Art. XLVIII.—Discovery of Artesian Water-supply, Ruatniwha Plain.

Transactions and Proceedings of the Royal Society of New Zealand, Volume 25, 1892, Page 350

Art. XLVIII.—Discovery of Artesian Water-supply, Ruatniwha Plain. By H. Hill, B.A., F.G.S. [Read before the Hawke's Bay Philosophical Institute, 28th November, 1892.] About five years ago, when portions of the Hawke's Bay District had been suffering from an unusual drought, the question was asked whether there would be any probability of an artesian water-basin on the Ruataniwha Plain. This plain flanks the lower slopes of the Ruahine Range on its eastern side, and for the eastern boundary it has a limestone range which is in reality a portion of the Puketoi Range. The latter range is of considerable interest, because in its turn it is flanked to the eastward by the chalk-marls which are met with in so many places between Te Aute and Waipawa. The head of the Ruataniwha Plain is at the Guavas, and it extends to the south-west for some distance beyond Takapau, where a fan-like shingle-deposit from the Ruahine separates the plain from the valley of the Manawatu River. The length of the valley from north-east to north-west may be set down at twenty miles, and the average width at eight miles. There is every appearance that at one time the Ruataniwha Plain was directly connected with the Heretaunga Plain, for the beds in the vicinity of each are identical in their structure and arrangement; but at that time the Kidnappers and Te Mahia Peninsula were joined together, and what is now Hawke's Bay was covered with shingle and conglomerates and pumice from the western watershed. A period of subsidence followed, since which time the Heretaunga and Ruataniwha Plains have been formed, by the deposition of new gravel, sands, and débris, from which material similar to the Kidnapper conglomerate and pumice beds had been washed, and which now

flank both plains, and afford complete evidence of the sequence of change such as I have described. The hills between Maraekakaho and the Guavas form the dividing-line at the present time between the Heretaunga and Ruataniwha Plains, and if a cutting were made in the hills so as to unite the plains the bedding which would be exposed would be identical with the beds at Redcliffe and the Kidnappers. The whole country between the Guavas and Maraekakaho, and between Havelock. Pakipaki, and the Kidnappers, received its supplies of pumiceshingle, which it has in abundance, from the same source and at the same time; and wherever there are great breaks in the continuity of the beds the causes are due to changes in the land-surface brought about by earthquakes and earth-movement. The beds flanking the limestones on the eastern border of the Ruataniwha Plain were formed at the same time, and are a part of the beds which are exposed on the banks of the Tukituki and Waipawa Rivers. And the same remark applies with respect to the beds at Norsewood, for they correspond in every particular with the beds forming the hills between the Guavas and Maraekakaho. But between these boundaries comes the Ruataniwha Plain, which is covered with more recent deposits, and has been filled up since the subsidence took place with the material that the streams and rivers brought down from the fractured surfaces of the country now forming the Ruahine Mountain-range. From this brief explanation it will be seen that the Ruataniwha Plain, like our own plain, is of comparatively recent origin. Both plains have grown and are growing, but the evidence as to their origin is sufficient to determine the possibility of their containing an underground water-supply. Of the Heretaunga Plain, including the area even within the limits of our own town, we have full proof, and it is through the accumulation of facts such as are available with regard to the water-supply in this district that it is possible to make inferences as to the prospects of finding water in other districts. Nature carries out her work with precision and regularity. The materials with which the work is carried on may differ, but moving water acting under similar conditions produces similar results everywhere, and this is why it is possible to generalise when dealing with changes brought about by ordinary physical laws. It has been already pointed out that limestones form the higher boundary hills on the eastern side of the plain, and traces of a limestone range are met with in the lower hills towards the Ruahine Range. But the limestones in the latter locality have been partly denuded, and replaced by the Kidnapper beds. Underlying the limestones everywhere are blue clays locally known as “papa,” and of course these clays may be looked upon as impervious to

water. The Kidnapper series is formed of many varieties of beds, most of them being highly pervious to water: hence we may assume that some of the beds which have assisted in the filling-up of the Ruataniwha Plain are saturated with water, seeing that wherever flowing water passes over beds at or near their outcrop a large quantity must pass away in the direction of the dip of the beds. Until sinkings had been made it was impossible to tell the depth of the more recent shingle and débris which have been deposited within the Ruataniwha basin; and it was with pleasure that I learnt in the early months of this year that Mr. Harding, of Mount Vernon, had determined to try for artesian water on the plain. The contract was undertaken by Mr. J. Gilberd, who consulted me as to the probable depth it would be necessary to go. He was informed by letter than an artesian flow must not be anticipated before 300ft. had been pierced. During the sinking some valuable experiences were met with, because the beds to be passed through necessarily differed from those within the Heretaunga artesian basin; but, although the difficulties were many, a bed of flowing water was reached when the tube-bore had pierced nearly 300ft. of the heavy shingle-deposits, and at a depth of 316ft. a good flow was obtained, the water rising 16ft. above the surface. During the sinking three watertight dividing-beds separating the shingle-deposits were met with. The first watertight bed commenced at 60ft. and continued to 80ft.; the second one began at 120ft. and was passed at 135ft.; and the third one was met with at 236ft. and was passed at 268ft. These three watertight beds separated three distinct water-basins, for during the first 60ft. of sinking there was an unlimited supply of water which rose to within 6ft. of the top of the pipe, and continued there until the first watertight bed had been pierced. Whilst passing through the 40ft. of shingle which separates the first and second watertight beds, there was still an unlimited supply of water in the pipe; but, curiously, it stood at an average height of 33ft. from the top, and remained at that height until the second watertight bed had been pierced. The water then fell to 60ft. from the surface, and remained constant until the third watertight bed was passed, when the water at once rose to the top of the pipe, and the flow continued to increase in quantity each foot that the pipe was lowered. As the quantity of water obtained at the depth of 316ft. is sufficient for Mr. Harding's requirements the work was stopped, but there is every appearance from the material brought up that the true Kidnapper beds are not far away. At present the flow is at the rate of 18 gallons a minute, the tube-bore being only 2in. diameter, and telescoped through a 3in. pipe, the latter having bent at 275ft. The rise of water

in the three separate beds is a curious circumstance, and suggests the possibility of the change of dip in portions of the beds from 90ft. to 260ft.; or the lowering of the water in the tube-bore may arise from underground springs. The latter is the more likely reason, as the creek known as the Waipawamate rises from several strong springs some distance from the well, and I am aware of several very large springs at a lower level in the plain in the direction of Ashcott Station. The water is very clear and tasteless, and is much softer than the water from the Hawke's Bay wells, but is similar to the Wanganui artesian water. The value of an artesian supply to the residents on the plain cannot be estimated, and now that the question has been settled the residents of Woodville may look with confidence to the possibility of finding an artesian supply within the limits of the borough. The diagram I exhibit shows the character of the deposits, and gives information which will be of value to sinkers should other wells be put down. Well completed the 3rd September, 1892. Situated a mile or so to north-east of Te Ongaonga Township.