In early medieval times it was seen as a major obstacle in land travel between the regions, and so the Baltic Sea was largely used instead. Together with the extensive forests of Tiveden and Tyl�skog to its west, Kolm�rden formed the crucial and wartorn frontier between Svealand to the north and G�taland to the south during the formation of the Swedish nation in the 13th century.
In the Icelandic text S�gubrot af nokkrum forn konungum, Kolm�rden between Svealand and �sterg�tland is described as the border between Sweden and �sterg�tland (Kolmerkr, er skilr Sv��j�� ok Eystra-Gautland). It also appears as Mirkwood in some editions of S�gubrot, in the legend of Helgi Hundingsbane and probably also inV�lundarkvi�a.
Today the forest is traversed by the Norrk�ping�Nyk�ping railway, the Norrk�ping�Katrineholm railway, and the E4 highway, all connecting southern Sweden with Stockholm and the M�laren region. Along the former there is also a Kolm�rden railway station, at the village of Krokek by Br�viken
