Adkin. — Readjustment of Drainage on the Tararuas. 191 



Mangaharakeke Stream : fig. 3) which produced the highest waterfall* 

 yet recorded in the Tararua Range, and a ravine exhibiting all the criteria 

 of extreme youth — precipitous, crumbling rocky sides, and a narrow 

 ungraded stair-like bottom down which the stream plunges in a series of 

 falls and cascades. The main fall is situated at the head of the ravine, and 

 descends the 300 ft. of its height in three leaps, separated by narrow rock 

 ledges. 



The northern end of the Poruriri Ridge is even more asymmetrical than 

 the Arapaepae Ridge, its spurs being so deeply truncated on its western 

 side as to present an almost unbroken face, the exceptions being the gash- 

 like Mangaharakeke ravine and some minor gullies. Like the Arapaepae 

 Ridge in the vicinity of " The Heights," the Poruriri has long branching 

 lateral spurs on its eastern side, and physiographically the two ridges have 

 much in common. 



Considered as a single feature, the former and present catchment areas 

 of the Mangaharakeke Stream have a topographic form intermediate 

 between that of the Waireka Stream (Plate XI, fig. 2) and that at " The 

 Heights " (Plate XI, fig. 1). Some of the topographic details of this dis- 

 membered catchment area are of considerable interest, but only one which 

 has a direct bearing on my argument can be touched on here. The sudden 

 spilling-over of a fair-sized stream like the Mangaharakeke liberated an 

 enormous amount of erosive power, with the result that the spurs on either 

 side of the ravine were shorn away longitudinally, leaving them as- half- 

 spurs — i.e., having a concave precipice on the one side and the normal form 

 on the other. None of the spurs enclosing any of the neighbouring minor 

 gullies possesses a similar configuration, a fact emphasizing the special origin 

 of the ravine. 



Formerly, the Mangaharakeke Stream took its rise on the northern side 

 of the Poruriri Trigonometrical Station, and flowed north and north-east 

 into the upper valley of the Tokomaru River. By the excessive alluviation 

 of the upper part of its course a state of great instabihty ensued, and while 

 swinging to and fro on its alluvial flat the stream found a low place in the 

 main Poruriri ridge-crest and flowed down the western slope, there pro- 

 ducing the ravine and falls described above. A sluggish shrunken remnant 

 of its former trunk, tributary to the Tokomaru, still drains the eastern part 

 of the deserted alluvial flat. 



List of Papers cited. 



Adkin, G. L., 1911. The Post-Tertiary Geological History of the Ohau River and of 

 the Adjacent Coastal Plain, Horowhenua County, North Island, Trans. N.Z. 

 Inst., vol. 43, pp. 496-520. 



1919. Further Notes on the Horowhenua Coastal Plain and the Associated 



Physiographic Features, Trans. N.Z. Inst., vol. 51, pp. 108-18. 

 Cotton, C. A., 1916. The Structure and Later Geological History of New Zealand, 

 Geol. Mag., dec. 6, vol. 3, p. 246. 



1918. The Geomorphology of the Coastal District of South-western Wellington, 



Trans. N.Z. Inst., vol. 50, pp. 212-22. 

 Heim, a. Quoted by Lord Avebury, 1902, in The Beauties of Nature, pp. 159-62 and 



maps. 

 Marshall, P., 1912. Geology of New Zealand. 

 Park, J., 1910. The Geology of Neiv Zealand. 

 Taylor, Griffith, 1919. The Physiographic Control of Australian Exploration, Geog. 



■Journ., vol. 53, p. 177. 

 Thomson, J. A., 1917. Diastrophic and other Considerations in Classification and 



Correlation, &c., Trans. N.Z. Inst., vol. 49, pp. 397-413. 



* From the WeUington-Manawatu Railway line just south of Tokomaru this fall is 

 a conspicuous and striking object. 



