Page:Popular Science Monthly Volume 43.djvu/264

252 Lining the shores of the head waters of the bay and spreading far inland up the valleys of its river tributaries are extensive tracts of alluvial marsh land of remarkable fertility, and differing in their origin from other so-called marshes. In general, alluvial deposits are formed in river basins, by materials washed down from higher levels by fresh-water floods; but here the whole deposit is of tidal origin, the result of a landward rather than a seaward transportation. Every incoming tide is freighted with a finely comminuted sediment, the product of the wearing action of the currents upon the sides and bottom of the bay. During the interval between the flood that covers the undiked river and basin margins and the ebb that leaves them bare again, the sediment is deposited as a film of soft and glistening mud upon the somewhat hardened material left by previous tides. Thus layer after layer accumulates, until the flat becomes too high for any but extraordinary tides to cover.

Instructive illustration these marsh flats often give of Nature's methods in the preservation of the records by which the geologist reads the physical history of the earth. So plastic and impressionable is the mud which an outgoing tide has left that it easily takes and holds the tracings of any disturbing contact. A wind-blown leaf, a resting insect, a drop of rain, may make in it a tiny mold which, hardened somewhat before the next incoming flood, receives thereafter successive linings to which it gives its form and markings. In this way even the rain-prints of a passing shower have been fixed, and then completely covered up; and yet when subsequently exposed, so perfectly were the spatter marks preserved, that one could tell in which direction the wind was blowing when the shower fell.

It is obvious that the deposition of tidal sediment can, in general, be made only between the lower and the higher limit levels of the ebb and flow. The accumulation of greater depths of mud than such a range permits can only be accounted for by the supposition of a gradual subsidence of the littoral areas—a movement which would also widen the area of tidal inundations. That such a steady and prolonged subsidence of the Fundy marsh-lined shores has been in progress since the marsh began to form is attested to not only by the surprising depths of mud accumulated, but also by the occurrence in many places, especially along the shores of the Cumberland Basin, of deeply buried forests which were clearly once above the coexistent tidal levels.

A general idea of the geological features of the great depression in which the Bay of Fundy lies is necessary to a fuller understanding of the nature of these Acadian marshes, and especially of the sources of their wonderful fertility. In early geological times, and until long after the close of the