Now if we combine all this and consider the total effect of all three factors in the Lower Thames valley we shall understand why no great road ever attempted to cross it, and why no line of travel runs transverse to it to-day. A mere examination of the contours would be almost sufficient, presenting, as they do, great flats in most places, stretching for miles from the main stream of the river. But beyond this you have the great variation of the tide and the type of _surface_ soil with which the stream deals. Civilisation has so considerably changed the aspect of all our streams, it has so embanked them and drained their neighbourhood, that in order to appreciate the original conditions which made it impossible to find a crossing place below London one must consult the new sheets of the Ordnance Geological Survey. They give us the drift or top-soil--which alone of course concerns travel. In this new survey the area covered by alluvium and the line where that alluvium impinges upon the older and harder soils to the north and the south are very clearly marked. That area with its boundary line gives one the original area and the original boundary line of the Thames’ marshes, and it is very instructive. The problem is one of approach from the south. From the north there is no firm soil at all within the neighbourhood of the riverbed from the sea upwards until one reaches the slight eminence of the City, unless one counts the isolated patches at Purfleet and Grays, the approach to which from the north was not only originally difficult but connected with no reasonable line of travel. One has but to look at the map to see that Purfleet could have been approached from the north by no considerable road. It might have formed some sort of terminal for an eastern road but only that at the expense of a long detour such as is made by the main road to-day through Ockendon or by the railway, for immediately behind lies a belt of what was originally marsh. Moreover, even if primitive travel had drifted by this somewhat circuitous route to the hard patch at Purfleet, it would have found no crossing there; immediately opposite lay the very wide belt of marshy land which flanks either side of the Darent. That river comes in almost exactly opposite the small belt of natural hard on which Purfleet stands. Primitive travel, then (and for that matter modern travel too, unless it is at a great expense of engineering), could not approach the northern bank of the Thames between the sea reaches and the City of London save, and that with difficulty, by the very small exceptional patches of Purfleet and Grays, and at Purfleet would have discovered no opportunities for a crossing: the bank opposite being a particularly wide and impossible stretch of marsh at this point.