350 



FIRST YEAR SCIENCE 



little rainfall, they yield themselves with peculiar facility 

 to irrigation, as ditches can be easily led out from the apex 

 of the cone in all directions. Southern California offers 

 many examples of easy irrigation due to such cones. 



162. Deltas. When a river enters a body of quiet water, 

 its current is gradually checked and it deposits its material 

 in somewhat the same way as on emerging upon a flat 



CONE-SHAPED DELTA IN LAKE GENEVA. 



country. But here the deposition is more gradual and 

 the slope of the deposited material less steep. The sedi- 

 ment, too, is sorted by the water, and the finer material is 

 carried far out from the river mouth. Formations of this 

 kind are called deltas, from the Greek capital letter Delta 

 (A) which has the shape of a trianglec Few deltas have 

 this ideal shape, but there is a general correspondence to it. 

 If the delta-forming stream descends steeply, it may 



