Longitudinal valleys.
Longitudinal valleys, by reason of their length and their branching lateral valleys, are the natural avenues of communication within the mountains themselves. They therefore give a dominant direction to such phases of the historical movement as succeed in passing the outer barrier. The series of parallel ranges which strike off from the eastern end of the Tibetan plateau southward into Farther India have directed along their valleys the main streams of Mongolian migration and expansion, heading them toward the river basins of Burma and Indo China, and away from India itself.[1217] While Tibetan elements have during the ages slowly welled over the high Himalayan brim and trickled down toward the Gangetic plain, Burma has been deluged by floods of Mongolians pouring down the runnels of the land. A carriage road follows the axis of the Central Alps from Lake Geneva to Lake Constance by means of the upper Rhone, Andermatt, and upper Rhine valleys, linked by the Furca and Oberalp passes. The Roman and Medieval routes northward across the Central Alps struck the upper Rhine Valley above Coire, (the ancient Curia Rhaetorum); this natural groove gave them a northeastward direction, and made them emerge from the mountains directly south of Ulm, which thereby gained great importance. The trade routes from Damascus and Palmyra which once entered the Orontes-Leontes trough in the Lebanon system found their Mediterranean termini south near Tyre or north near Antioch, and thus contributed to the greatness of those ancient emporiums. The Great Appalachian Valley used to be a highway for the Iroquois Indians, when they took the warpath against the Cherokee tribes of Tennessee. Later it gave a distinct southwestward trend to pioneer movements of population within the mountains, blending in its common channel the Quakers, Germans and Scotch-Irish from Pennsylvania, with the English and Huguenot French of the more southern colonies. In the Civil War its fertile fields were swept by marching armies, all the way from Chattanooga to Gettysburg.
Passes in mountain barriers.
The barrier nature of mountains depends upon their height and structure, whether they are massive, unbroken walls like the Scandinavian Alps and the Great Smoky range; or, like the Welsh Highlands and the Blue Ridge, are studded with low passes. The Pyrenees, Caucasus and Andes, owing to the scarcity and great height of their passes, have always been serious barriers. The Pyrenees divide Spain from France more sharply than the Alps divide Italy from France; owing to their rampart character, they form the best and most definite natural boundary in Europe.[1218] Epirus and Aetolia, fenced in by the solid Pindus range, took little part in the common life of ancient Greece; but the intermittent chains of Thessaly offered a passway between Macedon and Hellas. The Alps have an astonishing number of excellent passes, evenly distributed for the most part. These, in conjunction with the great longitudinal valleys of the system, offer transit routes from side to side in any direction. The Appalachian system is some three hundred miles broad and thirteen hundred miles long, but it has many easy gaps among its parallel ranges, so that it offered natural though circuitous highways to the early winners of the West. The long line (400 miles) of the Hindu Kush range, high as it is, forms no strong natural boundary to India, because it is riddled with passes at altitudes from 12,500 to 19,000 feet.[1219] The easternmost group of these passes lead down to Kashmir, and therefore lend this state peculiar importance as guardian of these northern entrances to India.[1220] The Suleiman Mountains along the Indo-Afghan frontier are an imperfect defence for the same reason. They are indented by 289 passes capable of being traversed by camels. The mountain border of Baluchistan contains 75 more, the most important of which focus their roads upon Kandahar. Hence the importance to British India of Kandahar and Afghanistan. Across this broken northwest barrier have come almost all the floods of invasion and immigration that have contributed their varied elements to the mixed population of India. Tradition, epic and history tell of Asiatic highlanders ever sweeping down into the warm valley of the Indus through these passes; Scythians, Aryans, Greeks, Assyrians, Medes, Persians, Turks, Tartars, and Mongols have all traveled these rocky roads, to rest in the enervating valleys of the peninsula.[1221]
Breadth of mountain barriers.
Mountains folded into a succession of parallel ranges are greater obstructions than a single range like the Erz, Black Forest, and Vosges, or a narrow, compact system like the Western Alps, which can be crossed by a single pass. Owing to this simple structure the Western Alps were traversed by four established routes in the days of the Roman Empire. These were: I. The Via Aurelia between the Maritime Alps and the sea, where now runs the Cornice Road. II. The Mons Matrona (Mont Genevre Pass, 6080 feet or 1854 meters [Transcriber's Note: printer's error incorrectly printed as kilometers]) between the headstream of the Dora Riparia and that of the Durance, which was the best highway for armies. III. The Little St. Bernard (7075 feet or 2157 meters), from Aosta on the Dora Baltea over to the Isère and down to Lugdunum (Lyons). IV. The Great St. Bernard (8109 feet or 2472 meters) route, which led northward from Aosta over the Pennine Alps to Octodurus at the elbow of the upper Rhone, where Martigny now stands. Across the broad double rampart of the Central Alps the Roman used chiefly the Brenner route, which by a low saddle unites the deep reëntrant valleys of the Adige and Inn rivers, and thus surmounts the barrier by a single pass. However, a short cut northward over the Chalk Alps by the Fern Pass made closer connection with Augusta Vindelicorum (Augsburg). The Romans seem to have been ignorant of the St. Gotthard, which, though high, is the summit of an unbroken ascent from Lake Maggiore up the valley of the Ticino on one side, and from Lake Lucerne up the Reuss on the other.
Mountains which spread out on a broad base in a series of parallel chains, and through which no long transverse valleys offer ready transit, form serious barriers to every phase of intercourse. The lofty boundary wall of the Pyrenees, a folded mountain system of sharp ranges and difficult passes, has successfully separated Spain from continental Europe; it has given the Iberian Peninsula, in the course of a long history, closer relations with Morocco than with its land neighbor France. It thus justifies the French saying that "Africa begins at the Pyrenees." The Andalusian fold mountains stretching across southern Spain in a double wall from Trafalgar to Cape Nao, accessible only by narrow and easily defended passes, enabled the Moors of Granada to hold their own for centuries against the Spaniard Christians. The high thin ridges of the folded Jura system, poor in soil and sparsely populated, broken by occasional "cluses" or narrow water-gaps admitting the rivers from one elevated longitudinal valley to another, have always been a serious hindrance to traffic.[1222]