The width of the coast zone is not only prevented from contracting by dredging and canaling, but it is even increased.  By deepening the channel, the chief port of the St. Lawrence River has been removed from Quebec 180 miles upstream to Montreal, and that of the Clyde from Port Glasgow 16 miles to Glasgow itself, so that now the largest ocean steamers come to dock where fifty years ago children waded across the stream at ebb tide.  Such artificial modifications, however, are rare, for they are made only where peculiarly rich resources or superior lines of communication with the hinterland justify the expenditures; but they find their logical conclusion in still farther extensions of sea navigation into the interior by means of ship canals, where previously no waterway existed.  Instances are found in the Manchester ship canal and the Welland, which, by means of the St. Lawrence and the Great Lakes, makes Chicago accessible to ocean vessels.  Though man distinguishes between sea and inland navigation in his definitions, in his practice he is bound by no formula and recognizes no fundamental difference where rivers, lakes, and canals are deep enough to admit his sea-going craft.

Such deep landward protrusions of the head of marine navigation at certain favored points, as opposed to its recent coastward trend in most inlets and rivers, increase the irregularity of the inner edge of the coast zone by the marked discrepancy between its maximum and minimum width.  They are limited, however, to a few highly civilized countries, and to a few points in those countries.  But their presence testifies to the fact that the evolution of the coast zone with the development of civilization shows the persistent importance of this inner edge.

[Sidenote:  Outer edge in original settlement.]

The outer edge finds its greatest significance, which is for the most part ephemeral, in the earlier stages of navigation, maritime colonization, and in some cases of original settlement.  But this importance persists only on steep coasts furnishing little or no level ground for cultivation and barred from interior hunting or grazing land; on many coral and volcanic islands of the Pacific Ocean whose outer rim has the most fertile soil and furnishes the most abundant growth of coco palms, and whose limited area only half suffices to support the population; and in polar and sub-polar districts, where harsh climatic conditions set a low limit to economic development.  In all these regions the sea must provide most of the food of the inhabitants, who can therefore never lose contact with its waters.  In mountainous Tierra del Fuego, whose impenetrably forested slopes rise directly from the sea, with only here and there a scanty stretch of stony beach, the natives of the southern and western coasts keep close to the shore.  The straits and channels yield them all their food, and are the highways for all their restless, hungry wanderings.[420] The steep slopes