. Elements of geology : a text-book for colleges and for the general reader. Geology. 28 AQUEOUS AGENCIES. illustration of the manner in which each mouth pushes its way into the sea. Its area is estimated at 12,300 square miles. The materials of which deltas are composed are usually the finest sands and clays, all the coarser materials having been deposited higher up the stream. Deltas are formed only in lakes and tideless or nearly tideless seas. In tidal seas, the sediments brought down by the rivers are swept away and carried to sea by the retreating tide; and instead of the land encroachin
. Elements of geology : a text-book for colleges and for the general reader. Geology. 28 AQUEOUS AGENCIES. illustration of the manner in which each mouth pushes its way into the sea. Its area is estimated at 12,300 square miles. The materials of which deltas are composed are usually the finest sands and clays, all the coarser materials having been deposited higher up the stream. Deltas are formed only in lakes and tideless or nearly tideless seas. In tidal seas, the sediments brought down by the rivers are swept away and carried to sea by the retreating tide; and instead of the land encroaching upon the domain of the sea by the formation of deltas, the sea encroaches upon the land by the erosive action of the tides, and forms bays or estuaries. Thus in tideless seas or lakes the rivers empty by many slender mouths, while in tidal seas they empty by wide bays: thus, for example, all the rivers emptying into the great Canadian lakes, and all the rivers emptying into the Gulf of Mexico, form deltas, while all the rivers emptying into the Atlantic in both North and South America form estuaries. In Europe all the rivers emptying into the Black, the Caspian, the Mediterranean, and the Baltic, form deltas, while those emptying into the Atlantic form estuaries. Process of Formation.—The process of formation of a delta may be best studied by observing it on a small scale, in the case of stream- lets running into ponds. In such cases we observe always a sand or mud flat at the mouth of the streamlet, evidently formed by the sand and clay brought down by the current. As soon as the current strikes the still water of the pond, its velocity is checked, and its burden of sediment is deposited. Through the sand-flat thus formed the stream- let ramifies, as seen in Fig. 20. The ramifica- tion seems to be the re- sult of the choking of the stream by its own deposit, which forces it to seek new channels. The sand-flat is gradu- ally extended farther and farther into the pond by suc
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectgeology, bookyear1892
