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 western Wallonia. The Silva Carbonaria was a vast forest that stretched from the rivers Zenne and the Dijle in the north to the Sambre in the south. Its northern outliers reached the then marshy site of modern Brussels.
Further to the southeast, the higher elevation and deep river valleys were covered by the even less penetrable ancient Arduenna Silva, the deeply folded Ardennes, which are still partly forested to this day. To the east, the forest was possibly considered to extend to the Rhine. It was there in Cologne in 388 CE that the magistri militum praesentalis Nannienus and Quintinus began a counter-attack against a Frankish incursion from across the Rhine, which was fought in the Silva Carbonaria.
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.
There are signs that the Silva Carbonaria represented the boundary between the Roman provinces of Gallia Belgica and Germania Inferior. In the Middle Ages, these provinces were still represented by the church dioceses of Reims and Cologne. On a smaller level, the forest served as a boundary between the Roman civitates of the Tungri to the east and the Nervii to the west.