There is a little note on the crossing of water courses which is curious and interesting in the history of roads. Since the crossing is always an effort, or, in economic terms, an expense, to be avoided as much as possible, the Road naturally avoids a _double crossing_, but, on the other hand, an island is a stronghold, and even a peninsula where two rivers meet is a potential stronghold. Therefore you have in the history of all early European roads a sort of dilemma, the first travellers debating, as it were, whether the occasion were sufficiently important to warrant the double crossing of the stream. At Reading, Lyons, Melun, notably at Paris, and in dozens of other places, the presence of the stronghold made it worth while for the Road to visit the place in spite of the double crossing, whether to an island or to the meeting of two streams. But in much the majority of cases the Road was deflected from its simplest line to a point below the meeting of two streams so as to avoid the double effort, and the occasion explains many a deflection which otherwise would seem to have no reason. CHAPTER III PASSABILITY _The Choice of Soils: Following the Gravel or the Chalk: Conditions in the South and East: The Obstacle of Gradient: The Early Vogue of Steep Gradients “The Other Side of the Hill”: The Modern Importance of Gradient: Passes or Gaps in Hill Country._ i To the next physical factor modifying the formula of the Road we have given the name: DIFFERENCES OF SURFACE OTHER THAN MARSH AND WATER COURSES. The differences of surface other than marsh or water courses affect the trajectory of a road in several ways: first and originally in its passability to human travel on foot or with beasts of burden, or later with wheeled vehicles, and here the two factors were hardness and evenness. But there was a great contrast in the obstacles of the North and the South of our civilization. In the North, and especially in England, damp was the enemy. For a trajectory to be used in all seasons and in all weather sand and chalk at once suggested themselves. Clay can be used only in the dry season. The various soils determined the first trackway and impose themselves visibly upon the map of our oldest roads. For instance, the road down the upper Wey to Farnham is, in its oldest form, a deliberate picking out of long gravelly stretches in the bed of the valley. On a geological map you can trace this road picking its way from gravel patch to gravel patch almost as a man crosses a stream by stepping stones. It leaps, as it were, from one gravelly stretch to another, and in each keeps to the gravel as long as it can. For the same reason a primitive road will follow the South, or sunny, side of a wood or of a ridge of land, so that the surface may dry as soon as possible after rain.