184 Transactions. 



this was followed by secondary folding (the Kaikoura deformation) accom- 

 panied by block-faulting on a large scale, and also by the deep dissection 

 of the penultimate erosion-cycle. The secondary folding corrugated the 

 Tararua earth-block into broad anticlinal and synclinal forms, and these 

 appear to have determined the trend of the principal drainage-lines, and 

 to have guided the agents of erosion in the production of the high reUef 

 of the present topography. The initial drainage-pattern is thus considered 

 to be mainly consequent gn the secondary deformation, and only to a very 

 slight extent due to adjustment to the original structure. 



With reference to the hills near Wellington City, which may be 

 regarded as a south-western extension of the Tararua Range, and perhaps 

 also to the main range itself, the longitudinal ridges have been explained 

 (Cotton, 1918, pp. 213-14) as being bands of resistant rock reinforced by a 

 network of secondary mineral matter sealing the joints ; and the valleys 

 between the longitudinal ridges as belts of shattered rock- — shattered by 

 the ancient folding of the strata — along which the erosional action of the 

 streams has been more efiective. 



While this hypothesis of resistant reiniorced bands of rock is accepted 

 as a probable contributory cause of the development of the longitudinal 

 ridges,* it appears to be inadequate when confronted by the notable linear 

 persistence of the longitudinal ridges of the Tararua Range, and also by 

 the associated physiographic features thereof — viz., the subsidiary trans- 

 verse ridges,t and certain arresting characteristics of the hydrography. 

 The present writer's explanation of the genesis of these land-forms may be 

 presented in detail later, and it will suffice to state here that there are 

 numerous and seemingly sound reasons for the belief that orogenic folding 

 and uplift, synchronizing with the production of the existing stream- 

 sculptured relief, is the most satisfactory explanation of the origin of the 

 longitudinal and transverse ridges, and also of the notable peculiarities 

 of the present hydrographical regime. Under this explanation the principal 

 rivers must be classed as anteconsequent and in part antecedent. 



3. Topography of the Arapaepae Ridge and of " The. Heights " 



' Basin. 



On the western side of the Tararua Range the outermost foothill ridge 

 is divided into sections by the vents of the rivers that issue from the moun- 

 tains on to the plain. Locally the ridge-sections bear names usually corre- 

 sponding to the trigonometrical stations situated on their highest points. 

 The Arapaepae Ridge, on which is situated the more striking of the two 

 examples of drainage-readjustment that form the subject of these notes, 

 lies between the vents of the Ohau and Mangaore Streams, which cross 

 the plain near Levin and at Shannon respectively. 



Towards its northern end the Arapaepae Ridge is markedly asymmetrical. 

 On its western side the spurs are deeply truncated, the hillside presenting 

 a steep face composed of short blunted salients. These features mark 

 the position of a former coast-line, they being ancient sea-cliffs (Adkin, 

 1911, p. 509 ; 1919, p. 109). On the eastern side of the ridge a different 



* The topographic details as etched out by erosion are undoubtedly due to the 

 presence of bands and patches of rock of varying resistance. 



t Tlie transverse ridges are not mere erosional features occurring at haphazard 

 intervals. In some cases they extend from lowland to lov/land right across the 

 mountain-system, and therefore must be attributed to a more profound causation. Nor 

 can it be due to mere chance that the principal transverse ridge of the Tararuas, if 

 produced eastward across the Wairarapa lowland, there coincides with the water- 

 parting from which the drainage of the lowland and adjacent mountains is directed 

 north and south respectively. 



