Thus, one can point out to many a path across the clay of the weald which picks its way from one shallow patch of sand, gravel, or stone to another, over country the main base of which is clay, and there is a similar example (with which I have dealt in another volume)[2] in the upper valley of the river Wey. There, once the primitive track has left the chalk and come to the marshy alluvials of the lower levels, it picks its way in this fashion from one long strip of gravel to another; and though these strips of gravel are shallow--mere casual drifts in many cases--they are sufficient for the purposes of the road. Now in the case of the crossing of the Thames at London, the new Geological Ordnance Survey, as it gives the drift as well as the rocks, shows us that a spit of sandy gravel projected into the alluvial mud of the Thames valley just opposite the “bluff” upon which the oldest part of London stands, and indeed projected so far towards the stream that the last traces of it are not lost until beyond Guy’s Hospital--that is, until within little more than a furlong of the present high-water mark. The causeway which might therefore be necessary to approach the stream from the south in all states of the tide need only have been such a hardening of the track over the mud as is necessary between the high- and low-water mark of any tidal river where a ferry is to be established, and we must believe that the river at high water washed the gravel spit. Upon the farther or northern bank traces of artificial embankment (indicating the original limit of alluvial mud upon that side) have been found upon the line of Thames Street, and the Roman wall ran just to the north of it. The total width, then, which had to be negotiated at this point was one at the very most of seven hundred yards, and perhaps much less than that, and it was one which at high water was flanked to the south, as to the north, by a hard surface across which the river could be approached. [Illustration: GREENHITHE] No such conditions were to be discovered between this point and the sea, and, far inland as this point was, it was therefore the lowest practicable crossing of the Thames. Thus it was that the Thames established London. It has also been maintained that this crossing formed not only the first practicable way to one coming up from the sea and seeking the _lowest_ passage of the Thames, but also that no practicable passage could be found for some considerable way _up_ the river either; in other words, that the opportunity for going over the Thames near the site of London Bridge was an isolated and all the more valuable one from the absence of similar opportunities _above_ as well as _below_ it.