FRESH-WATER STREAMS. 



659 



Alluvial Fans. When a flooded stream descends along a steep ravine, the detritus 

 carried down is piled up at the foot of the slope over the plain, making a section of a 

 verv low cone, usually 3° or less to 8° or 10°, called by Drew, from their shape, Alluvial 

 fans. The streams producing these "fans" are small ones, having more transporting 

 than denuding power. The material is bedded, but concentrically, or parallel with the 

 curved surface. When such "fans" are afterwards cut through by the little stream, 

 and then partly worn away by the floods of the river in the valley which they border, 

 and then formed anew at an outer and -lower level, and so on, the bedding becomes 

 quite complex in its directions and abrupt transitions; and there are parts of successive 

 ''fans " at different levels. (Q. Jour. Geol. Soc, xxix., 441, 1873.) 



3. Delta Formations. — The larger part of the detritus of a river is 

 carried to the ocean (or lake) into which it empties ; and it goes to 

 form, about the mouth of the stream, more or less extensive flats. 

 Such flats, when large and intersected by a network of water-channels, 

 are called deltas ; they reach a large size only where the tides are 

 quite small or are altogether wanting. The greatest river in China, 



