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 successive deposits, 

 as shown in Fig. 20. Fig. 21 shows the irregular stratified appearance 

 of the deposits as seen on cross-section. In all such cases of streams 

 flowing into ponds or lakes, the stream flows in at a muddy, but flows 

 out at b comparatively clear, having deposited much of its sediment in 

 the pond or lake. Evidently if this process continues without interrup- 

 tion, the pond will eventually be filled up, after which, of course, the 

 sediment will be carried farther down the stream. In this manner small 

 mountain-lakes are often entirely filled up. The Rhone flows into Lake 

 Geneva a turbid stream, but flows out beautifully transparent. The 

 whole of its sediment is deposited where it enters the lake, and it has 

 there formed a delta six miles long. We may confidently look forward 



