Bar, spit, and barrier.—Wherever the shore upon which a beach is building makes a sudden landward turn at the entrance to a bay, the shore currents, by virtue of their inertia of motion, are unable longer to follow the shore. The débris which they carry is thus transported into deeper water in a direction corresponding to a continuation of the shore just before the point of turning (see [Fig. 259], [p. 238]). The result is the formation of a bar, which rises to near the water surface and is extended across the entrance to the bay through continued growth at its end, after the manner of constructing a railway embankment across a valley.
Fig. 263.—Spit of shingle on Au Train Island, Lake Superior (after Gilbert).
Over the deeper water near the bar the waves are at first not generally halted and broken, as they are upon the shore, and so the bar does not at once build itself to the surface, but remains an invisible bar to navigation. From its shoreward end, however, the waves of even moderate storms are broken, and the bar is there built above the water surface, where it appears as a narrow cape of sand or shingle which gradually thins in approaching its apex. This feature is the well-known spit ([Fig. 263]) which, as it grows across the entrance to the bay, becomes a barrier or barrier beach ([Fig. 264]).
The continuation of the visible in the usually invisible bar, is at the time of high winds made strikingly apparent, for the wave base is below the crest of the bar, and at such times its crescentic course beyond the spit can be followed by the eye in a white arc of broken water.
Fig. 264.—Barrier beach in front of a lagoon on Lake Mendota at Madison, Wisconsin. The shallow lagoon behind the barrier is filling up and is largely hidden in vegetation.
The construction of a barrier across the entrance to a bay transforms the latter into a separate body of water, a lagoon, within which silting up and peat formation usually lead to an early extinction ([p. 429]). The formation of barriers thus tends to straighten out the irregularities of coast lines, and opens the way to a natural enlargement of the land areas. While the coasts of the United Kingdom of Great Britain have been losing some four thousand acres through wave erosion, there has been a gain by growth in quiet lagoons which amounts to nearly seven times that amount. As evidence of the straightening of the shore line which results from this process, the coast of the Carolinas or of Nantucket ([Fig. 459]) may serve for illustration.
The land-tied island.—We have seen that wave erosion operates to separate small islands from the headlands, but the shore currents counteract this loss to the continents by throwing out barriers which join many separated islands to the mainland. Such land-tied islands are a common feature on many rocky coasts, and upon the New England coast they usually have been given the name of “neck.” The long arc of Lynn Beach joins the former island of Nahant, through its smaller neighbor Little Nahant, to the coast of Massachusetts. A similar land-tied island is Marblehead Neck. The Rock of Gibraltar, formerly an island, is now joined to Spain by the low beach known as the “neutral ground.” The Spanish name, tombola, has sometimes been employed to describe an island thus connected to the shore.