These, however, as cultivation, settlement, and drainage have advanced, are disappearing types of Frontier, of which no more need now be said.
From Natural Frontiers I pass to the category of Artificial Frontiers, by which are meant those boundary lines which, not being dependent upon natural features of the earth's surface for their selection, have been artificially or arbitrarily created by man. These may be classified as ancient and modern, the distinction between them—which is one of method only and not of principle—roughly reflecting the difference between the requirements of primitive and of civilized peoples. Primitive society, where not assisted by natural features in the determination of its limits of occupation or conquest, but being nevertheless desirous to protect its boundaries from external aggression, commonly either erected a barrier or created a gap. Under one or other of these headings will be found to fall all the Artificial Frontiers of the ancient and mediaeval world.
The commonest type of the barrier-frontier was a palisade or mound or rampart or wall; elsewhere use might be made of an existing road or canal or ditch. Of the latter class a familiar illustration is the great Roman road of Watling Street, in this country, which, by the Treaty between Alfred and Guthrum, was made the boundary between the English territories and the Danes. An early English example of the other type was Offa's Dyke, the huge earthwork constructed by the Mercian king of that name (about 780 A.D.) from the mouth of the Wye to the mouth of the Dee as a Frontier