Silva Carbonaria

Silva Carbonaria, the "charcoal forest", was the dense old-growth forest of beech and oak that formed a natural boundary during the Late Iron Age through Roman times into the Early Middle Ages across what is now Belgium. The forest naturally thinned out in the open sandy stretches to the north and formed a barrier—trackless to the outsider—on the heavier soils to the south. Yet further to the south, the higher elevation and deep river valleys were covered by the even less penetrable ancient Arduenna Silva, the deeply folded Ardennes, which are still forested to this day. The Silva Carbonaria was a vast forest that stretched from the rivers Senne and the Dijle in the north to the Sambre in the south. To the east Silva Carbonaria extended to the Rhine, where near Cologne in 388 CE the magistri militum praesentalis Nannienus and Quintinus counter-attacked a Frankish incursion across the Rhine in the Silva Carbonaria. Its northern outliers reached the then marshy site of modern Brussels.

A great Roman road forming a "strategic axis" linked the Rhine crossing at Cologne with Maastricht, where it crossed the Maas at the head of navigation. Skirting the northern edges of the Silva Carbonaria, it passed through Tongeren, Kortrijk and Cambrai to reach the sea at Boulogne. The highway was the main east–west route in a landscape where the river valleys, tributaries of the Meuse and the Scheldt, tended southwest to northeast. It remained viable through the Early Middle Ages as the chaussée Brunehaut, the "Road of Brunehaut". As a public work its scale had become unimaginable in the Middle Ages: the chronicler Jean d'Outremeuse solemnly related in 1398 that Brunehaut, wife of Sigebert I, had built this wide paved road in 526, and that it was completed in a single night with the devil's aid.

...
Wikipedia