64 Transactions. 



part of tlie Aorere Valley indicates that it was first formed along a line 

 of earth-fractuvf, trending in the general direction of the present valley, 

 and having its ddwntlii-ow on the south-east side of the line of rupture " 



2 ]ie W olcamarama Fuult-scary. 



The scarp front of the Wakamarama Range, which forms the north- 

 western side of the Aorere Valley depression, belongs to a class' of com- 

 posite fault-line- and fault-scarps common in Xew Zealand, its upper 

 part being a true fault-scarp, and its lower part, from which the covering 

 strata of T]:e downthrown Ijlock have been in part stripped by erosion 

 since faulting tudk place, being a fault-line scarp. This " comiaosite 

 fault-scarp,"' as it may be called, rises to a height of about 3,000 ft. 

 above the valley lowland. Only one stream — the Kaituna — breaks 

 through the scarp and has its head far back in the range, and, with 

 the exception of this break, the divide between the heads of the numerous 

 steep-grade conse([uent ravines of the scarp and the north-westward- 

 flowing rivers of the back slope of the Wakamarama block lies at no 

 great distance back from the fault-line. The edges of the facets into 

 which the scarp is divided by the ravines which dissect it are rounded 

 oft", and no doubt tlie steepness of the facets has been much reduced 

 by weathering, slipping, and soil-creep. In spite of a covering 

 of forest Avliicli oljscures the details, however, the alignment of the 

 blunt-ended spurs is still very striking : and, even in the absence 

 of the geological evidence of the existence of a favdt along this 

 line, whicli is afforded by the presence of the covering strata in the 

 valley lowlands along the base of the scarp, the morphological 

 evidence would indicate faulting. In that case it would l)e neces- 

 sary to give very careful consideration to an alternate hypothesis 

 that the scarji is tlie result of lateral cutting in the course of normal 

 erosion by the Aorere River, which flows at its base, or perhaps that it 

 has Ijeen formed by glacial erosion. The most convincing argument 

 against either of tliese hypotheses is the absence of a similar scarp on 

 the opposite side of the Aorere Valley, lliough the whole scarp cannot 

 by any stretch of the imagination be regarded as the work of fluviatile 

 erosion, the streams of the Aorere system, while they were engaged in 

 eroding away the initial floor of the depression, undoubtedly swung occa- 

 sionally against tlie base of the scarp, which niay thus be expected to be 

 now a little way back from the fault-line. 



For the greater part of the length of tlie scarp the undermass rocks 

 alone occur in the npthrown block in the neighbourhood of the fault-line, 

 and the crest appears from the valley as a line of rounded prominences 

 of roughly accordant height, while, according to Bell, there are on tlie 

 highland surface patches of imhilil (flat open spaces) (p. 24), suggesting 

 that the range is a dissected block of an uplifted deniulation plain. 

 From the crest of the range, which, as stated above, is but a short dis- 

 tance back from the fault-line, the summit-levels descend towards tlie 

 north-west : and the north-western slope is dissected by large sub- 

 parallel streams of apparently consec[uent (more strictly, probably, super- 

 posed consequent) origin. The lower ground to the north-west is formed 

 of a continuous sheet of tlie covering strata, maturely dissected, and 

 there ai'e outliers of the same towards the crest of the range.* All 



* Cox, S. H., " On the District between CoUingwood and Big River," Geol. Surv. 

 of N.Z.. Rep. Oeol. Expl. dur. 1882, pp. 62-74, 1883; Park, J., "On the Geology of 

 Collingwood County. Nelson," ibid., 1888-89, pp. 186-243, 1890 ; Bell, J. M., toe. ciU 

 map opp. p. 89. 



