140 RIVER DEPOSITS 



newer deposits. It should further be observed that the older 

 gravels do not actually overlie the newer ones, but are merely at 

 higher levels. 



Unsymmetrical terraces, which are either confined to one side 

 of the river, or if present on both sides, are on different levels, are 

 formed when a stream is widening its valley by steadily cutting 

 away the bank on one side, shifting the channel toward that side, 

 and at the same time deepening it. This will result in the forma- 

 tion of terraces representing the former positions of the stream. 

 If the lateral movement be all in one direction, the terraces will 

 all be on the side away from which the channel is shifting ; if it 

 be alternately in opposite directions, terraces will be formed on 

 both sides, but at different levels. 



Still a third method of terrace formation should be mentioned. 

 If a river which has excavated a deep valley, have its velocity 

 checked by a slow subsidence of the country, it will commence to 

 fill up its valley with gravel or other sediment, and may thus 

 accumulate material of great thickness and extent. Should a re- 

 elevation of the country now occur, the river will acquire new 

 destructive power and cut a terraced channel down through its 

 own deposits. In such a case the material is a continuous mass, 

 and the gravels of the higher terraces are newer (not older) than 

 those of the lower. The rivers Mersey and Irwell in England are 

 believed to be examples of this mode of terrace formation. 



Deltas are accumulations of river deposits at the mouths of 

 streams, land areas which the rivers have recovered from the body 

 of water into which they flow. The factors which determine the 

 formation of a delta are not altogether clear. The presence or 

 absence of a strong tide is evidently one of these factors, for in 

 lakes and in seas with little or no tide, almost all streams form 

 deltas, while those rivers which empty into the open ocean almost 

 invariably do so by means of estuaries, in which the sea encroaches 

 on the land. In North America the rivers which discharge into 

 the Gulf of Mexico form deltas, while the Atlantic streams nearly 

 all have estuaries. In Europe the delta-forming rivers empty into 

 the Mediterranean, the Baltic, and the Black and Caspian seas. 



