ALLUVIAL DEPOSITS 



117 



Uvme a nearly dry basin, the lowest part of which is about 300 

 i'<vt In-low sea -k- vol. The Skagit River, in Washington, has built 

 its (Klta out so as to surround what were high islands in Puget 

 Sound, thus joining them to the mainland. The deltas of the 

 Miissippi (Fig. 118), the Nile and the Hoang-Ho Rivers are 



Fig. 121. Diagrammatic profile and section of a delta. 



well-known. The united delta of the Ganges and Brahmaputra 

 is also a great one, having an area (above water) of some 50,000 

 square miles. The Po has built a delta 14 miles beyond the former 

 port of Adria, which gave its name to the Adriatic Sea. The Rhone 

 Rivt-r (France) has advanced its delta some 15 miles' in as many 

 centuries. 



The effect of delta-building is to increase the area of the 

 land; but it is to be noted that the processes which lead to delta- 

 building reduce the volume of the land-masses, even though they 

 increase their area. 



The outline of some deltas is determined by the surroundings 

 in which they are built. When, for example, a delta is built into 

 a bay, the form of the bay-head determines the shape of the delta. 



Fig. 122. Diagrammatic profile and section of an alluvial fan. 



The normal form of a delta built on an open coast is somewhat 

 semicircular, though there is in many cases a fringe of delta fin- 

 gers which together have some resemblance to the Greek letter A. 

 which gave these terminal deposits of streams their names. 



