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. This boundary continued to be used into the Middle ages as the boundary between the bishoprics of Liège and Cambrai.
With the collapse of central Roman administration in the fourth century, Germanic Franks living along the Rhine border established kingdoms within the empire, and settled in less populated areas. The Salian Franks expanded their settlements from a starting point near Nijmegen until they pressed into the more populated and Romanized areas in the Silva Carbonaria and near the Maas. The Romanized population came to be known as *walh?z or "strangers" to the Germanic Franks--continued speaking a Late Latin, whose name survives in Walloon. In the past the Romance-Germanic linguistic division that marks Belgium to this day has been perhaps too facilely linked to these geographic parameters.
For a time in the sixth century, the Silva Carbonaria formed a barrier between the West Frankish kingdom of Clovis and the East Frankish kingdom of Sigebert the Lame, centred on Cologne, until he was defeated some time after 507, and Clovis joined the two kingdoms. 
Throughout the rule of the Merovingian dynasty, founded by Clovis, the Silva Carbonaria thus became the boundary between their two kingdoms of Austrasia and Neustria. The Silva Carbonaria is mentioned in the Salic Law of the Franks, where it marked "the boundary of the territories occupied by the Frankish people".
Extensive tracts of the untamed woodlands belonged to monasteries. The Benedictine Abbey of Lobbes lay in the Silva Carbonaria and that of Saint Foillan, in the Sonian Forest (Forêt de Soignes/Zoniënwoud) not far from Nivelles.
The charcoal--which gave the forest its name and into which the once seeming inexhaustible woods were slowly converted--was required to fuel the scattered smelting furnaces that forged the plentiful iron found in outcroppings laid bare by riverside erosion. Even before the Romans arrived, iron weapons forged in the Silva Carbonaria were traded by the Belgae to their cousins in the southeast of Britain. In the High Middle Ages further woodlands were cleared. Today the most significant remnant of the Silva Carbonaria is the Sonian Forest, preserved because it had been set aside as a noble hunt. At the start of the nineteenth century the area of this remnant of the primeval forest still covered about 100 square kilometres, but due to timber cutting its area has diminished to its current protected area of 44.21 km².