The Appalachian mountains separated the Ohio-Mississippi hinterland from the Atlantic seaboard, and overcoming this separation was the task of early trails (p.3). The efficient sea communication on the Atlantic shore had the curious effect of providing "a stretch of ocean ... over which goods could be transported at less expense than they could be carried overland for distances which to-day seem inconsiderable." (p.3)
... the colonies throughout the colonial period were commercially closer to Europe than they were to one another. (p.3)Trade routes into the interior were still necessary, however, as the "seaboard communities could not themselves produce the commodities required by this traffic".
Prior to the Revolution intercontinental commerce was inconsiderable, and intercolonial trade-routes, where they existed, were entirely inadequate. (p.4)The coastal plain was well opened up by rivers, whose branches led into the valleys and gaps of the mountains.
A study of the spread of settlement up to 1775 shows that the immigrant population ran up the river valleys as far as the fall-line, and there generally stopped. (p.4)After the Revolution, the twin patterns of the westward migration and the eastward flow of products made the question of how to traverse the Appalachian system "a dominant issue in national politics, involving the economic welfare and growth of every section of the country" (p.4)
The parallel ranges of the Appalachian, a system 300 miles wide and 1300 miles long, reached from the Green Mountains in Vermont, to the pine-hills of Alabama. The rivers could not be navigated up to the mountains, but the headwaters revealed good passes, though in the South these paths ended up being somewhat roundabout.
In Pennsylvania the chief route to the Ohio followed the West Branch of the Susquehanna. Farther south was another route by the Juniata to a tributary of the Allegheny. (p.4)
Balthasar Henry Meyer, History of Transportation in the United States before 1860, Washington (Carnegie Institute of Washington) 1917.