The present rock that makes Otago is schist, while the tors are a distinctive part of both the Otago lowland and the upland landscape. The making of what we know of Otago today is greatly influenced by how New Zealand came about in the beginning.
It all started several hundred million years ago when marine deposits of sand, mud and eroded land sediment were laid down on the ocean floor as separate masses of continental sediment. These separate sediment deposits were very deep and at first they formed layers of sandstone and mudstone with the bottom layers formed into greywacke as a result of immense pressure. Sometime during the Rangitata Orogeny, (derived from the Rangitata river which draines the eastern flanks of the central Southern Alps and oros = mountains) 140-120 million years back, two of these ocean continental deposits of greywacke (Caples and Torlesse deposits) were on the move and collided at a subduction zone. Today this collision impact is now known to lie along a line from Queenstown, Kawarau Gorge, Cromwell and on down close to Alexandra.
At the collision point, immense pressure combined with the great depth, heated the rock to very high temperatures - the result known as metamorphic, such that the rock flowed and the chemicals - the minerals, separated out and the larger crystals of rock were formed that contained quartz, some quartz with gold veins, feldspars and mica's. The rock at the collision boundary when cooled became known as Otago schist with the Caples terrane to the South and the Torlesse greywacke (Rakaia Rock) terrane to the North. (Torlesse is named after Canterbury's Mt Torlesse.) Greywacke forms the backbone of the South Island. As more sediment built up, this deformed land mass was pushed up - floated, such that it at first become a continental shelf attached to the eastern margin of the ancient super-continent of Gondwana. Later a large part of the shelf emerged as a huge new land mass that included New Caledonia, New Zealand and the intermediate land to form its own micro-continent.
Some 80 million years ago this vast attached land south of Australia and Antarctica, came adrift and separated, drifting over the southern oceans, eroding in size till 37-25 million years ago another tearing and rifting movement so violent, that New Caledonia was torn away from New Zealand and both countries went their separate ways.
Over this first period 80-25 million years, covering some 55 million years, while this initial large land mass was subject to massive erosion and then finally torn apart, going on all this time there was deep weathering of rock by the infiltration of warm water and the humid climate. The water dissolved some of the chemicals in the rock and leached them away leaving behind clay with some still solid rock showing the characteristic layering of schist. Although it looks like ancient river or lake deposits of gravel, closer examination shows that all the pebbles are sharply angular ie. they are not water worn but broken up layers of bed rock still in their original location.
25-20 million years ago the New Zealand land size was for a period very much smaller than it is today. The landscape was flat, a series of islands and inland seas - Otago was a peneplain. The climate was considerably warmer and plants such as kauri, nikau and casuarina (evergreen pines) grew here in Otago. In Central Otago, the Otago western region, covering the lower lying peneplain was a very large lake - Lake Manuherikia. In time, Lake Manuherikia was gradually filled with both wind blow sediment - loess and a massive amount of river gravel from the rivers of the greywacke mountains to the north that once flowed south. This influx of greywacke gravels in time provided for some 10 metres of sediment that is known today as the Maori Bottom.
In Otago, weathering continued on between 20 and 5 million years ago, but now not as fast as before, as most of the hill surface has now gone.
At the lower slopes at Butchers Dam and along at Chapman Road, can been seen where the early miners have sluiced away the surface of between three and thirty metres, exposing ancient soils - pedosals, that were once buried deep, reburied and escaped from the millons of years of constant erosion.
Coinciding when New Zealand was drifting north during the Pliocene times, 5-2 million years ago, major earth movements began all round the Pacific - the ring of fire. New Zealands drift stopped to nearly a standstill when lodged up against the undersea collisions boundaries of the Pacific Plate and the Indian-Australian Plate. This major influence was the remaking of New Zealand - the Kaikoura Orogeny 5-2 million years ago, though the Orogeny continues on today still shaping and redefining the landscape but at a slower rate.
Finally when these plates when being pushed over each over at the subduction zone, the pressure caused immense buckling, shearing and faulting, which not only gave the uplift of the Southern Alps but ripping apart the crust, shunting it sideways for hundreds of kilometres so that the "red hills" of Otago on the eastern side of the fault now precisely matched by the "red hills" in Nelson on the western side.
It was during these last million of years that these Central Otago mountains and basins were formed from the pressure waves from the continuing collisions of these two great plates. These collisions has had the effect, that on the eastern side of the Southern Alps fault, the peneplain has crumpled and folded the surface of Central Otago. The result is a ripple effect on a gigantic scale such that location and alignment of each successive range is influenced by deep seated fault lines.
Then the climate grew colder from about 2 million years ago, making for a series of glacial periods comprising of several ice ages. The most recent ending just sixteen thousand years ago. In Central Otago, the climate during the glaciations was very cold but dry. The mountain high country was thickly ice caped, but only a few small glaciers developed on the tops of the highest ranges such as the Pisa Range, while the huge glaciers from the Main Divide pushed into western parts of Otago forming the big lakes - Wakatipu, Wanaka and Hawea and the modern Clutha Valley down as far as Lowburn.
During these glaciation periods, gale force winds and severe frost action only allowed for a sparse covering of low-growing plants such that any surface water run-off would have caused a considerable amount of erosion. During these periods from the combined action of the weather, the hills were stripped of all the soft deeply eroded rock and deposited further down the sides and onto the valley floors. In places were the rock was more chemically resistant along old fault lines, from where the ancient heating 140 million years ago was more extreme, the rock remained standing above their surroundings - "tors", while in the valleys, the soft soils away from the rivers and streams, have been protected by the material that has come off the hillsides.
Today nearly all of Central Otago geology history can be physically be seen right through from Middlemarch close to Dunedin, and to the west of Glenorchy on Lake Wakatipu. Lowland tor landscape covers most parts of central with mountain tor landscape on most of the Central Otago mountains. The northern mountains from St Bathans Range north into Canterbury are Torlesse greywacke. Evidence of the remains of the peneplain can be seen on all the mountain tops and the higher flats / hills centred around Alexandra while the sediments of Maori Bottom shoOtago Landscape up at the Galloway Cliff face at the first Galloway Bridge. Schist rock (Creek for split), was used as a construction material and can still been seen in buildings built by the early miners and settlers, by the early bridge makers - Skippers, Kawarea, Shaky, Manuherikia (Ophir bridges), dried fence walls and as cladding on todays buildings.
Landscape tours, heritage trails, general/scenic tours and wild flower walks can be arranged with John Douglas of Safari Excursions
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