[Sidenote:  Passes determine trans-montane roads.]

In plains and lowlands highways may run in any direction expediency suggests, but in mountain regions the pass points the road.  In very high ranges there is no appeal from this law; but in lower systems and especially in old mountains which have been rounded and worn down by ages of denudation, economic and social considerations occasionally transcend orographical conditions in fixing the path of highways.  Scarcely less important than pass or gap is the avenue of approach to the same.  This is furnished by lateral or transverse valleys of erosion.  The deeper their reentrant angles cut back into the heart of the highlands, the more they facilitate intercourse and lend historical importance to the pass route.  The Alpine passes which are approached by a single valley from each side are those crossed by railroads to-day,—­Mont Cenis, Simplon, St. Gotthard and the Brenner.  The Alpine chain is trenched on its inner or southern side by a series of transverse erosion valleys, such as the Dora Baltea, Sesia, Tosa, Ticino, Adda, Adige, and Tagliamento, which carry roads up to the chief Alpine passes.  The coincidence of the Roman and medieval roads over the Alps with the modern railroads is striking, except in the single point of elevation.  Railroads tend to follow lower levels.  Modern engineering skill enables them to tunnel the crest, to cut galleries in the perpendicular walls of gorges, and to embank mountain torrents against the spring inundation of the roadbed, where it drops to the valley floor.

[Sidenote:  Navigable river approaches to passes.]

Where gaps are low and the approaching waters are navigable, at least for the small craft of early days, they combine to enhance the historical importance of their routes.  The Mohawk River, navigable for the canoe of Indian and fur trader, greatly increased travel and traffic through the Mohawk depression.  The Pass of Belfort is the greatest historic gateway of western Europe, chiefly because it unites the channels of the Rhone, Saone and Rhine.  Lake Lucerne brings the modern tourist by boat to the foot of the railroad ascent to the St. Gotthard Pass, as the long gorge of Lake Maggiore receives him at the southern end.  Lake Maggiore is the water outlet also of the Simplon Pass from the upper Rhone, the Lukmanier (6288 feet or 1917 meters) from the Hither Rhine, and the San Bernadino (6766 feet or 2063 meters) from the Hinter Rhine.[1235] This geographical fact explains the motive of Swiss expansion in the fifteenth century in embracing the Italian province of Ticino and the upper end of Lake Maggiore.  A significance like that of the Swiss and Italian lakes for the Alpine passes appears emphasized in the Sogne Fiord of Norway.  This carries a marine highway a hundred miles into the land; from its head, roads ascend to the only two dents in the mountain wall south of the wide snowfield of the Jotun Fjeld, and they lead thence by the valleys of Hallingdal and Valders down to the plains of Christiania.