Adkin. — Post-tertiary Geological UiUorij of Ohau River. o09 



The southern extremity of tlio newly uplifted coastal plain just de- 

 scribed lies about two miles south of Paekakariki. The cliffs which border 

 the sea fiirther south begin at this point to run inla)id. These cliffs attain 

 in some places a height of 800 ft., and do not rise perpendicularly from 

 the sea-margin, but form precipitous scarps, fringed at the foot with jagged 

 rocks which lie slightly above high-water mark. As these cliffs strike in- 

 land (as a result of the uplift), and recede more and more from the present 

 shore, they are, by the formation of talus slopes and other effects of sub- 

 aerial denundation. gradually transformed into steep hills, which present 

 comparatively unbroken faces. The best examples of the former sea-cliffs 

 are to be seen near Otaki and Ohau. Although the flat land (marine 

 sediment) first appears near Paekakariki, there are abundant signs of 

 uplift, and also of prior depression, of the country as far south as Porirua 

 Harbour. 



The coastal plain of marine deposit varies in the different localities 

 between Paekakariki and Wanganui in both width and elevation. The 

 uplift was probably greatest midway between the two extremities, and the 

 altitude of the upper edge of the sandstone increases at the southern end 

 apparently with some irregularity as that point is approached. This irre- 

 gularity may really be only apparent and not actual, for in some cases it 

 is certaiii that subaerial denundation has proceeded so uniformly that, 

 though the original surface of the sandstone has been considerably lowered 

 the original superficial slope remains unaltered, and the denuded surface 

 is liable to be mistaken for the original one. 



The projection of the coast (Kenakena) near Waikauae is, with the 

 exception of the numerous river-mouths, the only irregularity in the grand 

 sweep of the present beach lying between Paekakariki and Wanganui. 

 This projection, which lies exactly opposite the Island of Kapiti, has doubt- 

 less been formed by the flow of the tides round the ends of that island. 

 The meeting of the tidal currents on its lee has caused the suspended 

 sediment to be precipitated, thus forming a sandy ridge, partly sub- and 

 (by the rise of the land) partly supra-marine. Should the uplift of the 

 land continue, Kapiti, by the extension of this ridge, will eventually be- 

 come a peninsula. 



In the immediate neighbourhood of the Ohau River the principal stream 

 which dissected the coastal plain of marine sediment was the Koputaroa. 

 Taking its rise in the southern end of the Arapaepae Range, and receiving 

 many affluents from the western slopes of the same, it flows northwards 

 into the Manawatu River. During its, geologically speaking, short life 

 the Koputaroa has passed through many vicissitudes, and, though the 

 stream is rather insignificant, a review of its life-history will perchance 

 serve to indicate the nature of similar sediments and operations of larger 

 watercourses, the conditions mider which such sediments were deposited, 

 and the manner in which their actions and operations were effected. 

 When seen in miniature, origins, modes of action, and cause and effect 

 are more easily studied and understood than when the same phenomena 

 occur on a larger scale. 



Towards the termination of the Pleistocene period — that is, when the 

 Ohau River flowed in the shallow " north-west " channel, and this portion 

 of the country was at a somewhat lesser elevation than at the present day 

 — ^the valley which is now occupied by the Koputaroa Stream was trans- 

 ^'^erselv divided by a low sandstone ridge. This ridge lay directly between 



