The back slope of Rough Ridge is probably the most perfectly preserved inclined fossil plain in Otago. The surface, as shown in fig. 3 of the paper referred to above, is in the stage of erosion when the overmass has been stripped off and the ancient planed floor is undergoing dissection by numerous parallel consequent streams none of which is master. While a few feet of rock have been removed from the stripped floor since the removal of the cover, as is shown by the presence of abundant residual tors of schist, the form of the ancient eroded plain, now tilted with an inclination
Fig. 1.—Northern end of Rough Ridge (from maps by Department of Lands).
Fig. 2.—Diagram of a splintered fault dislocating a plane surface.
of 10°, is preserved almost perfectly by the existing surface of the inter-fluves. Though these are narrow, they slope towards the numerous stream-lines only near their edges, which are rounded off. The profiles of the little valleys of the dissecting streams are roughly graded, though incised to but a small depth below the general slope, this depth being, as would be expected, greatest about the middle of the slope. Thus, if the former presence of a cover be disregarded, the cycle of erosion for the surface is still in the stage of youth; and it may remain, and probably has remained, at this stage for a long period, for the conditions are here extremely favourable for long persistence of such pseudo-youthful topography.* In this way only can the frequent recurrence of similar back slopes be explained.
[Footnote] * C. A. Cotton, loc. cit., p. 259.
Though the back slope of the Rough 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 front, 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 structure of the schist forming the Rough Ridge block is a broad, open 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 foldscarp 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 separates 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 southeastern 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 remnant of the cover is preserved.* A glance at figs. 3 and 4 shows that the drainage of the depression behind the splinter, 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 bending-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.
[Footnote] * A. McKay, Report on the Older Auriferous Drifts of Central Otago, Wellington, 1897 (see map).