Southland Land District
The western part (Fiordland) consists of a huge, broadly domed glaciated mountain mass of plutonic and metamorphic rocks. Between Fiordland and the Otago schists late Paleozoic and Mesozoic rocks are folded into the Southland Syncline, a major structural feature which curves in an arc through the district from the Hollyford area to the Mataura Valley and thence through south-east Otago to the coast. In Southland the Alpine Fault runs from Lake McKerrow (in Otago) to the entrance of Milford Sound and then probably continues just off shore, thus accounting for the spectacularly cliffed Fiordland coast and the deep water close inshore. The southern part of Stewart Island contains granites correlated with those of south-west Fiordland, whereas Ruapuke Island and the northern part of Stewart Island contain rocks correlated with the volcanic and igneous rocks associated with the late Paleozoic rocks of the Southland Syncline.
The mountain summits of Fiordland rise northward along the main divide from 3,600 ft to over 6,000 ft until they terminate in the Darran Range at 9,000 ft. The main rock mass was originally composed of quartzite, greywacke, tuffaceous greywacke, and limestone between 6 and 8 miles thick and probably ranging in age from Cambrian to Devonian, or possibly Carboniferous. The sequence later became highly metamorphosed, probably in the Carboniferous; the most altered rocks are now sillimanite gneisses and garnet-hypersthene gneisses. In a coastal strip in the extreme north-west of Southland near Milford Sound a narrow belt of these rocks has been intensely deformed by movements along the Alpine Fault, and mylonites similar to those in Westland have been formed.
In the later Carboniferous and Permian, the Fiordland rocks were invaded by a sequence of syenitic and granitic dykes and batholiths. The granite batholiths of the southern part of Stewart Island were formed at about this time. The oldest fossiliferous rocks known from Fiordland are Ordovician graptolite slates found at Preservation Inlet, but some of the adjacent metamorphic rocks are probably Cambrian.
Although Fiordland's ancient dome originally had radial drainage, Pleistocene glaciation has created a complicated rectangular network, the most impressive features of which are the elongated, high-level lakes gouged out in pre-existing valleys by the ice. Lake Te Anau, the largest, has an area of 132 sq. miles. Other residual evidences of glaciation abound, chief of which is the magnificent forested fiord terrain extending for some 170 miles round the south-west coast; a succession of drowned U-shaped valleys in the resistant rocks which have retained their precipitous sides, frequently plunging thousands of feet into the sea along the whole of their 10–20 mile branched courses inland. Freeze and thaw acting on the bleak uplands during the Pleistocene have produced extensive scree slopes, filling in many of the topographic irregularities.
Numerous small deposits of metallic minerals in Fiordland, such as copper, zinc, lead, and molybdenum ores, are of only marginal economic value. Marble occurs in quantity in Doubtful and Caswell Sounds, mica was once mined at Mount Elwood, and the Maoris used to obtain greenstone from the ultrabasic rocks of Anita Bay in Milford Sound.
The most extensive structural feature of the land district is the Southland Syncline, which formed the southernmost extension of the Permian-Jurassic New Zealand Geosyncline. The Upper Paleozoic and Triassic rocks of the Southland Syncline are closely similar to those of the Nelson Syncline and it is thought that both structures were once continuous, but have been moved horizontally by the Alpine Fault.
The oldest sediments in the Southland Syncline are Permian tuffaceous greywackes which delimit the northern margin of the syncline flanking the Otago schists and extend across Southland as a broad south-east-curving belt from the Humboldt Mountains through the Thomson Mountains to the Waikaka area (and thence through Otago to the Kaitangata coast). These sediments become more strongly folded and partly schistose as they approach the Otago schists which are at least in part their metamorphosed equivalents.
Rocks of similar age form the other margin of the Southland Syncline flanking Fiordland and extending from the Hollyford Valley southwards and southeastwards through the Takitimu Mountains to Riverton, Bluff, Ruapuke Island, and the northern part of Stewart Island (where they flank Fiordland-type granites to the south). These rocks are dominantly volcanic – basalts, tuffs, agglomerates – but with thin sediments, and attain a thickness of about 14 miles, attesting to the most intense and prolonged period of volcanicity known in the New Zealand region. Linear intrusions of igneous rocks are associated with these volcanic rocks (for example, Longwood Range, Bluff Peninsula, Ruapuke Island, and the northern part of Stewart Island) and are also known along the northern margin of the syncline (as at Livingstone Mountains, Otama Hills, and the Clinton-Waipahi area of Otago). Triassic and Jurassic rocks form the core of the syncline and strike across eastern Southland to the south-east Otago coast. Subdivisions of the New Zealand Permian and Triassic rocks are based on the fossil sequences exposed in the Takitimu Ranges (Permian) and Hokonui Hills (Triassic). The Jurassic rocks contain many fossils a famous locality is Curio Bay near Waikawa, where a fossil forest is preserved with petrified tree trunks reaching up to 40 ft in length.
Section J shows Permian and Triassic rocks of the Southland Syncline flanked by the Otago schists to the east and Fiordland igneous and metamorphic rocks to the west. The schists shown are representative of those found in northern and eastern Southland and throughout Otago. They are characterised by complex folding, shown diagrammatically in the cross section. The igneous and metamorphic rocks shown in the Te Anau area are representative of those found throughout Fiordland – granites, diorites, and gneisses. In the Te Anau area the older rocks are mantled by Tertiary sediments, remnants of probably more extensive deposits elsewhere removed by erosion.
The Rangitata Orogeny folded the sediments of the Southland Syncline, very steeply on the northeast side where the beds become overturned towards the Otago schist belt. This land was quickly eroded to low relief and the bituminous coal measures now mined at Ohai were formed from late Cretaceous swamps. Similar conditions prevailed during the Eocene (seediagram 7) when the thick lignite deposits of Mataura Valley and eastern Southland were formed. Contemporary freshwater sandstones and conglomerates (Arnold Series) were laid down around Lake Te Anau.
During the Oligocene (seediagram 7), the sea was able to transgress over much of Southland and extensive limestone was laid down. This is quarried for agricultural use and a particular type occurring in the Waiau Valley is valuable in cement manufacture. Much of the mid-Tertiary marine mudstones and sandstones which underlie the widespread recent alluvium in the lowland areas was also deposited. Towards the end of the period the sea slowly retreated to a small bight in the lower Waiau Valley, where shelly sands and deltaic deposits are now preserved.
The lowlands of Southland comprise the huge Waiau Valley west of the Takitimu-Longwood barrier and the Plains area to the east, consisting of the Central Plains extending over some 1,000 sq. miles and the smaller more dissected Waimea Plain to the north. Although Pleistocene ice did not extend more than about 15 miles from the southern and eastern limits of the present lake basins, the remainder of Southland was subjected to a rigorous cold climate with the production of much coarse detritus. This material, together with glacial outwash gravels, forms most of the filling for the Southland Plains. The valuable lowland soils, however, on which the economy is mainly based, are formed on a blanketing deposit of fine yellow silt (loess) which originated during the last glacial period of the Pleistocene when clouds of dust were blown from the river beds of the region and trapped by vegetation on the surrounding plains. Alluvial gold is widespread in the Pleistocene deposits, but only at Te Waewae Bay and Preservation Inlet are the deposits likely to be economic. Gold was, until recently, won by sluicing at Round Hill (west of Riverton).
A number of earthquakes are known from the Fiordland coast and the neighbouring sea, and these are probably related to the Alpine Fault, lying just off shore for most of the length of western Fiordland. An earthquake accompanied by uplift was recorded in the Doubtful Sound area by early whalers.