Under examination, even our familiar term coastline proves to be only an abstraction with no corresponding reality in nature.  Everywhere, whether on margin of lake or gulf, the actual phenomenon is a coast zone, alternately covered and abandoned by the waters, varying in width from a few inches to a few miles, according to the slope of the land, the range of the tide and the direction of the wind.  It has one breadth at the minimum or neap tide, but increases often two or three fold at spring tide, when the distance between ebb and flood is at its maximum.  At the mouth of Cook’s Inlet on the southern Alaskan coast, where the range of tides is only eight feet, the zone is comparatively narrow, but widens rapidly towards the head of the inlet, where the tide rises twenty-three feet above the ebb line, and even to sixty-five feet under the influence of a heavy southwest storm.  On flat coasts we are familiar with the wide frontier of salt marshes, that witness the border warfare of land and sea, alternate invasion and retreat.  In low-shored estuaries like those of northern Brittany and northwestern Alaska, this amphibian girdle of the land expands to a width of four miles, while on precipitous coasts of tideless sea basins it contracts to a few inches.  Hence this boundary zone changes with every impulse of the mobile sea and with every varying configuration of the shore.  Movement and external conditions are the factors in its creation.  They make something that is only partially akin to the two contiguous forms.  Here on their outer margins land and ocean compromise their physical differences, and this by a law which runs through animate and inanimate nature.  Wherever one body moves in constant contact with another, it is subjected to modifying influences which differentiate its periphery from its interior, lend it a transitional character, make of it a penumbra between light and shadow.  The modifying process goes on persistently with varying force, and creates a shifting, changing border zone which, from its nature, cannot be delimited.  For convenience’ sake, we adopt the abstraction of a boundary line; but the reality behind this abstraction is the important thing in anthropo-geography.

[Sidenote:  Gradations in the boundary zone.]

All so-called boundary lines with which geography has to do have this same character,—­coastlines, river margins, ice or snow lines, limits of vegetation, boundaries of races or religions or civilizations, frontiers of states.  They are all the same, stamped by the eternal flux of nature.  Beyond the solid ice-pack which surrounds the North Pole is a wide girdle of almost unbroken drift ice, and beyond this is an irregular concentric zone of scattered icebergs which varies in breadth with season, wind and local current; a persistent decrease in continuity from solid pack to open sea.  The line of perpetual snow on high mountains advances or retreats from season to season, from year to year; it drops low on chilly northern slopes and recedes to higher altitudes on a southern exposure; sends down long icy tongues in dark gorges, and leaves outlying patches of old snow in shaded spots or beneath a covering of rock waste far below the margin of the snow fields.