188 Transactions. 



available evidence appears to indicate that it was sufficient. There was, 

 however, a contributory cause. The rejuvenated Makahika River has cut 

 its present trench-like channel at the foot of the long eastward-trending 

 lateral spurs of the Arapaepae Ridge, truncating them ; and to maintain 

 accordant junctions the Wainui and Waireka Streams were thereby forced 

 to cut shorter, deeper gorges than they would have done had the Makahika 

 River cut its present trench in the middle or on the opposite side of its 

 former widely-opened valley. Of itself the action of the Makahika was 

 quite insufficient to produce the bottle-neck, basin-like valleys of the 

 tributary streams referred to, but, in conjunction with the broad arching of 

 the secondary deformation, conditions came into existence that were favour- 

 able to the production of the tributary valleys in their present form. 



When " The Heights " basin had been opened to nearly its present 

 extent a period of alluviation followed. This alluviation covered the former 

 valley-bottom to a considerable depth, and ultimately formed a wide alluvial 

 flat (Plate XI, fig. 1) having a very gentle surface slope to the south-east. 

 The alluvium consists of stifi cream-coloured and yellow clays resting 

 on a thick mass of fine gravel. Towards the close of the deposition of 

 alluvium many of the exposed ends of the half-buried spurs round the rim 

 of the basin were levelled ofi by lateral corrasion, and the flat was thus 

 further enlarged. The current of " The Heights " stream was at this 

 time extremely sluggish, flowing in meanders of small radius, and the 

 surface of the flat must have been diversified by cut-ofi ox-bows and 

 deserted swampy channels (fig. 2). On the whole, the topographic form 

 of the basin at this stage was one of late maturity or even old age. 



Alluviation of a somewhat similar character took place also in the 

 Waireka, Wainui, and other adjacent bottle-necked valleys. Thick masses 

 of clay containing scattered angular fragments of rock, often of large size, 

 were laid down, the master streams became sluggish and winding, and 

 the topography reached an advanced stage of maturity.* The alluviation 

 of these tributary valleys took place in harmony with the alluviation of 

 their trunk valley, that of the Makahika : this is proved by the corre- 

 sponding accordant levels of the surfaces of the valley-fill in the several 

 parts of the valley-system. The alluviation of the large river-valleys of the 

 Tararua Range was caused by the failure of the rivers to transport the 

 excessive amount of waste derived from the then more extensive alpine 

 and subalpine areas in which these rivers took their rise during the 

 " glacier period," the snow-line (and consequently the timber-line) being 

 at that time, by reason, in part, of the greater elevation of the country, 

 relatively very much lower than now.f In my previous papers (Adkin, 

 1911, pp. 497-98, 520 ; 1919, p. 112) it was shown that the building of the 

 valley-plain of the Makahika and Ohau Valleys, and also the construction 

 of the Ohau fan, took place during that period of great elevation of .the 

 North Island — viz., in the early Pleistocene (Park, 1910, pp. 156-57, 250 ; 

 Marshall, 1912, p. 210^ The initial opening -out by rosion of "The 

 Heights " basin and the other adjacent bottle-necked valleys is therefore 

 of some antiquity — -certainly of not later date than middle or perhaps late 

 Tertiary times. 



The alluviation of " The Heights " basin was followed by further 

 changes. The s(^urces of a tributary of the Mangaore Stream, situated on 



* The last physiographic event in each of these valleys except that of " The Heights " 

 was rejuvenation, by which their alluvial bottoms were trenched to depths up to 100 ft. 



t For fuller reference to this subject see " The Discovery and Extent of Former 

 Glaciation in the Tararua Ranges, North Island, New Zealand," Trans. N.Z. Inst., 

 vol. 44, p. 315, 1912. 



