THE ROCK FANS 



211 



ceive of the preservation of a convex form by a retreating alluvial fan ; 

 one involuntarily imagines that its surface must be trenched along one 

 or another of its radiating elements ; but the difficulty of the problem 

 is lessened when it is remembered that the retreat of the fan must be 

 slow, since it can only follow the degradation of the graded rock floor 

 in the lower part of the ravines. Such degradation cannot progress at 

 a rate at all comparable with that of the incision of a rock-walled gorge 

 by a powerful stream, for the agencies of transportation in the ravine 

 are always taxed almost to the full measure of their capacity by the 

 receipt of creeping and washing waste from the upper Avails and the 

 tributary slopes. Moreover, the transportation of the coarser gravels is 

 chiefly the work of intermittent wet-weather streams, which work im- 

 petuously when flooded, and tluen cease working altogether for consid- 

 erable intervals. Such streams actively erode the upper ungraded floor 

 of the ravines ; they very slowly rasp down the graded floor of the ra- 

 vines, and they distribute the waste to the right and left on the convex 

 surface of their fans, clogging channels and filling slight depressions, 

 as they now follow one radius, now another. At the same time, the 

 fan surface weathers and wastes where not renewed by gravel veneer, 

 and the fine soil thus produced is removed by unconcentrated rain- 

 wash. 



Thus always wasting, here or there, the surface is nevertheless always 

 maintained in fan-like form, because any undue depression will be filled 

 with gravel at the next visit of the wandering stream. Regularly con- 

 tinued retrogression, aided by the lateral swinging of the graded streams 

 in the lower part of the ravine, should tend to carry the head of the fan 

 back into the solid rock mass ; and it seems as if this condition had 

 been actually brought about in the Lepini mountains. The side slopes 

 of the ravines are undercut near their mouths by the swinging streams, 

 and thus the rock floor of the ravine is widened. The wider it becomes, 

 the more manifest must be the convexity of its surface, especially if the 

 floor be steep, for all the lines of descent must have equal declivit}' 

 from the point where the widening begins. A considerable fraction 

 of a fan may thus come to be carved in solid rock, and eventually 

 the whole of it may be thus placed, as in the examples described by 

 McGee. 



At the base of the X*epini front perhaps a fifth or a sixth of the distance 

 from the apex of a fan to the lower margin of the piedmont slope lies on 

 firm limestone ; the remainder of the fan should be sought for in a con- 

 tinuously descending slope on the other side of the Tertiary fault line 

 by which the mountain mass is bordered ; and it may be added that 



