292 



Proceedings of the Royal Physical Society. 



the streamlet in fine weather often slips down one side of the 

 semicircle, between the cone and the bank. When swollen 

 with rains it pours straight over the apex, adds to it what of 

 gravel it may carry, and splits into radiating runnels that 

 shift hither and thither, building wherever they best can, and 

 strained of their finer sediment by the grass. 



The forces producing alluvial fans being in the condition 

 of delicate adjustment described by Capt. Button, the periodi- 

 cal destruction of their margins through river planation at 

 once upsets the equilibrium. When the stream finds the 



Fig. 15. 



Fan Terraces (after F. Drew, Quart. Jour. Geol. Soc., vol. xxix, , p. 454). 



A Delta or Alluvial Fan shed from ravine on the side of a valley and cut 

 by the river into a Delta Terrace at T ; A' Second Delta thrown out through 

 the first, and also converted into a terrace ; A" Third Delta spread on the 

 existing river level, partly cut into at T' ; R, river. 



edge of the river-bank where it was wont to find its own 

 gentle slope, it pours over, cuts a gully back into the heart of 

 the fan, and never again resumes its conical distribution of 

 materials at that level without first building up to it. The 

 old delta is left as a fan terrace ; and successive interruptions 

 of the fan-forming processes, which may be resumed at one 

 level after another, if the river, in making its own terraced 

 valley, should happen to prepare them, will result in a series 

 of fans with their edges clipped off into terrace-fronts (see 

 Fig. 15, in which the fans are small enough to be very 

 distinct). 



