The immediate effect of the Roman conquest, which was far more rapid than in Britain, was to stop for a time the influx of German tribes by the establishment of a solid barrier along the Rhine. The colonists of German origin were soon absorbed by the old inhabitants of the country, and were subjected with them to the powerful influence of Roman culture. Celts and Germans alike became Belgo-Romans, and adopted the trade and the institutions of their conquerors. As far as we can make out from the scanty documents at our disposal, Roman civilization moved along the Rhine towards Cologne, whence a great Roman highway was built towards the West, crossing the Meuse at Maestricht and, following the edge of the Coal Wood, through Tongres and Cambrai to Boulogne. This road, known through the early Middle Ages as the "Road of Brunehaut," was for a long time the main way running from east to west in a country where all the important streams, such as the Meuse, the Scheldt and their tributaries, ran from south to north. The extent of Roman influence may be gauged by the position which the various parts of the country occupied towards this highway. Tongres and Tournai still possess Roman remains. The foundations of Roman villas are found in the provinces of Namur, Hainault and Artois, while all traces of Roman occupation have disappeared from Flanders. The sandy and marshy nature of the soil in Northern Belgium may to a certain extent account for this fact, and we know that, in some instances, the stones provided by old Roman structures were used, in the Middle Ages, for the construction of new buildings. But it can nevertheless be assumed that, generally speaking, communications remained the principal factor of Roman civilization in these far-away marches of the Empire, and that Roman influence, so strongly felt on the Rhine and along the Meuse, became gradually less important as the distance increased. The country was almost exclusively agricultural, but it is interesting to note, in view of later developments, that, even at this remote period, the Menapii, who dwelt in Flanders, had acquired a reputation for cattle breeding and manufactured woollen mantles which, under the name of "birri," were exported beyond the Alps. Though strongly influenced by Rome in their trade and methods of agriculture, the Belgo-Romans had retained their language and religion. Romanization, in the full meaning of the word, only began during the last years of the third century, under the influence of Christianity. During the third century, the bishopric of Treves included the whole of "Germania Inferior." A special bishopric was established subsequently at Cologne, and, about the middle of the fourth century, at Tongres. Others appeared later at Tournai, Arras and Cambrai. This gradual spread of Christianity, which moved along the same roads as Roman civilization, from Cologne towards the West, only reached Flanders half a century later.