210 W. M. DAVIS — FAULT SCARF IN THE LEPIN] MOUNTAINS 



base of the mountain front nearly all of the ravines broaden and their 

 lloor.s become distinctly convex, thus imitating the form well known in 

 alluvial fans, though rarely matched in an eroded surface of solid rock. 

 These convex floors will be called rock fans. 



Although the existence of rock fans, as appropriate elements of re- 

 treating escarpments, especially in arid regions, might have been deduct- 

 ively inferred, no observations of such forms were announced, so far as 

 I have learned, until the appearance of McGee's essay, l{ Sheetflood 

 Erosion,"* in which rock fans are described as occurring at the base of 

 Coyote mountain, in the arid Sonoran region of Mexico. Several low 

 fans there "have the form of alluvial accumulations, but actually con- 

 sist of sharply carved mountain rocks, veneered thinly with granitic 

 loam and gravel littered with great boulders " (page 112). The context 

 shows that the fans, like the mountains above them and the plains 

 below them, have been carved out of a once much larger rock mass; 

 and hence it must be concluded that the fan form has been preserved 

 during its retrogression from an earlier position, and that it will be 

 preserved during continued 'retrogression in the future if no disturbance 

 interferes. 



The rock fans of the Lepini front seem to possess essentially the same 

 characteristics as those of the Sonoran region. The rectilinear elements 

 of a fan all diverge from the point where the floor of the ravine widens 

 and changes from concave to convex form, and all the elements seem 

 to possess the same declivity. Veneers of gravel are strewn more or less 

 plentifully over the fan surface, but ledges of bare rock are seen not in- 

 frequently. The following explanation is offered to account for these 

 curious forms. 



It may be supposed that for a time during and after the general uplift 

 of the mountain mass the scoring of ravines in its northeast front sup- 

 plied material for the formation of advancing or aggrading alluvial fans 

 on the piedmont surface similar to the aggrading fans that are often seen 

 today in the once ice-filled valleys of the Alps. But there must have 

 come a time when the incision of the ravines reached such a depth that 

 the advance or aggradation of the fans ceased. This would have been 

 when a graded slope was established on the rock floor of the ravines in 

 such a position and at such an angle that the prolongation of its line of 

 descent was tangent to the surface of the fan ; then any further degrada- 

 tion of the ravine must cause a degradation of the fan ; at first near its 

 apex, but later over a greater part of its surface. The advance of the 

 fan is thus slowly reversed into a retreat. It is difficult at first to con- 



* Bull. Geol. Soc. Amor., vol. 8, 1897, pp. S7-112. 



