Cotton. — Rough Ridgt and its Splintcrtd Fuulf-scurp. 285 



Though the back slope of the Eough Ridge block is remarkably uniform, 

 the stripped surface arches over near the crest-line, becomes horizontal, and 

 then slopes down towards the south-eastern side, or fiont, of the block. 

 Around the north-eastern end this little-dissected surface has the form of a 

 plunging anticline, and, close to the north-eastern end, even on the front, 

 the strongly warped fossil plain seems to form the slope, though farther to 

 the south it undoubtedly passes into a fault-scarp. The eastward-sloping 

 stripped surface, or fold-scarp, has an inclination of 20°, and is more deeply 

 and maturely dissected than the back slope. Tors survive on the interfluves. 



The structuie of the schist forming the Rough Ridge block is a broad, 

 oi^en anticline with its axis transverse to the elongation of the block. On 

 the crown of this anticline, which is near the northern end of the block, 

 the ancient eroded surface truncating the anticlinal structure is practically 

 parallel with the foliation of the schist. The deformation to which the 

 uplift of Rough Ridge is due is here shown not only by the form of the 

 surface, but also by the dip of the schist foliation, which arches over the 

 top of the ridge accordant with the surface. Such an agreement between 

 the surface-slope and the dip of the foliation may occasionally be noted in 

 other parts of Central Otago where the schist is flat-lying ; but, on the other 

 hand, there are frequent changes of dip which are quite discordant with the 

 general slopes of the surface, and are obviously of more ancient origin. The 

 same is true of the broader structures of the schist — as, for example, the 

 broad anticline referred to above in the Rough Ridge block. 



As the eastern front of Rough Ridge is followed southward the fold- 

 scarp is found to be replaced by a fault-scarp. Here, however, near the 

 base of the scarp lies a strip of the schist undermass, the north-eastern part 

 of which is but little above the general level of the neighbouring portion 

 of the Maniototo depression and is separated from the scarp by a narrow 

 strip of planed undermass (see fig. 4). This low, flat outcrop of schist is 

 named Little Rough Ridge, on account of its tor-covered surface. Followed 

 south-westward its surface rises — the displacement on the fault which sepa- 

 rates it from the main block decreasing in that direction — so that it is seen 

 to be a splinter with its stripped surface inclined towards the main block 

 (see figs. 3 and 4). This first splinter is bounded on the front, or south- 

 eastern side, by a fault-scarp similar to that of the main block. At the 

 base of this scarp is another low-lying area of schist, which rises south- 

 westward to form a second splinter similar to the first. At the base of the 

 fault-scarp of the second splinter lies a schist surface which emerges from 

 beneath the overmass in the Maniototo depression and rises southward to 

 form part of the plateau into which the Central Otago block mountains and 

 depressions merge when followed southward. 



In the fault-angle behind the higher portion of the first splinter a rem- 

 nant of the cover is preserved.* A glance at figs. 3 and 4 shows that 

 the drainage of the depression behind the spUnter, after following what 

 appears to be the normal, longitudinal, consequent course in the fault-angle 

 for some distance, turns south-eastward and crosses the splinter in gorges. 

 This points to some l)ending-down of the initial surface adjacent to the 

 crest of the fault-scarp, for there is nothing to suggest that capture has 

 taken place, and the stream-courses are probably consequent throughout 

 their length. 



* A. McKay, Reiwrt on the Older Auriferous Drifts of Central Otago, Wellington, 

 1897 (see map). 



