C. A. Cotton— Block Mountains in New Zealand. 263 



surfaces, as has been pointed out by Davis (1892; 1912) and by 

 Gilbert (1909). In small facets of' fault scarps blunting of the 

 edges destroys the flatness of the whole facet, though'the dis- 

 section of the scarp by ravines may be incomplete. This 

 rounding of facets is particularly well illtistrated by alow scarp 

 at Waimate, South Canterbury (see fig. 8), which was lirst diag- 

 nosed as a fault or fault-line scarp by Thomson (1914). Sharp- 

 edged facets seem to occur only in the case of scarps resulting 

 from movements that have been renewed in very recent times. 



Fig. 8. 



Fig. 8. An eastward-facing composite fault scarp near the southern end 

 of the Hunter's Hills block in South Canterbury. 



If the crest line — the divide between consequent drainage 

 of the back slope and of the block front — is some distance 

 back from the base-line, the initial descent from an arched 

 crest to the top of a fault scarp may be gentle. Consequent 

 streams will arise on the upper slope, and the ravines will cut 

 deeply and rapidly into the upper slope and after removing 

 the cover dissect the undermass. Thus, in a comparatively 

 short time, the mountain front will be maturely dissected by 

 steep-graded ravines which will continue to work headward 

 and push the crest line divide down the back slope of the 

 block. On the steep mountain-face spurs will descend between 

 these ravines to end in a line of rapidly dwindling facets at the 

 fault trace, and the stage of maturity will rapidly be attained. 

 The front of the Kakanui-Horse Range facing the Shag Yalley 

 in eastern Otago appears to be a scarp of this kind (see fig. 9), 

 also the southeastern front of the Kaikoura Mountains (Cotton, 

 1913). 



