, the two causeways would be found still, hard and firm, beneath the mass of accumulated soil and rubbish, as the Romans left them.