Fig. 282.—Archipelago of steep rocky islets due to large submergence of a coast having strong relief. Entrance to Esquimalt Harbor, Vancouver Island (after a photograph by Fairbanks).
Fig. 283.—The submerged Hudsonian channel which continues the Hudson River across the continental shelf.
Submerged river channels.—The sinking of a coast of small relief be sufficient to completely submerge river valleys, whose channels then begin to fill with sediment and whose courses can only be followed in soundings. One of the most interesting of such channels is that which continues the Hudson River across the continental shelf into the deeper sea ([Fig. 283]).
Records of an oscillation of movement.—Because a coast is deeply embayed is no ground for assuming that a subsidence is now in progress, or is, in fact, the latest movement recorded upon the coast. In many cases it is easy to see that such is not the case. The coast of Maine is perhaps as typical of an embayed shore line as any that might be cited, but a study of the river valleys in the neighborhood shows clearly that the present submergence of their mouths is a fraction only of an earlier one which has left a record of its existence in beds of marine clay which outline the earlier and far deeper indentations ([Fig. 284]).
Fig. 284.—Marine clay deposits near the mouths of the rivers of Maine which preserve a record of earlier subsidence (after Stone).
If now we give a closer examination to the coast, it is found that there are marks of recent uplift in an abandoned shore line now far above the reach of the waves. There is here, then, the record, first of subsidence and consequent embayment, and, later, of an uplift which has reduced the raggedness of the coast outline exposed the clay deposits, and raised the strands of the period of deep subsidence to their present position.
In countries which possess a more ancient civilization than our own, the record of such oscillations in the level of the ground has sometimes been entered upon human monuments, so that it is possible to date more or less definitely the periods of subsidence or elevation. At the little town of Pozzuoli, upon the shore of the Bay of Naples, is found one of the mos instructive of these records.
