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.
Use as a border
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), and that this boundary continued to be used in 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 *walhōz 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.
- Or Carbonarius saltus, "the charcoal ravine or wildwood" — in the sense of "unfit for the plough" (Hoffmann 1698, s.v. "Carbonarius saltus"); the lexicographer Hoffmann found Carbonaria silva mentioned by Gregory of Tours, the twelfth-century chronicler Sigebert of Gembloux, and Johannes Trithemius.
- F. L. Ganshof, "Manorial Organization in the Low Countries in the Seventh, Eighth and Ninth Centuries" Transactions of the Royal Historical Society, 4th Series 31 (1949:29-59) p. 30.
- André De Vries, Brussels: A Cultural and Literary History, 2003:18.
- A. H. M. Jones, John Robert Martindale, J. Morris, eds. The Prosopography of the Later Roman Empire, 1971 s.v. Quintinus"
- According to Sulpicius Alexander, quoted in Gregory of Tours, History of the Franks ii.9: multos Francorum, qui Rhenum transierant, a Romanis apud Carbonariam ferrô peremptos tradit, quoted by Hoffmann 1698.
- Ulrich Nonn, Pagus und Comitatus, pp.226-234 and map.
- Van Durme 2002:11.
- The confused legendary origins of the chausée Brunehaut were unraveled and examined by J. Lestoquoy, "L'étrange histoire de la Chaussée Brunehaut", in Arras au temps jadis1946; see "Presentation of Brunehaut and its villages".
- Émile Cammaerts, A History of Belgium from the Roman Invasion to the Present Day, 1921: 34, in general following conclusions based on toponymy by the historian Godefroid Kurth.
- The historiography of this idea is traced by Luc van Durme in "Genesis and evolution of the Romance-Germanic language border in Europe", in Jeanine Treffers-Daller and Roland Willemyns, eds. Language Contact at the Romance-Germanic Language Border, 2002:39ff.
- "At that stage, the silva Carbonaria separated Clovis's Salian kingdom from Sigebert's eastern domain." (The Cambridge Ancient History, eo.loc.); "The Silva Carbonaria formed for a time a natural barrier between Salians and Ripuarians, though it may not have proved very effective," (John Michael Wallace-Hadrill, The Barbarian West, 400-1000 1996:70); in the chronicle of the Monastery of Saint Arnulf, under the year 690: adunatô exercitu Peppinus ad Carbonariam silvam pervenit: qui terminus utraque Regna dividit.
- Title 47 of Lex Salica specifies that the interested parties in a contested ownership meet within forty days if they live within the bounds of the Silva Carbonaria and the Loire; otherwise eighty days must be allowed. (T.M. Charles-Edwards, in Iorwerth Eiddon and Stephen Edwards, eds. The Cambridge Ancient History, vol. XV [Cambridge University Press] 2005:273.
- As Gregory of Tours noted, Rhenum transierunt, pluribus suorum in Romano relictis solo, ad repetendam depopulationem paratis, cum quibus congressua Romanis adcomodus fuit, multis Francorum apud Carbonariam ferro perimptis. (Historia Francorum ii.9.).
- Hoffmann 1698, Laubiense Monasterium in Silva Carbonaria esse situm, auctore Fulcuinô; esse et Coenobium S. Foillani in silva Soniaca parte Carbonariae non longe a Niviala:
- There are seven other forests in Belgium that are also remains of the Silva Carbonaria
- De Vries 2003:13; Hofmann, in the late seventeenth century, noted this remnant in writings of Gotefridus Wendelinus and also remarked on remnants in the Forêt de Mormaux or Mormal, the Bois de Cirau, and the woodland called Die Leu that stretched from Leuven to the gates of Diest, the forest-covered Hageland or Hagelanden.
- Hofmann, Johann Jacob. Lexicon Universale, Historiam Sacram Et Profanam Omnis aevi... (Leiden) 1698. on-line facsimile text on-line transcript.
- Duvivier, Charles, "La forêt charbonnière: Silva Carbonaria", in Revue d'histoire et d'archéologie 3 (1862:1-26).
- Freiherren von Richthofen, "Review of "Der lex Salica und der lex Anglorum et Werinorum Alter und Heimat, von Hermann Müller, ordentlichem Professor der Rechte zu Würzburg" Würzburg, 1840", Kritische Jahrbücher für deutsche Rechtswissenschaft, 5, p. 1000 (includes list of early references to the Silva Carbonaria)
- Vander Linden (1923), La Forêt Charbonnière (PDF)
- "Carbonaria Silva", Reallexikon der Germanischen Altertumskunde, 4, 1981
- Van Durme (2010) Genesis and Evolution of the Romance-Germanic Language Border in Europe
- Mentioned as a boundary in the Lex Salica. Various versions can be compared here and here on the dMGH.de (Monumenta Germaniae Historica) collection.
- The Liber Historiae Francorum repeats the story found in Gregory of Chlodio going through the forest to take Tournai. Latin is here on the dMGH.de collection.