Art. L.—Descriptive Geology of the District between Napier and Ruapehu Mountain viâ Kuripapanga and Erehwon.

Transactions and Proceedings of the Royal Society of New Zealand, Volume 22, 1889, Page 422

Art. L.—Descriptive Geology of the District between Napier and Ruapehu Mountain viâ Kuripapanga and Erehwon. By H. Hill, B.A. [Read before the Hawke's Bay Philosophical Institute, 7th October, 1889.] The geological character of the country between Napier and the extinct volcano Ruapehu, viá Kuripapanga, has not, as far as I am aware, been published, nor do I think that route has been traversed by any one connected with the Geological Survey. In January of the present year, in company with Messrs. Petrie and Hamilton, I visited Ruapehu for the second time, and the following passing notes were taken during my journey by this route:— The road might be described as running due west from Napier along the parallel of 39° 30', and for all practical purposes it might be taken as being on the same parallel as Ruapehu and Egmont, the two highest and largest extinct volcanoes in New Zealand. The distance between Napier and Ruapehu by road would be about 110 miles; but as the crow flies the distance is about seventy, Ruapehu being about midway between Napier on the east and New Plymouth on the west of the North Island. As far as Kuripapanga, fifty miles from Napier, the drainage belongs to the Hawke's Bay river-system; but beyond this the chief drainage is into Cook Strait and the South Taranaki Bight. The country between Napier and the Erehwon plateau resembles an inclined plane, through which run river-valleys transverse to what was at one time a great plain of denudation for most of the East Coast country, extending from Poverty Bay southward as far as the Wairarapa Valley. Generally it may be said that the road to Ruapehu and the volcanic district south of Lake Taupo by way of Kuripapanga passes through the country drained by

the Tutaekuri in its lowest and middle course, the Ngaruroro in its middle and upper course, the Rangitikei in its upper course, and the Whangaehu also in its upper course. Between Napier and Woodthorpe, situated fifteen miles up the Tutaekuri River, all the hills are composed of limestones and marls corresponding to the upper division of the Napier series. The river-terraces which are met with between Puketapu and Woodthorpe correspond to the Kidnapper pumice and conglomerate beds. At the latter place the Kidnapper beds have large exposures, and they occupy the hills to a height of more than 400ft., and in all cases blue clays and marls are seen to underlie the terrace-beds. From Woodthorpe the road passes for some distance through the bed of the Tutaekuri River, the banks of which are very high, and are composed of blue clays and marls on either hand. Overlying the marls are shingle-deposits of various thicknesses, mixed in places with thin bands of pumice-sands and clays, whilst the character of the surface of the old plain of denudation, at the top of the clay-marls, is clearly seen throughout the whole of the valley. In two instances the exposures show pumice-bands, which form such a characteristic feature of certain of the blue clay-marls over a great extent of country along the east coast. In places the blue clays are crowded with fossils, whilst in others they are few and very brittle, which makes it difficult to collect good specimens. The following were among the more common of those seen in this portion of the district: Dentalium giganteum, Natica (?), Ostrea edulis, Pectunculus laticostatus, Anomia (?), Waldheimia lenticularis. For some distance after the road leaves the bed of the river the uplying lands and river-terraces, through which pass several deep transverse creeks, show exposures of the same clays and shingle-deposits, whilst here and there traces of limestone are met with, and limestone is also seen to top all the highest hills which form the watershed of the smaller tributaries on the right and left banks of the river. As far as Waikonini the country presents little or no change in the rocks from what may be seen a hundred times exposed in the river and streams near Woodthorpe; but from this point new physical aspects present themselves, as the plateau which is now reached, and which is known as the Matapiro and Aorangi Plain, shows limestone outliers in the small hillocks which now and again appear by the roadside. The surface-soil appears to be composed mainly of grit, pumice, and scoria, which between here and Karioi, situated to the south of the great Ruapehu, takes the place of the fine porous pumice which is found over the country further to the eastward. This plateau appears to have been formed by the breaking up of the limestones when the elevating forces caused the

chain of the Ruahine Mountains to appear through what was at the time a limestone country, extending from the east to the west coast. At the time when this elevation was in progress, denudation was great, and the fractured limestones were much worn by the shattered and indurated slates, which had probably been subjected not merely to great pressure, but also to heat-zones, such as might be expected at the beginning and during the progress of the volcanic period which caused such marked and extensive changes in the physical aspects of the North Island. These limestones correspond, and, in fact, are identical, with the Maunga-haruru limestones as seen so well exposed in the great scarp at Te Waka, on the Taupo road overlooking Pohui. They have, however, been so worn, and ground, and powdered within the area under notice that only in isolated spots one sees—just as one sees on the flanks of the Whakarare Mountains, in the Kereru district, further south—the remnants of an extensive limestone range, of which few traces remain, so great was the denudation which succeeded the earth's movements referred to above. In the Otaharuru Creek, at Glenross, near the fellmongery of Mr. Williams, lenticular limestones appear, interbedded with the blue clays and clay sands, as seen near the crossing. Terebratula, Ostrea, and Anomia shells are common, and in several places broken portions of Pinna shells were also numerous. From this place the road passes over the hills to Mount Blowhard. Many of the exposures by the way show limestones, but as the top of the range is reached fossiliferous sandstones appear. The sandstones over a large extent of the elevated country have comparatively recently become quite denuded of vegetation, the fires which raged over the country for the destruction of fern and scrub having destroyed all the roots which bound the soil to the decomposing rocks. The bared sandstones provide an excellent collecting-ground for fossils, as in many places they are crowded with specimens of coral in an excellent state of preservation. At a spot where the road passes the joining saddle of the range some very remarkable limestones appear. They resemble a series of organ-pipes, are fluted like them, and arranged in columns and tiers as in an immense organ-chamber, presenting a very grand and imposing appearance, the concave and convex flutings being exceedingly perfect. These flutings have no doubt been caused by chemical and aerial agencies: it seems to me, however, somewhat difficult to account for the perfection of so many hundreds of flutings, with such regular concave and convex surfaces, by chemical means alone, as in texture the limestones to all appearance were the same, and the concave and convex surfaces appeared equally hard.

It is difficult, somewhat, to trace the relationship between the limestones and the sandstones, but possibly they pass one into the other, and are the corresponding beds to the fossiliferous grits and sands as seen when approaching Te Hauroto from Mohaka, on the Taupo road. The Te Hauroto limestones, however, which overlie, and in places pass into, the grits, abound with fossils; but, as already remarked, no fossils were noticed in the fluted limestones such as were exposed on the top of the saddle near Mount Blowhard. Between this place and the top of the next hills, which really separate the basins of the Ngaruroro and Tukituki Rivers, the country is somewhat broken, but there are few exposures in the roadside. A little to the southward great scarps are seen facing to the westward, and overlooking the valley in which Kuripapanga Hotel is situated. Such rocks as are exposed in the cuttings by the roadside appear to be of a tufaceous character, brownish in colour, and almost identical with tufaceous rocks found on Ruapehu at a height between 5,000ft. and 6,000ft. Although somewhat porous in texture, it is a difficult rock to fracture, and appears to have plenty of wearing-power. The soil covering the hills is of a reddish-brown colour, with pumice grits and pebbles. Here and there over the surface of the ground are white-quartz pebbles, well rounded, and generally in small heaps of a dozen to a score. These pebbles, Mr. Hamilton informs me, are moa crop-pebbles; and if such is the case it would seem that moas not so long ago must have been extremely numerous in this district. It might be mentioned in support of this that many moa-bones have been found scattered over the country between Mount Blow-hard and Karioi, a small settlement a few miles south of Ruapehu Mountain; and as the country becomes better known and caves explored no doubt evidences of the moa will be largely increased. From this place to Kuripapanga, which stands upon the Ngaruroro River, the country is very broken, and there are accumulations in places of pumice pebble-deposits, which would seem at one period to have choked up all the old water-ways and formed plain-terraces of rather large extent. The terraces on this line of road begin on the same longitude, or very nearly so, as on the Taupo-Napier Road, where pumice sands and pebbles are first met with as terrace-deposits near the Mohaka Bridge; and after the Taurangakumu Hill has been crossed the terraces are traceable right on to the Kaingaroa Plains. At Kuripapanga the pumice-deposits fill up the valley, and overlie coarse bouldery shingle of a semi-conglomerate character. Underlying the shingle on the left bank of the Ngaruroro are blue clay-marls

full of bivalve shells, a Kellia predominating. In the bed of the river near the bridge is an excellent section showing the fossiliferous clays resting on the slates, which for the first time make their appearance in this direction, although some distance above and below the bridge the slates are the only rocks exposed on either bank of the river. I remember that a similar section, showing clay and sands resting on the slates, is met with in the right bank of the Tukituki River at a place known as the Khyber Pass, on the flanks of the Ruahine Mountains. There the slates and blue clay-sands meet, the latter at their junction with the slates presenting the appearance of the bow of a boat. On the right bank of the Ngaruroro, at Kuripapanga, large deposits of pumice are found for at least 200ft. above the river, and there are no traces whatever of the blue clay-sands in this direction. The high hills between Kuripapanga and Ohauko scarcely change in their rock-characters the whole way, except that on the former side of the hills the slates show cleavage-structure, whilst on the Ohauko side they seemingly run into sandstones and present the appearance of having been affected by igneous agencies. Traces of pumice are to be met with over all the hills in this direction, and in places where pockets have been formed it is many feet in depth. About midway between Kuripapanga and what is known as the Taruarau Spur the blue clays reappear, and are overtopped here and there by bands of lenticular limestones. These clays and limestones cover a large part, so it would seem, of the Ohauko plateau; but the pumice which covers the surface diminishes greatly the value of what otherwise would be a rich district. The clays are fossiliferous, but the limestones, though composed mainly of broken shells, do not provide any specimens which it is possible to identify with certainty. As the road leads into the Taruarau River the pumice-terraces reappear, and they overlie the marls on the left bank and the slates on the right just as they do at Kuripapanga. On the latter bank the pumice is very deep, and large trees of charcoal, with branches and roots intact in some cases, are exposed in the pumice, and present an appearance as if a forest had been suddenly submerged by a vast deposit of hot pumice, and ignition had taken place as it does in the ordinary process of charcoal-manufacture. As at Kuripapanga, there is no trace of marls or limestones on the right bank of the river; nor do these rocks again appear until towards the top of the ascent of the Taruarau Spur, where several blocks of limestone are seen at a height of about 2,600ft., or about 450ft. below the top of the hill known as Otupi. On Otupi the rocks resemble those which flank the western side of the Kuripapanga Range, except that they are harder and of a somewhat deeper blue. When

broken the grain is coarse and the fracture irregular, with extremely sharp edges. Between this place and the Rangitikei River, near Kelly's accommodation house, the distance is nine miles. The country is bossy, and is a plateau varying between 1,800ft. and 2,500ft. in height. Exposures by the way-side show limestone scarps, and in several places the scarps show the dip of the rocks to be towards the S.E. and S. at a-low angle. But the dip, as might be expected in a district which has been so much affected by earth-movements and volcanic explosions, varies very rapidly as one passes along the plain, and little value can be set upon the dip of the rocks in this locality. When nearing the Rangitikei the road descends rapidly towards the deep, narrow valley through which the river has cut its way. On the left bank, below the crossing, great scarps of blue clay-marls overtopped with limestones are seen, and they appear to be similar to those at Ohauko, and which disappear in the left bank of the Taruarau Stream, as described above. Towards the top of the Rangitikei Hill, by way of the newly-formed road leading to Erehwon, coarse limestone blocks are exposed by the wayside. These contain rather large-sized pebbles and many fragments of bivalve shells, of which Ostrea ingens and Pectunculus laticostatus could be distinguished. In this hill slates and fine and coarse sandstones are found, the latter being in connection with the limestones. At the top of the hill the Upper Patea district is reached, and limestones again top all the hills in the whole of this somewhat extensive plateau, of which Erehwon is the centre. The height of the limestones would vary from 2,700ft. to 3,000ft.; but on the margin of the volcanic basin, towards the north-west, limestone is found in the scarps of the hills which overlook the volcanic basin at a height of not less than 3,500ft. above sea-level. At Moawhanga, a native settlement five miles or so beyond Birch's homestead, the river has cut out for itself a deep channel in the blue clay-marls which are largely exposed in the whole of the valley. These clay-marls are very fossiliferous in certain places, and sometimes concretionary limestones are found interbedded with them. The same fossils were collected from them as from the blue clay marls in the Tutaekuri River already described. From Moawhanga to Mapouriki, on the Whangaehu River, five miles or so due east of Ruapehu, the distance is twenty-two miles. For about half the distance the country partakes of a limestone character, the limestone being interbedded with calcareous sands, which are fossiliferous, but the shells are so broken that it is difficult to give with certainty their specific names, except Ostrea edulis, Pecten triphooki, Gardita zealandica, Hemimactra elongata, Anomia(?). On the border

of the volcanic basin the limestones present a series of remarkable scarps in the direction of Ruapehu, and it would seem as if the limestones had at one time covered the country now occupied by volcanoes and volcanic products. Our party pitched camp at Mapouriki, and spent several days in the vicinity of Ruapehu and the Rangipo Desert. The whole of the volcanic district had been visited by me two years before, and I had ascended Ruapehu on the western side for a height of nearly 6,000ft. From Mapouriki no attempt was made to get to the top of the mountain, but our party, if I may say so, went into it. Ruapehu is an immense truncated cone, whose crater has been blown out on the eastern side. There now remains a kind of amphitheatre, with perpendicular walls on three sides not less than 1,200ft. in height. In the middle of this vast cinder-heap stands the neck of what was at one time a volcanic vent. From this isolated pedestal, the top of which is more than 7,000ft. above sea-level, an observer commands a full view of the everlasting snow-fields, and can see the sources of the Whangaehu, separated from that of the Waikato by a low saddle. The grandeur and sublimity of the scene, combined with its awful weirdness and desolateness, need not be described here. My purpose was to see the rocks, to collect specimens, and to observe the effects of volcanic phenomena. I had now traversed the whole of the volcanic district, and it will be my aim to deal with the question of volcanic phenomena in a second paper. From what has already been stated with regard to the rocks between Napier and the volcanic basin, it will be inferred that no rocks of the younger Secondary formations are met with, nor even of the Lower Tertiaries. The last appearance of these is between Te Aute and Waipawa, so that they do not even come within the range of the district under notice. The blue clays and marls are similar to those so common along the east coast, more especially in the northern part of Hawke's Bay, and which in a former paper I ventured to term Miocene. The limestones between Moawhanga and the volcanic basin are younger than most of the other limestones, and are closely allied to those of the Pukekuri Hill, near Taradale, which contain sandy clays, and belong to the Petane or Napier upper series. The other limestones are intermediate between these and the clay-marls of the Miocene rock. The Woodthorpe beds are the youngest of the Pliocene rocks, and have their representatives over a large extent of country eastward of the Ruahine and Kaweka Mountains. The pumice-terraces appear to belong to the same period of deposition as the Woodthorpe beds. The slates I am doubtful of, as, although carefully sought after, no traces of fossils

have yet been met with in any of the slate rocks of this district. The members of the Geological Survey have classed them as Palœozoic, and the name has been retained here, though its meaning is so wide and indefinite. Addendum.—Since the above paper left my hands I have again visited the volcanic district by way of Kuripapanga, and I find that the sands referred to as being seen at Blowhard underlie the limestones and pass into streaky clay-sands, which are the upper beds of the slates. In two places near Blowhard Secondary rocks appear similar to those seen at Kuripapanga. On the hill known as “Gentle Annie,” beyond Kuripapanga, there is to be seen on the roadside a large limestone boulder. This rests on the slates, and is, I suppose, a remnant of the limestones which once covered the whole of the hills in this district. The highest limestones appear about midway between Erehwon and Ruapehu, at a height of 4,720ft. They rest on the Secondary rocks, and are full of fossils, mostly bivalves.