We have seen that the winds and the currents carry the detritus of the coasts reduced to sand to great distances. The currents however do not arrest the motion of the waves and the ground swell; they bend to their forms, and as their direction necessarily tends to the shore, the sands clear the currents with the ground swell which contains them, and which thus conducts them to the shore. When the direction of the waves is oblique to the coast, the sands are borne to a distance, but when it is perpendicular to the coast, the waves raise the sands brought by the ground swell into dykes and banks, which protect the low shores. The most minute and lightest portions are accumulated at the more elevated points of the flats, where, being dried by the sun, they are soon carried away by the wind, which leaves them, in its turn, in the shape of downs. The ground swell, therefore, furnishes the materials of those downs, which usually border flat shores, and it is that which has drawn from the depths of the sea the sands of those immense deserts of Africa, and of so many other plains which are found in various parts of the globe.
“Often,” says M. Jomard, “have I remained for whole hours, contemplating the origin and progress of the phenomenon of the formation of sands. I saw the waves break and deposit a small line, scarcely discernible, of very fine sand. Another wave came, burdened like the first, and this new line of sand pushed the first slightly on. This, once beyond the reach of the water, and exposed to the rays of a burning sun, was quickly dried, and became the prey of the wind, which immediately seized and carried it off into the air; the less light particles of gravel did not reach so far, but, subjected to the alternate motion, diminished more and more, and were converted by degrees into sand.”
We may also say with Colonel Emy, that
“All river bars are deposits, brought or arrested by the ground swell, and without it these deposits would be repelled into the main as far as the rivers extend their course. The Delta of the Nile, those of the Mississippi, of the Ganges, of the Scheld, of the Meuse, of the Rhine, and the Camargue of the Rhone, were originally bars formed by this same ground swell.”
The tongues of sand which separate the lake of Thau from the gulf of the Lion, the tongue of earth upon which Alexandria is built, those which separate the lakes Bourlos and Menzaleh from the Mediterranean, are bars of sand formed by the ground swell. The sand bank which separates from the Red Sea the vast basin of the Bitter Lakes, was, without any doubt, a ford elevated by the ground swell, which, in tempestuous weather, ascends this sea with the current of the tides charged with sand. The ford, which answers at the present time at Suez, was certainly formed in this manner by the ground swell.
We may say also, that the whole Isthmus of Suez was formed by the maritime deposits of the Mediterranean and of the Red Sea. We believe that, previous to historic times, the two Seas were in communication with each other, that the detritus of the chains of mountains situated to the right and left, carried down by rain, filled up the space which separates them, and that when that space was elevated to such a height that the ground swell could reach it, its action was applied in such a way that by the meeting of the swell of the two Seas, a bank was formed, which is no other than the bar of El Guisr. After the formation of this bank, the combined action of the ground swell, both on one side and the other, and the accretions from the neighbouring mountains continued until the Isthmus was dry. Then the soil thus constituted was covered by the downs, which advanced upon it from the direction of Pelusium, driven by the north winds, and from the direction of Suez, driven by the winds and currents from the south.
In this state the Isthmus is at present, and the numerous soundings which we have asked for from His Highness the Viceroy, will prove whether our hypothesis is well founded or not.
The same theory may, as Colonel Emy observes in his remarkable work, throw a new light on important geological facts:—
“For instance, those ancient and elevated plains, composed of sand and pebbles, the formation of which, it has been attempted to explain, by the revolutions of the globe and violent convulsions of nature, or which have been regarded as deposits left by rivers, appear to be maritime accretions. If is, indeed, easy to conceive rivers capable of bringing down the fragments detached from mountains, by shocks, and by the decomposition of the rocks; but how could they extend those fragments uniformly, and over spaces so extensive as the plains in question? Moreover, was not the course from the summits of the mountains generally too short for the fragments of the excessively hard rock found in some of those plains, to have time to acquire their roundness? The rivers have prolonged their courses through these accumulations of pebbles; they may, in overflowing, have covered them with sand and earth, but it is more probable that they contributed in nowise to the formation of these accretions, unless it were by transporting the rough materials to the sea. Nothing but the ground swell could spread these fragments of mountains so uniformly as they are, convert them into shingle and sand by a long trituration on the shores, where it had driven them; gather them either into banks or plains, and thus fill up spaces over which the sea formerly extended.
“The ancient collections of shingle, pebbles and sand are owing, like those at present forming in a similar manner, to maritime accretions, and must henceforth be regarded as an incontestable proof that the ocean formerly reached and was long stationary at different heights far exceeding its present level.”