The Industrious Heart A History of New Plymouth / 18


APPENDIX A New Plymouth-A Comment from an Earth Science Viewpoint by V. E. Neall, B.Sc. (Hons.) Ph.D., Department of Soil Science, Massey University, Palmerston North Long before the first Europeans set foot in Taranaki, local Maoris had discovered oil seeps on the foreshore near the future settlement of New Plymouth. Over the next 150 years the continued search for oil and natural gas reserves was to become one of the notable features that accompanied the European settlement of the Taranaki Province. The first settlers to arrive established a thriving settlement where the distal slopes of the ancient Pouakai Volcano dipped to the sea. Here was a safe anchorage that was to become the future harbour alongside the Sugar Loaf Islands and the coastal monolith of Paritutu, rocky vestiges of a former volcano now largely denuded by the sea. Preserved within the adjacent coastal cliffs chaotic accumulations of volcanic debris testify to the vigorous volcanic processes that once built these edifices. Blocks of andesite mobilised by past volcanic activity became admixed with sand and mud to flow as turbid submarine currents, which contain occasional marine fossils at Mikotahi. It was then probably during the last stages of volcanic activity at this centre that viscous lava rose to solidify as plugs that we see today with the ancient crater. Not each' Sugarloaf is a single lava plug, but rather each is part of a group of extrusions, the one exception being Paritutu which is preserved in its entirety. This origin is supported by evidence of tiny crystals within the lavas tending to form concentric ring patterns. Unfortunately some of the best exposures on the foreshore have since been covered by fill in the construction of the New Plymouth Power Station. The land bordering New Plymouth to the south largely comprises surfaces related to the Pouakai Range. About 220-250,000 years ago the Range was part of an active volcano of comparable size to the present Mt Egmont. The volcano created large volumes of andesitic debris which were spread across the landscape as gigantic mudflows to bury older sedimentary strata. The result was the creation of a landscape of circular outline that rose from a near planar surface near the coast to gradually steepen to the high slopes of the volcano. The lower two-thirds of this landscape is referred to as the ring plain. Ring plain formation accompanying volcanic activity at Kaitake, Pouakai and Egmont has been largely responsible for the formation of the present Taranaki Peninsula that we see today above sea level. The deposits of the initial period of ring plain formation are rarely exposed, mainly in deep stream channels and road cuttings mainly between Lake Mangamahoe and Inglewood. Egmont, Upland, King and Hursthouse Roads traverse ancient segments of this ring plain to the north of State Highway No.3. After this initial period of volcanicity, erosion seems to have cut deeply into this landscape removing much of its former extent. A second period of ring plain construction then led to deposition of a wide sector of the existing Pouakai ring plain especially from New Plymouth to the southwest, along the coast to Okato. The mud flow deposits that resulted are well exposed in the coastal cliffs and near Oakura evidence is seen of the forces involved in the emplacement of the mudflow deposits. Siltstone blocks have been ripped from the underlying sedimentary formations to be incorporated within the basal parts of the mudflows. Since the Pouakai ring plain was constructed, the earth has undergone a number of climatic fluctuations. During warmer periods in the past the sea was at or above present sea level. Such high sea levels combined with gradual tilting and uplift of the land led to the formation of the marine benches which cut into the older Pouakai volcanic surfaces which now extend from the centre of New Plymouth northeastwards. In the heart of the city a narrow marine bench which is part of the Ngarino Terrace is preserved. The cliff cut by this high sea level forms the prominent scarp running through New Plymouth, from near the port, past Devon Intermediate School and just to the north of New Plymouth Boys High School. East of the Waiwakaiho River the bench is much wider, its inner margin extending 8km inland of the coast. Its outer margin is then terminated one km from the coast by the younger Rapanui Terrace. On the northern outskirts of New Plymouth the cliffline between these two terraces is obscured by large sand dunes which accumulated between Bell Block and the Waiwakaiho River as the sea retreated. Resting directly on these marine terraces are thin deposits of marine sands which were deposited on the marine cut bench as the sea retreated. Upon aerial exposure, lignite deposits accumulated, which in places contain much wood. Overlying the lignites are terrestrial lithologies comprising largely volcanic mudflow deposits, sands, loess and volcanic ash. Beneath some of the mudflow deposits are well preserved buried forests, which are frequently encountered around Bell Block in deep drain laying operations, as well as at Airedale Reef near Waitara. Ten kilometres to the south-south-west of New Plymouth lies the Kaitake Range, a volcanic centre that was active about half a million years ago, and thus of intermediate age between Pouakai and the Sugar Loaf Islands. The upper parts of the Kaitake Volcano have been largely removed by erosion and former remnants of the ring plain are difficult to identify because they have been largely buried by younger mudflows from Pouakai that completely encircled the Kaitake Range.

The deep inner core of the volcano is now exposed in a few streams that radiate from the summit point of Patuha, and these show highly altered rocks which resulted from percolating hydrothermal solutions when the volcano was last active. These rocks contain trace amounts of the more precious metals and the search for more substantial deposits in the Range began about 110 years ago. Between 1868-1870 a group of prospectors worked in the Boar's Head Creek in the hope of making their fortune. A company was established, the Perseverance Company, and 2Vz tons of quartz were crushed but it yielded no gold or other precious metals. Prospecting in Ahuahu Stream in 1888 also produced no promising results. Then in 1898 a Captain Capel continued work in one of the old shafts in the hope of encountering a silver lode. In a report of a visit to these working by N. D. Cochrane, no gold or silver was found from Capel's workings but other lodes in Ahuahu Creek yielded some small amounts.
In 1909 it appears the Director of the Geological Survey, Dr J. M. Bell, visited workings in the Kaitake Range, probably in Ahuahu Stream. A Mr P. F. Hughes had been prospecting and had sunk a shaft to 12 m depth and then excavated eastwards for about 15 m. It appears the prospector was following a thin seam of silicified andesite with much pyrite and gypsum. An assay showed traces of silver and no gold, but a sample from earlier workings by Dr Bell yielded much more silver. Another prospect before the end oflast century was Godsal's Mine situated on the western slope of the Kaitake Range, about I km north-east of the Boar's Head Mine, but it was short lived. Although many samples have been collected from the Kaitake Range since, most have shown only minute traces of gold and silver which have never proved economic. It was only in the last 100,000 years that volcanic activity broke out at the Egmont centre. Most of the mudflows from Egmont were confined to southern and central Taranaki due to the protection Pouakai and Kaitake afforded to the north. Only two major rivers, the Waiwakaiho and the Waitara were large enough to pierce their way through the Pouakai ring plain to the north. Since their path was incised they have been responsible for guiding mudflows from Egmont towards the coast but few reached the northern coastline. However, the continued activity at the Egmont centre has been responsible for the volcanic ash which has been intermittently spread across the north Taranaki landscape on many occasions in the past. The net result was the accumulation of large volumes of ash in the New Plymouth district which are visible in most road cuttings. Some of the oldest ash deposits may have been derived in fact from the Pouakai Volcano. The ash is readily eroded and this accounts for the deeply incised rivers and streams around New Plymouth which have become entrenched down to the underlying volcanic mud flow deposits. The ash is up to 33 m thick at many localities and is characteristically a yellow-brown colour with strong prismatic jointing. Interbedded within the ash are dark-grey coloured layers which fret easily in road cuttings and are friable. These are buried soils within the New Plymouth Ashes, and in the larger road cuttings on State Highway 3, they can be seen to span the exposures paralleling the present ground surface. Thirteen buried soils have so far been recognised to the south of New Plymouth, the best exposures being north of the Oakura River. The minerals in the ashes and buried soils have been studied in detail from this area as well as from test drill holes made at the site of the Taranaki Base Hospital. Accompanying weathering of the ash, the more susceptible compo- nents, such as volcanic glass, break down to the gelatinous clay mineral called allophane. This mineral has little internal cohesion or shear strength and does tend to collapse when pressure is applied. This is why many vertical cuts in the ash seem to fail, and why the ash forms a poor base fill to many of the roads in the district. The very latest volcanic ash spread around Taranaki is colloquially referred to as 'Egrnont Ash'. It is in fact a sequence of ash showers from Mt Egmont that vary according to direction and distance from source. In the New Plymouth area the ash is largely Oakura Tephra that overlies Okato Tephra, both being less than 15,000 years old. On the southern outskirts of town, these ashes total 1.5 m thickness but thin to about 0.5 m along the New Plymouth foreshore. They are the main parent materials of the soils in the district, except near the coast where sands have blown inland. Most of the soils are New Plymouth loams, which are subdivided on the basis of their topsoil colours. Near the coast is a strip of New Plymouth black loams which originally had a scrub, flax, toi toi vegetation whereas inland, those soils that were once under forest display a brown topsoil. The warmer climate combined with these deeply friable soils, which have few limiting factors, provides one of the principal natural resources of the New Plymouth region. It will be the conflict of uses between urban and industrial growth on the one hand versus the value of these soils for food production (as market gardens or for dairying) on the other, which will be one of the principal decision making issues for the people of north Taranaki over the next 100 years.

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