The dominant Harbour Cone was not actually a volcano, erupting ash and lava from a vent at the top, as is commonly believed. It was actually part of the much larger Dunedin volcano that formed the Otago Harbour. The Otago Harbour volcano began erupting around 16 million years ago in three main phases and was approximately 1000 metres high. Its form is called a “shield” volcano that is built up from fluid lava flowing down its gently sloping sides. The Dunedin City area, Otago Peninsula and Otago Harbour were all part of the volcano. The sea has drowned the lower slopes of the volcano.
The conical shape of Harbour Cone is the result of being a small protective cap of very hard rock at the summit from the Dunedin Volcanic Group. Columnar basalt has formed around the summit of the Cone from pyroclastic flows and these are still visible today. The high points of Peggys Hill and Harbour Cone consist of hard basaltic lava flows, which have protected the softer rocks underneath from erosion. There are several other basalt outcrops, recognised in the landscape as rocky knobs. Most of the Hereweka property has rocks that belong to the initial phase (based around Portobello and Port Chalmers), while Peggys Hill has rocks of the first main eruptive phase, and Harbour Cone has a small cap of rock from the second main eruptive phase.
Much of the property is underlain by volcanic ash and rubble, which weathers readily to clay-rich materials. This clayey rubble, together with the loess (wind-blown silt) which also mantles the Peninsula, is responsible for the widespread landsliding. This is particularly noticeable along the “winding section of Highcliff Rd adjacent to the current woolshed. Very few parts of the property are free from this hazard.
There is also a small area of older sedimentary rock which lies beneath the Dunedin Volcanic Group. This includes the limestone which was quarried on Sandymount Road. It occurs only in this part of Otago Peninsula and across the harbour on the way to Aramoana. On the Akapatiki Block, it is well exposed near the historic lime kilns, on the north side of Sandymount Road. This limestone is a hard rock that forms small bluffs, but the Burnside Mudstone that lies immediately beneath it is more likely to contribute to local landslides.
On the lower slopes and in the stream valleys is the clay-rich material, often with large basalt boulders embedded, that has been slumping down the slopes for thousands of years. This is termed colluvium; it tends to form a slurry when saturated and is very prone to slope failure. Removal of forest has probably accelerated this natural process. Planting or retention of forest, and careful disposal of runoff from hard surfaces, are the best ways to reduce the amount of landsliding and sedimentation into streams.
The Hereweka/Harbour Cone property contains five patches of indigenous forest that are ecologically significant to the property and the wider Otago Peninsula. Two of these patches occur on Harbour Cone, two in the upper part of the Stewart’s Creek catchment and one on the northeast face of Peggy’s Hill. Additional stands of kanuka tree land and forest occur in the northern part of the property, and are adjacent to more extensive areas of indigenous forest on adjoining properties. Smaller patches of broad-leaved forest and shrub-land are also scattered across the property.
The most comprehensive previous assessment of the vegetation of the entire Otago Peninsula was carried out by Botanist Peter Johnson, commencing in 1976 and reported in 1982. The work of Johnson was re-surveyed for the Hereweka/Harbour Cone property in 2007. The re-survey found that 113 indigenous vascular plant species are present on the property, representing 30% of the total recorded for the entire Otago Peninsula. This indicates high species richness given that the property constitutes just 3.4 % of the land area of the Peninsula. Two plants found on the property are regarded as nationally threatened, and these Raukaua edgerleyi and the wetland herb Epilobium chionanthum, both with the threat status of ‘Gradual Decline.’ Several other species are significant in a local sense, occurring elsewhere on the Peninsula in only one or two sites (Blechnum colensoi, Olearia bullata, Raoulia subsericea, Neomrytus pedunculata, Muehlenbeckia complexa ) and sometimes with only one or two individuals.
There are two major freshwater catchments on the Hereweka property. The central Smiths Creek catchment comes from two central gullies on the west-facing side of the property above Broad Bay and runs down into Bacon Street and out into Turnbull’s Bay. The Stewart Creek Catchment runs from a broad gully at the head of Sandymount Road below the old lime kilns and runs into Hooper’s Inlet, with narrower ephemeral run off from behind the rocky knob south of the Cone itself. Both are major catchment areas for the property and contribute significant habitat for native freshwater fish, eels and crayfish.
In Smiths Creek, three species of native fish have been observed, the Banded Kokopu (Galaxias fasciatus), Inanga (Galaxia maculatis) and Red-finned bully (Gobiomorphus huttoni). Both short and Long finned eels have also been recorded in previous surveys and are reported by landowners. In the Stewarts Creek catchment koura or native freshwater crayfish (Paranephrops zealandicus) have been observed along with short and long fin eels, Banded kokopu , Inanga, and the Red-finned bully. Long fin eels are endemic to N.Z and are classified as being in “Gradual decline“. The Red-finned bully, although not threatened, is also starting to disappear from our fresh water areas.
Significant restorative work is ongoing in the Smiths Creek catchment with fencing, weed control and replanting. This will have a significant positive impact on the opportunities for freshwater species on the property, by providing spawning habitat and shade to lower water temperature.
The birds of the Hereweka property are typical of those found in similar habitats in the Dunedin coastal area. The farmland is dominated by introduced species, while the native bush areas and regenerating kanuka fragments have a wider selection of native species. Fantail (Rhipidura fuliginosa) and Grey Warbler (Gerygone igata) can be found in both habitat types, while Rifleman (Acanthisitta chloris) and Brown Creeper ( Mohoua novaeseelandiae) prefer the larger bush remnants. Kereru (Hemiphaga novaeseelandiaeare) are relatively rare on the property, but over recent years Kereru sightings have increased on the Peninsula due to possum control and the presence of urban food sources. Tomtit (Petroica macrocephalaare) are also rare, probably due to the relatively small size of the fragmented bush areas on the property and wider Peninsula. However, the property maintains a good selection of well-known native species, such as Bellbirds, fantails and riflemen. The eastern edge of the property borders estuarine wetlands around the outlet of Stewarts Creek and this is useful habitat for Pukeko (Porphyrio melanotus) and Paradise Duck (Tadorna variegata).
While the Hereweka property does not have currently support large populations of native bird species the long-term restoration and enhancement of the bush remnants and riparian areas will provide greater opportunities for native species. Currently, the value of the property lies in its connectivity from private properties to wider open space within the Peninsula that allows bird species to move progressively from habitat to habitat searching for food and suitable breeding sites.
The slopes of Harbour Cone are typical habitat for New Zealand’s only noliid moth. The exquisite Celama parvitis (Noliidae) has larvae that are specialist feeders on the daisy shrub Helichrysum lanceolatum. This species has a patchy distribution in the eastern and central South Island and are rare south of Otago Peninsula. Endemic butterflies, normally inconspicuous in New Zealand in general are a feature of this area including:
- Red admiral Vanessa gonerilla has its main New Zealand stronghold on Otago Peninsula where its larvae feed on tree nettle Urtica ferox.
- Two species of orange-coloured copper butterfly – both undescribed – are abundant on Peggy’s Hill and Harbour Cone and elsewhere in the area in question. Both the glade and common copper representatives have larvae that feed exclusively on Muehlenbeckia australis or M.complexa. Both butterflies are locally common from late December till early February, and are confined to the greater Dunedin area.
- The tussock butterfly Argyrophenga antipodum is common in mid summer on Otago Peninsula over sand hills and pasture. The larvae have expanded their natural choice of native food to include exotic tall grasses and so it is now quite common in this area
- Tree nettle hosts an amazing insect fauna of moths, flies, weevils and bugs that are specialist feeders on this plant alone. Thirteen moths alone feed exclusively on the foliage and flowers; species such as Pasiphila urticae, Pseudocoremia pergrata (Type locality Sandymount), Udea marmarina and two Meterana species.
- A newly described flightless caddisfly species that is endemic to Otago Peninsula Pseudoeconesus paludis breeds in Otago Peninsula streams such as those in the property. It is endemic to Otago Peninsula and was only discovered a decade ago.
Generally the shrub, forest and wetland remnants left on Otago Peninsula support a nationally significant range of endemic invertebrates including two species of Peripatus, a high diversity of stick insects, and high species richness of moths, beetles, bugs and wasps. What is additionally significant is the number of day-active insects such as the elegant orange moth Cephalissa siria.