How the road crossed the marshes of the Wandle we can only conjecture, as we can only conjecture where it lay exactly between Epsom and those marshes. Why it should disappear in the marshes is evident enough. The causeway sank in. Why it should disappear under the plough to the south of the marshes, as Roman roads nearly always do on arable land, can also be explained. But why it should wholly disappear on the last mile or two of chalk is inexplicable. One theory put forward is that in the great wars of the Dark Ages portions of the road were deliberately destroyed to impede the progress of an enemy, just as a railway may be destroyed in modern warfare. But this theory will hardly hold water. The gaps that have disappeared thus, often come just where you have the best soil for marching independently of artifice, and where, therefore, an interruption of them would least incommode an advance. For instance, they are perpetually found on high chalk; and, further, the disappearance is hardly ever connected with a defensive position. From the point of view of the development of the English road system much the most interesting point in the fate which befell the Roman roads is to be found at the crossings of rivers, especially of rivers which have marshy banks or flow through wood or sodden valleys. The neglected Roman embankment across the marsh fell out of use in the Dark Ages. Probably the bridge first broke down, and the barbarous time had not the energy or skill to repair it; then the mere process of time caused the swallowing up of the Roman viaduct, unrelieved by repair, in all marshy land. It is difficult to affirm a negative, but I can recall not one example of a long Roman viaduct still wholly in use across such an approach to water. [Illustration: A DERELICT ROAD SCOTTISH HIGHLANDS] What happened, then, in these sections was this. The bridge and the viaduct disappeared in the Dark Ages--that is, some time between the fourth and the eleventh centuries. Sometimes this gap led to the complete isolation of the district immediately concerned. The best example I know of this is the breakdown of the crossing of the Arun at Romans Wood, in the county of Sussex. There the Roman road was a hard causeway over very thick clay land, quite impassable for armies in winter, and rapidly overgrown by oak scrub and thorn when neglected. The result of the breaking down of the Roman bridge at the “Romans Wood” crossing was to isolate West Sussex. There was no other way from the north, for the clay and thorn scrub rapidly arose and obliterated the road. It was in use as far south from London as Ockley; but the breakdown of the bridge at Alfoldean broke the continuity further on, and that, I believe, is one of the reasons why Sussex was so isolated as only to be converted to the Christian religion a hundred years later than the rest of the country.