Dunedin’s hills are virtually all of volcanic origin. Several eruptions occurred near Port Chalmers from 10-13MYA. The lava flows from this volcano and its many smaller flank vents extend in a roughly circular pattern, of some 15 km radius, over the sediments.
Subsequent erosion and subsidence have resulted in, among other things, the formation of the Otago Harbour, a gash that runs across the middle of the volcano, separating the island that is now the Otago Peninsula from the mainland. This gash tends in a north-east south-west direction, a direction similar to many of the faultlines of the basins and ranges of Otago and the uplifted Southern Alps. The island became the Otago Peninsula when a build-up of sand drifting up the coast from the Clutha River mouth joined it to the mainland.
The three peaks of Mt Cargill are all of volcanic origin. The Organ Pipes on Mt Holmes are one of the most spectacular landforms still visible. They were formed as a result of the period of volcanism. They were formed into their column shape during the cooling of lavas that flowed across the summit.
Until quite recently, the hills of Dunedin were covered in forests – the bulk of which were rain forest or southern beech forest. Large areas of these forests disappeared some time before European settlement. Repeated burning by the Maori was the major cause of this although the climate also became unfavourable for some trees. Tussock grassland now thrives where these forests once stood, with the Flagstaff Walk crossing a large area of the grassland. Successive fires in the area have prevented regrowth of scrub and forest.
Since whaling stations were set up on the shores of Otago in the early 19th century and in particular since original European settlement in 1848, cutting, sawmilling, clearing and fire have destroyed the greater part of the forest vegetation. Fortunately, pockets of the once extensive forest are still evident in the watersheds of the Leith, Silverstream, Waikouaiti, and at Silverpeaks, Maungatua and Mt Cargill.
Parts of Flagstaff and Swampy Hill have been repeatedly burnt yet change is not as evident as expected.
The rain forest grew from sea level to about 600 m above sea level.
Demands for land for settlement and agriculture quickly reduced this amount of forest. A few remaining areas have not been greatly changed and are found in two distinct belts: Podocarp-hardwood forest grows from sea level to 365m and mountain cedar-totara forest grows up to the bush-line. The Mt Cargill Walk passes through remnant podocarp-hardwood forest. Southern beech forest is confined to small patches hemmed in by rain forest, such as the one seen on the Mt Cargill Walk.
The present forest margins were mainly established over a century ago by a process of deforestation.
While possums and rats are abundant on Mt Cargill, they are rarely seen in daylight by the casual visitor. Much more readily visible is the wide diversity of birdlife. Once yellow-crowned parakeets, kakas and laughing owls lived in these forests. Nowadays, as humans have dramatically altered the area, these birds are gone. Today, pigeons, once driven from the hills by sawmilling, have returned and are a common sight on the walkways, as are bellbirds, grey warblers and tomtits.
Nearer the summit, in swampy gullies, the fernbird with its spine-like tail feathers, is occasionally seen. The eastern Rosella, a many-coloured parakeet released in Dunedin from a visiting ship at the turn of the century adds an exotic flavour to the lower regions of the Mt Cargill Walk.
Throughout the area the Red Admiral and the less common Yellow Admiral butterflies can be seen. At lower altitudes, the small blue butterfly (Lycaena baldenarium), with is purplish wings spotted with black, is abundant. Tussock butterflies, their brownish wings spotted with a large orange area, and the tiger moths, are found in the tussock on the skyline. The adult male tiger moths have black forewings and orange hindwings while the females are wingless and live under rocks.
The basement rock for most of Otago is schist: a flaky layered rock. Along the coast, areas of this schist became submerged about 90MYA, during which time deposition of sediments began. Subsequent uplift, or retreat of the sea has since exposed these sediments. Part of these deposits can be seen in the cliffs below the Tunnel Beach Walk.
– From DoC information Sheet hard copy