ALLUVIAL CONES AND FAN ft 



349 



early settlements, like Philadelphia, New York and Provi- 

 dence. 



161. Alluvial Cones and Fans. When a stream having 

 a steep grade and bearing a heavy load of . sediment 

 emerges upon a flat country where the grade is suddenly 

 reduced, it so quickly deposits its sediment as to be con- 

 tinually obstructing its own course and forcing itself to 



LAKE DELTA. 

 Notice the triangular formation. 



find new channels. It thus builds a fan or cone-shaped 

 deposit pointed toward the place where the stream reached 

 the plain. If the material is coarse, the deposit will have 

 a steep slope ; if fine, a gentle slope resembling a spread- 

 ing fan. Sometimes these fans so overlap each other 

 as to form an irregularly sloping plain. 



Plains of this kind are found along the base of many 

 mountain regions. If such formations occur in regions of 



