Art. XLVI.—Artesian-water Prospects at Wanganui.

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

Art. XLVI.—Artesian-water Prospects at Wanganui. By H. Hill, B.A., F.G.S. [Read before the Hawke's Bay Philosophical Institute, 11th July, 1892.] Having a few days to spare at Easter, I paid a visit to Wanganui. My object was to form an opinion as to the prospect of an artesian water-supply for the town. In company with Mr. A. Atkins, architect, I examined Shakespeare Cliff, Castle Cliff, Landguard, and the area within what is locally known as No. 3 Line as far as Kaitoke Lake. The country extending in a north-west and north direction, as far as the Kai-iwi Stream, and the Kaiwaka stone-quarries on the Wanganui River, were also visited, including the cliffs along the sea-coast towards the mouth of the Kai-iwi. In addition to this formal inspection of the country I have consulted the following papers which bear more or less directly upon the geology of Wanganui and surrounding district:— (a.) “Geology of Wanganui,” J. Buchanan, Trans., vol. ii. (b.) “Geology of the Province of Wellington,” J. C. Crawford, F.G.S., Trans., vol. ii. (c.) “The Wanganui Tertiaries,” C. W. Pearce, Trans., vol. iii. (d.) “The Wanganui System,” Professor Hutton, F.G.S., Trans., vol. xviii. (e.) “Geology of Western Part of Wellington Province and Part of Taranaki,” James Park, F.G.S., Geological Reports, 1886–87, pp. 30, 31. Each of the papers named has some reference to the district under notice; and the geological sections that are given by Messrs. Crawford, Park, and Hutton to illustrate their several papers have enabled me to correlate the rocks within a radius of at least twenty miles of the Town of Wanganui towards the east, north, and north-west. Wanganui itself is situated a little east of the 155th degree of east longitude, and a few minutes north of the 40th parallel of south latitude. It stands upon the right bank of the Wanganui River, and about three miles from the sea in a straight line. The district between the town and the sea is made up principally of sanddunes. At the mouth of the river, on the right bank, shingle-

conglomerates appear, just as they appear at the mouth of the Wairoa River in this district. They pass underneath the sands, as no other shingle is met with along the coast in a westerly direction. Two miles or so beyond the mouth of the river cliffs of blue fossiliferous clays make their appearance, topped by light-brown sands and shingle-conglomerates; and these continue along the coast in an unbroken line nearly as far as the Kai-iwi Stream, at a varying height from 15ft. to 40ft. The clays have a general dip to the south-east at a low angle—of not more than 4° or 5°. Proceeding up the Wanganui River from its mouth, the following points of interest claim attention: (a) Landguard, (b) Putiki, (c) Shakespeare Cliff, (d) Kaimatera, (e) Upokongoro and Kaiwaiki. Landguard is on the left bank of the river, almost due east from the freezing-works at Castle Cliff. The rock-sections are well exposed, and they provide a key to the distribution of rocks over the whole of the Wanganui district. Freed, or exclusive, of the blown and moving sands which cover the flats and the hills from Putiki to the sea, the following sequence of rocks in descending order is met with: 1. Pumiceous swampclays of a brown or fawn colour. 2. Shingle-conglomerates. 3. Sand and clay interbedded. 4. Fine sands. 5. Pumiceous brown sands and clay. 6. Strong conglomerates. 7. Fossiliferous calcareous sands (many fossils). 8. Thin blue claysand bands. This bed is characterized by what may be termed a black-oyster deposit. Mr. Park, in his report, draws attention to the existence of a fault about 5 chains from the extreme west end of Landguard Bluff. This may help to account for the conglomerates exposed at Castle Cliff, as the displacement which took place would bring these beds, which dip at a low angle, to near the sea-level at this place. The Putiki sections, as far as exposed, agree with those seen at Landguard, near the shooting-target. (c.) Shakespeare Cliff: This important section is situated a few chains above the bridge leading from Victoria Avenue. The section presents the following sequence, read from above: (1.) Pumiceous swamp-clays of a brown and fawn colour. (2.) Pale-yellow sands and clays. (3.) Shingle-conglomerates. (4.) Fine calcareous sands, crowded with fossils, and passing into indurated shell-bands and lenticular limestone-bands in places. (5.) Blue sandy clays, with many fossils in places. Beds corresponding to 4 are finely exposed in a recently-opened quarry between the cliff and the bridge. Beds 1, 2, 3, are not conformable to 4 and 5; nor, indeed, does there appear to be any conformability between the pumiceous swamp-clays and the two beds immediately succeeding. Proceeding up the River Kaimatera, about eight miles above the bridge, Upokongoro, ten miles, and Kaiwaiki, eighteen miles, are the only other

places of special geological interest. At the two former the exposures show beds younger than the blue sandy clays in Shakespeare Cliff, and corresponding in their main characteristics with the higher beds seen in the hills along the left bank of the river from Wanganui to the sea. At Kaimatera the rocks are made up mostly of grits, gravels, and pumiceous sands, passing into fine powdery pumice in several places. At Upokongoro the rocks are made up of mixed clays, conglomerates, grits, and pumice-sands, and they present in their bedding many varieties of movement during deposition. Between Upokongoro and the Kaiwaiki upper and lower quarries I saw no traces of fossiliferous rocks except in certain sand-beds, which here and there make their appearance. At Kaiwaiki, limestone similar to the Napier upper limestone is being quarried on the right bank, and half a mile or so further up the river similar limestone is found at a much higher level, whilst from 10ft. to 15ft. above water-mark blue sandy clay-beds are exposed, corresponding, apparently, with the Shakespeare Cliff beds, sixteen miles or so lower down the river. The several important points of vantage enumerated above will make it possible to deduce some facts as to the character of the underlying beds in the vicinity of Wanganui. The oldest rocks that are exposed in the district belong to the blue-sandy-clay series. These are met with, as pointed out above, at Shakespeare Cliff, Landguard, and along the sea-shore between the mouth of the Kai-Iwi Stream and the Wanganui River mouth. No exposure of the blue sandy clays is met with between Shakespeare Cliff and Kaiwaiki, and all the rocks met with between these two places belong—so it appears to me—to the upper beds of the series. The blue clays, with the calcareous sands overlying them, sometimes form, as at Kaiwaiki, lenticular bands of limestone, and in one place they thicken out into a true limestone mass, and resemble the Napier upper limestones, as they appear at what is known as Scandinavian Point. The beds overlying these correspond to the Kidnapper pumice and conglomerate deposits, which, with the Redcliffe conglomerates, form a syncline underneath the Heretaunga Plain. Thus, all the rocks in the vicinity of Wanganui belong to a recent period, and they are the actual equivalents of the beds from which the people of Napier and surrounding district derive their artesian watersupply. I am not aware of the actual distance between the mouth of the Kai-iwi Stream and the Kaiwaiki quarries, nor of the height above sea-level of the river at the latter place; but from what has been stated it will be readily understood that the slope or inclination of the blue clays from the places named

to the south-east in the direction of Wanganui is small, as the blue clays exposed in the Shakespeare Cliff are not less than 30ft. high, and they are about the same height on the beach between Castle Cliff and Kai-iwi. If we suppose a line running along the top of the blue clays from the Kai-iwi mouth or the coast cliffs to the Kaiwaiki quarries, and another from where the clay-sands terminate on the beach towards Castle Cliff to Shakespeare Cliff, there will be actually represented the old slope or plane of denudation over a given area. On this area calcareous and fossiliferous sands were deposited, such as are now exposed in Shakespeare Cliff and in other places along the left bank of the Wanganui River as far as Landguard. In some places the sands have been denuded, and their places are now occupied by shingle-conglomerates and other succeeding beds such as have already been enumerated as being exposed in the several places along the bank of the river. Now, the area between Shakespeare Cliff and the sea, including the spot on which the Town of Wanganui is built, contains traces that it also was, at a comparatively recent date, capped with shingle-conglomerates and pumiceous swamp-clays. In the direction of St. John's there are shingle-conglomerate beds of great thickness, and they appear to be the remnants of a line of hills that once extended from Aramoho to the coast. Whether the area now covered by dunes and shingle-conglomerates is one of denudation or of depression I do not know; but evidence is in favour of a combination of the two. If the former be the sole cause of the changes, then we may be sure that the blue clays of Shakespeare Cliff are not far below the surface, and artesian water would hardly be expected or possible under such conditions. But if the area under notice is one of depression—that is, if the country between Shakespeare Cliff, Landguard, and the cliffs along the coast in the direction of the Kai-iwi mouth be one of depression—then the conditions exist of a catchment- or water-basin, and the prospects of an artesian water-supply are good. Every deposit covering the hills in the immediate vicinity of the town, and to the north-west and north of the district, is a great water-carrier, except the claybeds, and the general slope of the beds from the Kai-iwi Stream and the Kaiwaiki quarries is towards Wanganui. These are essentials to the existence of an artesian water-supply; but there is yet another essential—viz., the existence of a basin or a depression-like area having such an arrangement of beds that water may drain into them, and be focussed, as it were, to a given place, somewhat like the depression for the gravy in a good grill-iron. The lower bed must be impervious, or nearly so, to water, and it must be channelled or troughed somewhat like the blade of a long-handled shovel, as used by navvies. In other words, whilst there may be a

slope of beds in one direction, there must be a tilting along the edges of the beds in the opposite direction, or in the line at right angles to the flow. In the case of the area between Wanganui and the sea, the long-handled shovel referred to above will fully illustrate what is wanted. If the handle of the shovel be placed in the direction of Aramoho, and the blade in a sloping direction towards Redcliffe, then the sides of the blade will represent the cliffs along the sea-coast on the one side and those between Shakespeare Cliff and Landguard on the other. These will tilt inwards, and water dropped on any portion of the blade, or the handle, were it flat, will flow in the direction of Castle Cliff. Now, the clays extending from Shakespeare Cliff to the coast cliffs appear to be so tilted; but whether as a result of denudation only, or of depression, or of a combination of the two, it is—unless the testing-bore be used—impossible to say. That there has been a downthrow along the strike of the beds is certain, for Mr. Park truly points out that “about 5 chains from the extreme west extremity of Landguard a fault extends across the strike of the beds. The gravel-bed which occupies a position up the middle of the cliff is faulted down to the water-level, the displacement being about 50ft.” This may account for the dip, as seen in the cliffs on the sea-beach, where the conglomerates suddenly dip underneath the sands, and it is certain that similar conglomerates at Castle Cliff dip towards them, as if forming a syncline. Here, then, there is evidence of what appears to be a limited area of depression; but whether the area in the immediate vicinity of Wanganui itself was affected cannot be stated. These are all the facts which it has been possible to correlate concerning the character and arrangement of the rocks in the vicinity and district around Wanganui. The area of the probable water-bearing basin is a very limited one, and of irregular outline, and great care will be necessary in the selection of a proper site for a trial-bore, should it be decided to put down a well. My own opinion is that two trial-bores should be put down, if any attempt is made at all. A bore put down near the site of the Girls' College, and another one at or near the Asylum, would test the question, once for all, whether or not there is an artesian basin in the vicinity of the town. The depth for testing purposes should not be less than 130ft. or more than 250ft., and, if the area be one of depression, no doubt water will be struck within these limits.