valleys were their chief battleground; elsewhere the broad Appalachian barrier held them apart.  Again in the Revolution, control of the Mohawk-Hudson route was the objective of the British armies mobilized on the Canadian frontier, because it alone would enable them to co-operate with the British fleet blockading the coast cities of the colonies.  In the War of 1812, it was along this natural transmontane highway that supplies were forwarded to the remote frontier, to support Perry’s fight for control of the Great Lakes.  The war demonstrated the strategic necessity of a protected, wholly American line of water communication between the Hudson and our western frontier, while the commercial and political advantage was obvious.  Hence a decade after the conclusion of the war, this depression was traced by the Erie Canal, through which passed long lines of boats to build up the commercial greatness of New York City.

[Sidenote:  Height in mountain barriers.]

Other structural features being the same, mountains are barriers also in proportion to their height; for, with few exceptions, the various anthropo-geographic effects of upheaved areas are intensified with increase of elevation.  Old, worn-down mountains, like the Appalachians and the Ural, broad as they are, have been less effective obstacles than the towering crests of the Alps and Caucasus.  The form of the elevation also counts.  Easy slopes and flat or rounded summits make readier transit regions than high, thin ridges with escarpment-like flanks.  Mountains of plateau form, though reaching a great altitude, may be relatively hospitable to the historical movement and even have a regular nomadic population in summer.  The central and western Tian Shan system is in reality a broad, high plateau, divided into a series of smoothly floored basins and gently rolling ridges lying at an elevation of 10,000 to 12,000 feet above the sea.  Its pamirs or plains of thick grass, nourished by the relatively heavy precipitation of this high altitude, and forming in summer an island of verdure in the surrounding sea of sun-scorched waste, attract the pastoral nomads from all the bordering steppes and deserts.[1228] Thus it is a meeting place for a seasonal population, sparse and evanescent, but its uplifted mass holds asunder the few sedentary peoples fringing its piedmont.  The corrugated dome of the Pamir highland, whose valley floors lie at an elevation of 11,000 to 13,000 feet, draws to its summer pastures Kirghis shepherds from north, east and west; and their flocks in turn attract the raids of the marauding mountaineers occupying the Hunza Valley to the south.  The Pamir, high but accessible, was a passway in the tenth century for Chinese caravans bound from “Serica” or the “Land of Silk” to the Oxus River and the Caspian.  Here Marco Polo and many travelers after him found fodder for their pack animals and food for themselves, because they could always purchase meat from the visiting shepherds.  The possibilities of the Pamir as a transit region are apparent to Russia, who in 1886 annexed most of it to the government of Bukhara.