Notes on the Geology of the Poor Knights Islands
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NOTES ON THE GEOLOGY OF THE POOR KNIGHTS ISLANDS By J. J. SUMICH Location of columns formed by shrinkage cracks on cooling. The islands, two main ones, several smaller ones and (ii) On Aorangi, on the east side, forming a rounded several stacks, are situated 1.4M miles off the North spur which passes down to sea-level from the ridge at Auckland east coast at Whananaki, between Tutakaka the eastern side of Amphitheatre Bay. and Whangaruru. Some members of the party were able to visit the landing on Aorangi and report that there is evidence of Geological Associations lava flows there. The islands are composed of rhyolitic lavas, flow breccias and tuffs and they possibly represent the northern part (b) Rhyolitic Breccias of the tectonic line which further south is marked by These seem to constitute the main parts of the islands. rhyolite outpourings at Moko Hinau, Great Barrier, They form distinct strata which dip westward at several Cuvier and Colville etc. places on the eastern side of the island. On the western side of Tawhiti Rahi the cliffs are high (300 to 400 feet) Topographical Descriptions and some beds along this section strike east-west and dip The group is strung out in a line which runs approxi north. mately north and south. The largest island, Tawhiti Rahi, which is about 1M miles long by. about % mile wide at (c) Rhyolitic Tuffs and Fine Pyroclastic Deposits its widest point is the, most northerly. Between this These are found on the southern end of Tawhiti Rahi, island and the other main one (Aorangi), there are about on some islands between the two main ones, and also four small islands and a few rocks varying in size. Some in a bay on the western side of the north-west tip of are high and most are vegetated. Aorangi is about a mile Aorangi. There are also scattered deposits at the southern long and % mile wide. South of Aorangi are two smaller end of Aorangi and on Mammoth Island. The pyroclastic islands, Mammoth Island and Aorangaia. Both are about material is usually fine grained and has a characteristic M mile long and about 100 to 200, yards wide. Some 5 pale creamy colour; the beds are poorly compacted and miles south of the main group is a group of stacks which in the process of being rapidly eroded by wind and salt obviously belong to the same formation and bear the spray. name of the Poor Knights Rocks. The islands are, without exception, steep to precipitous with cliffs from 50 feet to 400 feet high; depths of water GENERAL GEOLOGY immediately off-shore are at least 20 fathoms. At some time during Miocene Period of the Tertiary Era On Tawhiti Rahi the highest point is in the vicinity of volcanoes existed at or near where the islands are today. 600 feet and is situated near the centre of the island. Periods of quiet outpouring of lava seem to have been From this point there is a plateau stretching northwards infrequent. For most of the time the volcanoes were in for about li mile. On the margins of the plateau ridges a state of violent eruption as evidenced by the abundance run down to the sea in all directions except southwards of pyroclastic material. On topographic evidence there where a long spur forms the northern third of Tawhiti seem to have been several craters, but their sites cannot Rahi. On this island streams are almost non-existent but now be located precisely. From the dip of the breccias one or two seepages were located. the crater appears to have been located to the east of Aorangi has a highest point of about 500 feet which the islands. is situated near its southern end. From this point two One exception is the amphitheatre at the northern end ridges extend northwards and eventually form the dual of Aorangi. This is apparently part of a crater which northern points or horns of the island. Within these may have been far more extensive than appears today. ridges there is a huge amphitheatre which gradually It is possible that the crater walls extended through the slopes down to the sea to make ā rock shelf land area islands to the east coast of Tawhiti Rahi, almost as far on the island's northern shore. From the two northerly as a spur which runs into the sea halfway up this coast. leading ridges and from the peak, minor spurs extend The existence of tuffaceous and line pyroclastic material outwards and terminate in cliffs at the water's edge. In supports the possibility of a final explosion which de one or two places spurs of rounded profile run down stroyed the initial outline of this crater. It is not clear to the sea. whether the crater was then invaded by the sea or whether the sea has subsequently eroded its way through LITHOLOGY the wall. Erosion on the western side must have been less than on the more exposed eastern side. Nevertheless, (a) Rhyolite larva flows on the west side there is no shallow water in-shore, while These could only be identified positively in two places: on the eastern side there are several places where reefs (i) On Tawhiti Rahi on the west side, on Pimple Ridge are evident. It is suggested that extended viscous rhyo and extending up to the summit. The pimple is built litic lavas froze rapidly and formed the typical convex 64 outlines which later were mantled by pyroclastic debris. such a sea cave is about 100 feet high and runs in There has been a good deal of local faulting in the about 300 feet. In another place these arches and caves flow rocks and this has resulted in many sea arches, have formed a complicated series of tunnels and funnels blow holes and sea caves around the coast. In one case where the roof has collapsed. "•:n? 1 A BIRD CENSUS ON TARANGA (HEN ISLAND) By SOLITA A. KING The aim of this work, during a visit to Taranga Island suggests that territorialism is not restricted to the breed• from May 7th to May 14th was to count the number of ing season only. birds present in approximately 25 acres of coastal The silver-eyes flew around in bands of half a dozen forest. or more, often with several fantails in attendance. The Since the census was made during the winter, birds fantails were occasionally seen in pairs but more often were usually observed singly. However, in several singly or associated with the silver-eyes or saddlebacks. instances, e.g. saddlebacks, pigeons, pied tits and pipits, CENSUS OF LAND BIRDS the birds were seen in pairs. Song The island lies 10 miles east from the mainland and Observed Heard Total approximately 11 miles north-west from Bream Head. Blackbird Terdus merula 1 1 ' 2 Its steep rock terrain is composed mainly of andesitic Saddleback Philesturnus caruncu- breccia; the highest pinnacles rise to about 1420 feet latus rufusater 10 8 18 and in many places the land slopes steeply to the coast. Bellbird Anthornis melanura 21 5 26 The census area covered comparatively level land Tui Prosthemadura novae-seelandiae 12 , 3 15 lying at the foot of the steep escarpment. It had a Silver-eye Zosterops lateralis 12 12 south-westerly aspect and was flanked on both sides by — Pied fantail Rhiphidura flabellifera 13 7 20 steep ridges. White breasted tit Petroica toi toi 3 1 4 The vegetation is of a northern rain-forest type, grad• Grey warbler Pseudogenygone igata 3 2 5 ing from tall spindly (30 feet) kanuka and pohutukawa Kingfisher Halcyon sanctus 2 2 to the taller and more spreading kanuka, taraire and — Red fronted parakeet C novae- puriri trees with bush lawyer and supplejack. Nikau zelandiae 8 8 plants of all ages were common. — Brown haka Nestor occidentalis 1 1 The area, stretching approximately 100 yards from — N.Z. owl (Morepork) Ninox the coast, was covered daily by repeated traverses. In novae-seelandiae 2 2 places it was necessary to stop for several minutes to — Harrier Circus approximans 1 1 identify and to take a full count of all the birds that — N.Z. Pigeon Hemiphage novae- were singing or feeding in the tree canopy. zeelandiae 6 6 After the first day or so it was found that most of — the birds could be identified by their song, except for TOTAL 122 the tui and bellbird: hence an accurate count of these birds was not possible. The total number of land birds (14 species) found in There appeared to be a slightly higher concentration this area was 122, which using the unit proposed by the of species in the taraire-nikau vegetation than in the British Trust, and used by Turbott (1940), represents a coastal karaka-pohutukawa. This may be due to the density of 500 birds per 100 acres. greater variety of food offered by the more typical rain• The work done during the camp was baseu on the forest vegetation. previous census taken by Turbott during the breeding Two separate flocks of starlings were seen flying inland season in 1939. The density he estimated was 728 birds together late one afternoon. This was probably a tempor• per 100 acres. ary visit from the mainland. The harrier seen flying over this region should probably be regarded as a visitor REFERENCES from the mainland. Oliver, W. R. B., 1955. New Zealand Birds. Second edition. Recognisable pairs of saddlebacks, pied tits and Turbott, E. G., 1940. Bird Census on Taranga.