B 
176 ELEMENTARY GEOLOGY 
bottom, there are other kinds of river deposits that 
need explanation. 
Alluvial Fans or Cones (Cone Deltas).— When a 
river emerges from a mountain valley upon a plateau, 
its slope often quickly changes; and if it carries much 
sediment, some of this must be deposited. This is done 
Fic. 88. 
Alluvial cones at the base of small mountain valleys. 
at the outlet of the mountain valley, and an accumula- 
tion is built up at the mountain base (Fig. 88). This 
deposit, which is a part of a cone, with its apex up 
stream, spreads out fan-shaped. The river flows over 
its surface in different channels, so that in the course 
of time, it deposits sediment on all parts of the slope 
