214 W M. DAVIS — FAULT SCARP IN THE LEPINI MOUNTAINS 



the piedmont slope that the recent faulting, or at least the inequality of 

 its movement, is to be explained by an irregular depression of the pied- 

 mont Eocene mass rather than hy an uneven elevation of the Cretaceous 

 mountain block. No sufficient explanation has occurred to me for the 

 systematic irregularity of dislocation, whereby its greatest measures lie 

 on the lines of the ravines. 



The suggestion that the line of basal cliffs is due to the outcrop of par- 

 ticularly resistant members of the Cretaceous limestones deserves special 

 consideration, for cliffs of such origin are much more common than fault 

 cliffs. Outcrop cliffs are well known in many dissected plateaus, such as 

 the Allegheny plateau of West Virginia, where the hillsides are in general 

 reduced to a tolerably constant angle of slope,* yet where the slope is 

 frequently broken by cliffs that mark the outcrop of resistant sandstone 

 layers. Such cliffs always retreat from the front of the spurs or buttresses 

 into the ravines; it is the constancy of this familiar relation that estab- 

 lishes the general principle that erosion is more rapid on the trough line 

 of a concave ravine than on the slope line of a convex spur. But the 

 basal slope of the Lepini are abnormal in being least graded in front of 

 the ravines and most graded at the base of the spurs. Furthermore, if 

 the cliffs were the result of normal denudation, the ravines should con- 

 tract to narrow gorges in passing through the controlling hard strata 

 instead of opening to convex fan surfaces. But all consideration of ex- 

 cessive hardness in the basal strata is here irrelevant, for the limestones 

 are of essentially the same quality for hundreds of meters upward from 

 the mountain base. The lowest exposed layers have no such excessive 

 strength as to have resisted erosion through the long period during 

 which the ravines and slopes above them were so well graded. The 

 sudden break at the base of the ravines in particular admits of no ex- 

 planation save by a recent dislocation, presumably located on or near 

 the ancient line of fracture by which the mountain mass was originally 

 delimited. 



Some of the rock fans are newly and narrowly trenched by the streams 

 that formerly ran contentedly forward upon their sloping surface. As 

 far as observed the trenches are sharp-cut gorges, not yet graded on their 

 floors, and enclosed by steep and bare rock walls. The depth of the 

 gorges decreases rapidly upstream, as in the fourth and fifth ravines 

 northwest of Morolo, and their heads are not yet, as far as I examined 

 them, cut back to the apex of the rock fans. The fine fan shown in 

 plate 19 is practically untrenched, probably because of the underground 



*See the plates in Campbell and Mendenhall's paper, Seventeenth Annual Report (pt. II), U. S 

 Geological Survey. 



