Adkin. — ['ost-teriiarij Geological History of Ohau Eiver. 497 



1)01111(1 gorges. Here may be seen on a very extensive scale the effects 

 of fluviatile erosion. Tlie parallel ridges have been dissected, and their 

 corresponding valleys deepened to an enormous extent by the action of 

 high-grade streams and swiftly flowing rivers. 



The culminating peaks of the district are the Mitre, 5,154 ft.; Mount 

 Dundas, 4,944 ft.; and Mount Crawford, 4,795 ft. These are not within 

 or even contiguous to the Ohau catchment-area, but lie near enough to 

 intercept the moisture-laden westerly winds, and thus increase the rainfall 

 within that area. The principal salients of the Ohau water-parting are 

 Mounts Waiopehu, 3,588 ft., and Tawirikohukohu, 3,455 ft. It is near the 

 summit of the latter that the Ohau River proper has its source. 



In character the valleys of the Ohau and its two main tributaries differ 

 considerably. As already stated, the Upper Ohau occupies a valley which 

 is alternately longitudinal and diaclinal ; the lower portion of the Makahika 

 appears to flow in a deeply incised anticlinal valley, while that of the Ma- 

 karetu, though diaclinal immediately above its junction with the Ohau, 

 is for the most part longitudinal. 



Prior to its occupation by European settlers most of the Horowhenua 

 County was clothed in tangled virgin forest. The zone of heavy mixed 

 bush (almost entirely beech near its upper limits) extends from low alti- 

 tudes up to the 2,500 ft. contour-line. Above this elevation the bush usually 

 becomes stunted, and is partially — or more often wholly — replaced by 

 shrubs, forming a dense subalpine scrub. In most places at the altitude 

 of 3,000 ft. the scrub terminates more or less abruptly, and the alpine 

 meadow land is reached. The principal components of the latter are tus- 

 sock-grass, Astelia, and alpine flowering-plants, while the subalpine scrub 

 is for the most part Olearia Colensoi and Draccphyllums. 



Though the district under consideration presents many features of in- 

 terest to the botanist and the nature-lover, it is for the geologist that it 

 reserves its chief attractions. The successive events within its limits have 

 been so varied, and in some cases the action of the several natural agents 

 so complex, that an examination of these events and operations should 

 prove of interest to geological students. 



Owing to the entire absence, or perhaps non-discovery, of fossil remains 

 in the area under notice, the age of each of the geological formations de- 

 scribed in the following has been determined entirely by stratigraphical 

 considerations. Judging, however, from the views held by writers on the 

 Tertiary and post-Tertiary geological history of New Zealand, the recent 

 maximum elevation of the country occurred in the early Pleistocene, and 

 it is upon this conclusion that the chronological divisions in the following 

 are based. 



At the close of the Tertiary epoch a plain of the Upper Tertiary, most 

 probably Pliocene, strata was uplifted, and at the cessation of its emer- 

 gence extended seaward on the western coast of the Wellington Province 

 from the foothills of the Ruahine and Tararua Ranges to far beyond the 

 limits of the present shore. At the termination of this uplift the elevation 

 of our Islands was much greater than now, but, as the movement was 

 more pronounced in the South Island than in the North, the latter Island 

 did not experience so severe a "glacial period" as did the former. The 

 Pliocene (?) plain is now not visible in this district, more recent deposits 

 having completely covered it, and on this account its exact nature cannot 

 be ascertained from observations made in this locality alone, but that the 

 uppermost Tertiary strata had a comparatively plane^and gently sloping 



