182 



GEOLOGY. 



some of the water then seeks new courses to right and left of the apex 

 of the deposit. This expands the area of deposition to right and left, 

 while the water which flows over it lengthens it in the direction 

 of flow. 



The course and behavior of the water after reaching an alluvial 

 cone is instructive. As its velocity is checked, deposition often takes 

 place in the channel, diminishing its capacity. As the channel is 

 filled up, the water tends to overflow on either side. The overflowing 

 water, being shallow, has so httle velocity that much of its load is 

 dropped on either margin of the channel, building up levees. The 



Fig. 170. — A talus cone. North Greenland Coast. The talus cone reaches the sea- 

 level. Drawn from photograph, 



water ever and anon breaks through the levees, giving rise to dis- 

 tributary streams, each of which aggrades its channel and builds its 

 own miniature levees (Fig. 171). Not rarely this process of channel- 

 fiUing and levee-building goes on until the channels of the little rivulets 

 are above the general level of the cone on which they rest. The rivulet 

 then runs in a groove on the crest of a little ridge. The channels on 

 the surfaces of fans and cones are fewest and deepest at their heads, 

 and more numerous and shallower below. In some cases the surface- 

 water disappears altogether before the outer border of the fan is reached, 

 by sinking into the debris. 



Alluvial fans and cones have various forms, and often attain 

 considerable dimensions. Their angles of slope depend on the amount 

 of reduction of velocity which the depositing water suffers, and the 

 amount and kind of load which it carries. The maximum slope of the 

 cone is the angle at which the loose material involved will lie. The 

 minimum slope of the fan, on the other hand, approaches horizon- 



