The Otago Mountains are bounded in the north by the courses of the Waitaki and Ahuriri Rivers and the Haast Pass, and in the south by the Clinton-Waimea Plain depression which stretches from Balclutha to Lake Te Anau, and the Eglinton Saddle. Two distinct types of mountain topography are evident: the relatively flat-topped block mountains of eastern and central Otago, and the glacially sculptured mountains of the Otago Alps in the far west. The block mountains increase continuously in altitude from the east coast inland (2,500–6,000 ft). The Kakanui Mountains in the north are the highest near the east coast. Mt. Bitterness (6,739 ft) and Mt. St. Bathans (6,843 ft) mark the northern limit of the Otago block mountains, and Mt. Pisa (6,433 ft) and Double Cone (7,688 ft) the western and south-western limits respectively. Beyond these limits lie the glacially sculptured mountains of the so-called Otago Alps, the higher peaks of which include Mt. Earnslaw (9,250 ft) in the south and Mt. Aspiring (9,959 ft) in the north. These mountains still support permanent icefields and glaciers, the largest being the Olivine Ice Plateau and the Bonar, Volta, and Therma Glaciers.
The rocks of the Otago Mountains comprise typically dark-grey schist (of Upper Paleozoic-Lower Mesozoic age) with white quartz banding or foliae. In eastern and central Otago the block mountains are fault-folds in which a fault generally replaces one limb of the up-fold. Intermontane depressions have preserved in them Tertiary quartzose gravels, which once formed a continuous cover over the whole region. In some places they are preserved as remnants on the block mountains, or else their former existence is shown by scattered sarsen stones or chinamen boulders. In Central Otago the Pleistocene periglacial regime modified the block mountains, giving rise to the schist tors which dot their crests and flanks. In the far west a glacial regime prevailed during the Pleistocene, glaciers occupying all the valleys and those southern lakes which lie within this region. Subsequently the glaciers retreated to their present positions in late Pleistocene time, revealing the deep broad glacial valleys and the distinctive glacial sculpture of the mountains.
From the accompanying map it is evident that the block mountains of eastern and central Otago form a rough rectilinear cross pattern of NE and NW directions, due to earth movements in late Tertiary to early Pleistocene time. The natural lines of communication from the coast to this part of Otago, which the pioneers discovered and developed, were determined at this date because the major rivers followed the depressions which lie between the block mountains. In western Otago, however, access was gained via the glacial valleys. Most passes such as Dansey and Lindis, as well as that over the Crown Range in eastern and central Otago, are through Tertiary down-folds which cross the block mountains. In western Otago, on the other hand, glacial diffluences, such as the Haast Pass and Eglinton Saddle, afford the only easy access around the Otago Alps to the west coast region.
The central Otago block mountains support high-level silver and red tussock grasslands which contrast with the western mountain cover of varied snow tussock and subalpine shrubland between the native beech tree line below, and the bare rock, snow, and ice above. The tussock grasslands constitute important fine-wool sheep-grazing areas. Reefs, which transect the schist rock of the Otago Mountains, have yielded gold, copper, antimony, and scheelite but it is the gold-bearing quartz reefs that yielded the most mineral wealth. These have been eroded away over a large period of geological time, to produce the valley deposits with their large quantities of alluvial gold, which were discovered in the gold rushes of the sixties.
by Alexander Russell Mutch, B.SC., A.O.S.M., New Zealand Geological Survey, Dunedin.