Art. LIV.—On Artesian Water Prospects in Poverty Bay and Gisborne.

Transactions and Proceedings of the Royal Society of New Zealand, Volume 29, 1896, Page 567

Art. LIV.—On Artesian Water Prospects in Poverty Bay and Gisborne. By H. Hill, B.A., F.G.S. [Read before the Hawke's Bay Philosophical Institute, 12th October, 1896.] The Town of Gisborne, in the Poverty Bay district, is situated a little to the eastward of the 178th degree of east longitude, and about 38° 23′ south latitude. To the west and north-west of the town a plain extends of some 15,000 to 20,000 acres in extent, through which the Waipaoa River and several smaller rivers flow to the sea. The Turanganui River, on which the Town of Gisborne stands, is made up of two streams—viz., the Waimata and Taruheru. Neither of the rivers named as flowing into the bay is a shingle-carrier, and, as far as I have seen, there is no trace of shingle along the beach between the town and Young Nick's Head, on the south bend of the bay. Near the new freezing-works there is a pumiceous chalky-like material which rims the lower front of the Kaiti Hills, facing the town. The substance may be described as tough rather than hard. It is very absorbent, and weathers well on exposure. There is no trace of its junction with the adjoining rocks, although in the exposed bands of fine sandstone which are exposed along the eastern side of the breakwater at low-water there are traces of this rock met with, and it appears to be resting unconformably on the sandstones. A similar deposit is met with on the opposite side of the bay, between Te Arai and the Murewai, where there is an isolated hill of from 50ft. to 60ft. high, composed entirely of this pumiceous chalk-like material. The rock appears to strike across country from-north-east to south-west, for it appears along the coast in the vicinity of Gable-end Foreland, and it is also met with on the Gisborne-Wairoa Road, between Panikanapa and the Upper Patutahi. The tops of the hills eastward and north-eastward from Gisborne, as far as Tua-motu Point, are covered with a heavy

deposit of nearly pure, coarse pumice, overtopped by a thin coating of shingle, which passes towards the point into old sea-beach deposits made up of sands, boulders, and a kind of impure cemented limestone full of recent shells. The deposits, wherever exposed, rest unconformably upon the underlying beds, which are highly inclined towards the west and northwest. The pumice and beach deposits are of varying thickness, but in the exposed face of the hills opposite the native church on the Kaiti they must be at least 130ft. in thickness, and show bands of stratification as if laid down in a slowly-subsiding water-area. Similar deposits cover the tops of all the hills to the west and north-west of the Poverty Bay plain, and they continue to top the hills and fill the valleys as terrace deposits to the north-west beyond Ormond, in the direction of the district known as Mangatu. All these deposits are of fresh-water origin, and in some of the finer pumice beds beautiful leaf specimens are met with. All the beds are very porous, but they are of limited extent in their distribution, for, although they can be traced over the larger portion of the present basin of the Waipaoa River, they are limited in other directions, and form no area of accumulation of any extent. They are not met with in the direction of Patutahi, although the low hills near Mr. Sutherland's residence are capped with a heavy silt-deposit, under which traces of shingle might be expected, followed by limestones similar to those to be met with at the Ormond quarry. The higher hills to the north, northwest, and west are capped here and there by limestone, with which most Gisborne residents are familiar, as it is used for metalling the county and borough roads; and Mr. Oxenham, of Makauri, also uses it for the manufacture of lime. This limestone belongs to the same geological series as that at Patutahi (lower and upper), which crops out on the roadside as you proceed from the plain towards Waerenga-o-kuri. The same limestone is met with at the waterfall between Scott and Weatherhead's, and it strikes across the country in a north-east direction, and eventually merges further on into a fossiliferous sandstone. No limestones are met with on the Kaiti or on the hills in the immediate vicinity of Gisborne, nor is there any trace of limestone on the hills in the Murewai district, in the vicinity of Young Nick's Head. Turning to the lower or underlying rocks, the general character of these can be seen in the exposures facing the bay along the Kaiti Beach. The rocks are banded, being made up of blue clays and soft thin sandstones interbedded and dipping or sloping in the direction of the town. The clays wear quicker than the thin bands of sandstone, and the latter break away in slab-like masses as the clays wear down by the action of the atmosphere. The thickness of the beds is very great,

and cannot be less than 5,000ft. or 6,000ft. All the beds axe conformable, and dip at an angle of about 25° in a north-west direction, and pass beneath Gisborne in the direction of Ormond at a varying depth. The rocks which make up the series known as Young Nick's Head, along the south side of the bay, have no corresponding characters with those described as dipping beneath the Poverty Bay plain. They are younger, and are connected with the beds forming the hills on the Whautaupoko, although there is apparently a fault of some extent, as the dip of the beds at the Murewai is south-southeast, whilst those on the Whautaupoko is south-west. The rocks exposed on the right bank of the Waipaoa River, opposite Ormond, belong to the Nick's Head series, and they dip E. by N. towards the bay at a low angle. From the descriptive account here given it will be noticed that, although there are numerous porous rocks in the district, they are by no means generally distributed, nor do they form a syncline and dip generally in the direction of the plain. The limestones are not conformable with the clays, and unfortunately they dip north by west, and their drainage is mainly in that direction. The lower rocks in the vicinity of the plain have been much tilted and broken, and, except in the case of Kaiti and Ormond, underlying beds do not present any prospect of a syncline such as is required for an efficient artesian water-supply basin. As already explained, the rocks forming the lower hills on the Whautaupoko are porous, being made up of shingle, sands, lignite, and a coarse, fossiliferous, marly sandstone, which seems to pass into an impure limestone in the direction of the Ormond quarries. These beds are to be found in isolated spots on the Patutahi side of the district, and between them and over them the Waipaoa River passes on its way to the sea. All these beds correspond with what are known as the Kidnapper beds, in the Hawke's Bay district, and from which the artesian waters of Napier and the whole of the surrounding plain are drawn. In the latter district, however, they are much more extensive, and they form with the limestones a trough or syncline of large extent, and which constitutes the artesian basin in the district. The plain extending from Gisborne in the direction of Patutahi and Ormond is of very recent formation, and is made up chiefly of the débris carried down to the sea by the Waipaoa and the other streams which fall into the bay. There have been some differentiated earth movements in the district, and these, along with the deltoid deposits, have produced the present plain and coast lagoons. When these changes began the bay was a deep arm or inlet of the sea, and the Waimata River ran through the Kaiti Valley, which at the time of high water was washed by the sea. The areas have been slowly

filled up by the process of denudation from the hills, and by means of the material which the rivers have brought down in the wet season. An imaginary line from the top of the Kaiti Hill to the hills behind the Hon. Mr. Carroll's homestead, on the Whautanpoko, thence across the lower Patutahi Hills, near Mr. Sutherland's, and onward to the hills up the Ormond Valley, joins together all the porous beds which at one time covered the whole of the intervening area. It will be noticed how great the changes in the land features have been since these contemporary deposits took place. I am in doubt whether the whole of the enclosed area forming the plain is one of denudation, and deposition or of depression and deposition. It may be there is a combination of the two, as there is evidence in support of each. The sinkings for artesian water that were carried out in several spots within the Gisborne Borough some years ago show a denuded surface only, but the wells at Matawhero and Makauri imply an area of depression, followed by one of deposition, from the material which the Waipaoa River has brought down and deposited over a deltoid area. If the plain was the outcome of denudation only there would be little hope of finding an artesian-water supply, and even as an area of depression the enclosed basin may be expected to be of small extent, and special care is needed in the selection of a suitable place for putting down a trial bore. I have pointed out that the Waipaoa River passes across the trough of the porous beds which are to be found in different places-within the boundaries of the district. These beds are absent, except the upper very recent ones, within the limits of the Gisborne Borough, and this is the reason why water was not met with in the trial bores that have been put down. The beds that dip under the town are those belonging to the Kaiti beach series, and they are mainly made up of clay, and are impervious to water; besides, the outcrop of the beds cannot possibly have more than a tithe of the quantity of water needed for the supply of the town, as the area of drainage of the outcrop beds is too limited in extent. It is quite useless to attempt sinking for an artesian supply within the limits of the borough—at least, I could not recommend any one to do so, because the money would be wasted, as there is no real artesian basin in the immediate vicinity of the town. But within certain limits the prospects of obtaining an artesian supply are good, the limits being within what appears to me as the area of depression to which reference was made above. This area is not of great extent, but it is sufficiently large to allow of a fair number of wells being put down should the water be suitable for domestic purposes. The line of depression extends in a line from. Ormond to the most easterly

point of Young Nick's Head, and passing through Waerengahika, Makauri, Matawhero, and thence to the sea-beach. On either side of this imaginary line a. trough exists, and I have no doubt artesian water would be obtained from it. Already tests have been made, as in the wells at Makauri and Makaraka, and with proper appliances wells might be put down of sufficient bore to provide an ample supply for the wants of Gisborne. The best place for putting down a well would be near the native settlement, towards the sea-beach, in a line between the cape and Matawhero. I am unable to say anything as to the quantity or quality of the water likely to be obtained. A knowledge of these can only be gained by experience. The Waipaoa River is not a shingle-carrier like the rivers of Hawke's Bay, and I do not expect a flow equal to what is obtained in the Hawke's Bay district, nor do I suppose that the water will possess the same general qualities. But these matters can readily be put to the test at comparatively small cost. A trial bore, with a 2in. pipe, should be made by the local governing authorities, such as the County Council and the Gisborne Borough. The work could be done in ten days or so, and the information gained would provide information for the district, and it would set at rest the possibility of supplying Gisborne with water of a quantity and quality suitable for the needs of the town. As my description of the district is written in a popular form, and free from technicalities, I have omitted sections of any kind, as the account can be readily followed by any one having even a slight acquaintance with the district.