186 Transactions. 



set of land-forms occur: long fully-developed lateral spurs run out from 

 the main divide like great buttresses for a mile or more, and between 

 them lie broad, open stream-valleys, all diversified by a dendritic drainage- 

 pattern. One of these broad valleys, locally known as " The Heights " 

 basin, is situated at the junction of the Arapaepae Ridge with the trans- 

 verse ridge that connects it with the inner ridges of the range. By virtue of 

 its situation, this basin possesses such change-favouring features as superior 

 altitude and shallowness to a greater degree than the adjacent eastward- 

 facing lateral valleys of the Arapaepae Ridge. 



" The Heights " basin, in which the changes of drainage took place, lies 

 near the crest of the Arapaepae Ridge, its flat alluvial bottom (Plate XI, 

 fig. 1) having an average altitude of 1,015 ft. — only 200 ft. below the 

 trigonometrical station, Arapaepae No. 3, 1,210 ft., located at its north- 

 western corner. The basin is roughly rectangular in shape, and about 

 a mile and a half across from north to south. Forming its rim are hilly 

 ridges, varying in height from a few feet to more than 400 ft. above the 

 alluvial flat: on the west the Arapaepae Ridge, on north and south two 

 lateral buttressing spurs of the same, and on the east the western end of 

 the transverse connecting-ridge. To the north lies the catchment area of the 

 Mangaore Stream ; to the south and south-east that of the Makahika, a 

 tributary of the Ohau River ; and to the west that of the Koputaroa. 

 Formerly " The Heights " basin had only one outlet ; now there are no 

 less than three. 



4. Changes of Drainage in " The Heights " Basin. 



Originally the whole of the drainage of " The Heights " basin was 

 discharged through a comparatively narrow outlet situated at its south-east 

 confer, by a single stream, tributary to the Makahika River. The origin 

 of the basin is, in the present state of knowledge, somewhat problematical ; 

 but a tentative hypothesis is that it was formed by the denudation, first 

 by the subaerial agencies in general and afterwards principally by stream 

 erosion, of the crest of a broad anticlinal structure of crumpled strata pos- 

 sessing but little primary variation in hardness. Under this conception 

 the weakened crest of the anticline was widely opened, while the limbs 

 retained a sufficient degree of compactness to restrict erosion to narrower 

 limits. This hypothesis conforms to the conception of the orogenesis of 

 the Tararua Range briefly outlined in section 2 ; the general topography 

 of the Arapaepae and adjacent ridges is also favourable to its adoption. 



An identical origin must be ascribed to the neighbouring eastward-facing 

 lateral valleys of the Arapaepae Ridge (fig. 1)— the Waireka (Plate XI, 

 fig. 2), Wainui, &c. All these valleys are of the basin-like, bottle-neck 

 type, being gorge-like and narrow at their outlets, and broad and open 

 above. Valleys of similar form occur in the Blue Mountains* due west of 

 Sydney (Taylor, 1919, p. 177). 



It may be suggested that in the case of the Tararua foothills the broad 

 arching of the secondary folding would be quantitatively insufficient to 

 produce so sharp a distinction in the resistance to erosion of the crest and 

 limbs of a fold as to determine the ultimate form of the bottle-necked 

 valleys. For the present this may remain an open question, though the 



* Professor David's and Griffith Taylor's explanation of the Blue Mountain bottle- 

 neck valleys appears to be applicable to those of the Tararua foothill ridge, though in 

 the former locality this particular type of land-form was produced by a single rock 

 stratum in the limb of a large anticlinal fold, and in the latter by the compacted and 

 stronger strata of the anticlinal limb itself as compared with the weaker crest. 



