Vietnam or Thailand ? Vote for the TOP Country of the Week !
Updated: May 11, 2025
Soundings also, and the observations made in shallow waters by divers, tell something of its surface; but to learn more of its structures we must study those ancient sediments which have been lifted from the sea and dissected by subaerial agencies. From them we ascertain that sea deposits are stratified.
Nothing impresses the mind with the vast duration of time, according to our ideas of time, more forcibly than the conviction thus gained that subaerial agencies, which apparently have so little power, and which seem to work so slowly, have produced great results.
As the submarine delta grows near to the level of the sea the distributaries of the river cover it with subaerial deposits altogether similar to those of the flood plain, of which indeed the subaerial delta is the prolongation.
Besides the calcareous branches from the Cape of Good Hope, I have seen casts, of exactly the same forms, from Madeira* and from Bermuda; at this latter place, the surrounding calcareous rocks, judging from the specimens collected by Lieutenant Nelson, are likewise similar, as is their subaerial formation.
We may, I think, conclude that sediment must be accumulated in extremely thick, solid, or extensive masses, in order to withstand the incessant action of the waves, when first upraised and during subsequent oscillations of level, as well as the subsequent subaerial degradation.
Still the present escarpments so much resembling sea-cliffs have no doubt, for reasons above stated, derived their most characteristic features subsequently to emergence from subaerial waste by rain and rivers.
Croll, in 1867, and again, with more exactness, in 1868, deduced from the latest measurement of the sediment transported by European and American rivers the rate of subaerial denudation to which the surface of large continents is exposed, taking especially the hydrographical basin of the Mississippi as affording the best available measure of the average waste of the land.
At the rate of five feet per century a higher rate than obtains on the youthful rocky, coast of Britain it would require more than ten million years to pare a strip one hundred miles wide from the margin of a continent, a time sufficient, at the rate at which the Mississippi valley is now being worn away, for subaerial denudation to lower the lands of the globe to the level of the sea.
Scrope considers that the crater-cones of the chain of the Haute Loire give evidence of a somewhat earlier epoch of activity than those of the Puy de Dôme, as they have undergone a greater amount of subaerial erosion. The extent of this river erosion has been clearly brought out by Scrope, and is admirably illustrated by several of his panoramic views, such as that in Plate IX. of his work.
Both owe their origin to a sudden check in the velocity of the river, compelling a deposit of the load; both are triangular in outline, the apex pointing upstream; and both are traversed by distributaries which build up all parts in turn. In a delta we may distinguish deposits of two distinct kinds, the submarine and the subaerial.
Word Of The Day
Others Looking