The line of contact between French and the languages belonging to the Germanic group begins at the sea on French soil. Starting a few miles west of Dunkirk,[8] the linguistic divide follows a direction which is generally parallel to the political boundary between France and Belgium until, a few miles east of Aire, it strikes northeast to Halluin, which remains within the area of French speech. From this point on to Sicken-Sussen, near the German border, the line assumes an almost due east trend. This division corresponds broadly to the mountainous and depressed areas into which Belgium is divided. The upland has always been the home of French. Walloon is but a modified form of the old langue d’oïl.[9] Flemish, on the other hand, is a Germanic language which spread over Belgian lowlands as naturally as the Low German dialects to which it is related had invaded the plains of northern Europe. This east-west line also marks the separation of the tall, blond, long-skulled Flemings from the short, dark, round-skull Alpine Walloons. The remarkably straight course of the linguistic divide, in Belgian territory, is generally regarded as an effect of the plain over which it extends. Whatever ruggedness it may have once possessed has been smoothed away in the course of centuries by the ease with which either Flemish or French could spread in the low-lying flatland. The two languages have now been facing each other for about four centuries. Place names indicate that the variations of the line have been slight. It is a rare occurrence to find Roman village names north of its present extension. Teutonic roots, in locality names to the south, are likewise unusual. A few can be traced. Waterloo, Tubize, Clabecq, Ohain were once Flemish settlements. Tubize was originally known as Tweebeek and became a Walloon center in the fifteenth century. Ohain likewise is known in the form of Olhem in twelfth century documents.