Having established this, let us consider why the crossing place came just here upon the line of the river, and to solve that problem we must recollect what it is that acts as a barrier to communication, especially in early times. This barrier is certainly not to be found in hills of such a gentle sort as diversify south England. The breadth of a stream, up to half a mile or so, and apart from broken water, though of considerable importance in the problem, is again not the chief difficulty, for man has always been able to cross water if his approach to it was unimpeded. The true obstacle was marsh--bad going. It is still the chief impediment to modern engineering, and the difficulty which men found during earlier centuries in negotiating any considerable breadth of marsh translates itself to-day into the expense which a similar undertaking now involves. Belts of marsh impeding or actually forbidding travel across them have been formed upon one, the other, or both banks in so unbroken a chain down the Lower Thames valley as to make that valley the obstacle it was and is to transverse travel. The first factor in the formation of such marsh is the tide. If you have a considerable difference between high and low water, and if that difference is further complicated by great variations between the neap and the spring tides, the difficulty of the barrier of marsh affected by such a tide will be correspondingly increased. [Illustration: LIMEHOUSE] This consideration is often missed, and it will therefore be worth while to explain it. A fairly regular height of tide covering daily the same expanse lends itself to the establishment of a regular crossing place, and though that crossing place may not be in continuous use from the difficulty of using it at low tide, it can be regularly counted upon to serve twice in the twenty-four hours. But if there is any very considerable difference in the state of the tides throughout each fortnight, then the opportunities for using the crossing are very much reduced, unless, indeed, one has a steep shore to deal with. Under such conditions one might find a spot where crossing was easy enough at the springs, and yet impossible at the neaps; and instead of a ferry regularly in use twice in the twenty-four hours, you would have one which could only be depended upon a few times in a month. Now the Thames is a river in which this difference is considerable, and it has greatly strengthened the power of the waterway to act as an obstacle to travel from north to south. This, then, is the first of the factors which have combined to make the Lower Thames the obstacle it was and is to travel. If we compare the Thames in this respect with the other great rivers which we have seen to be its parallels upon the Continent, we shall be struck by the greater effect of the tide in its waters.