Central Otago comprises parallel fault-block mountain ranges and valleys with the metamorphic rock schist forming the bulk of the rock types. Metamorphic rock is formed from other rocks, both sedimentary and volcanic in origin, by alteration under intense heat and pressure. On summit ridges and lower slopes, the schist is exposed in tors that are blocks of tower-like outcrops of platy laminated rock. These are characteristic of the Central Otago landform and very distinctive.
In the northern boundary of the Upper Clutha Valley lie two large lakes, Wanaka and Hawea. Both occupy glacial troughs and which are impounded by ridges of end moraine. The Clutha River, New Zealand’s largest, rises from Lake Wanaka and is joined a few kilometres downstream by the Hawea River. At Cromwell, and before the formation of Lake Dunstan, the Clutha was joined by the Kawarau River (at The Junction) which empties Lake Wakatipu at Queenstown. The landforms and relief of the valley comprise three general types: Steep, hilly and rolling land; Fans; and Terraces.
The steep, hilly and rolling landform the valley and comprise the Pisa Range to the west and Dunstan Mountains to east. Fans are depositional landforms and may be geologically old or young, and are widespread in the Upper Clutha Valley. Terraces are extensive and have been formed by the main river systems, may be of various heights and degree of dissection by smaller river systems.
The geological history of the valley has been dominated by the intrusion of glaciers and ice erosion. The number of morainic deposits represent about five advances and retreats of ice into the valley, and possibly as far south as Clyde.
Apart from tors, the most unique geological feature of the valley are the terraces – especially the Lowburn, Bendigo and The Bend Terraces. In the “Geological Society of New Zealand Miscellaneous Publication No.77. Inventory of Important Geological Sites and Landforms in the Otago Region. 1993”, the significance of the Lowburn Glacial Outwash Terraces was described as “The best example of fluvio glacial outwash terraces in New Zealand. A sequence of widely spaced terraces. Classified as extremely well defined landforms of scientific/educational and scenic value”.
More detailed descriptions of the geology of the Cromwell district may be found in the following publications:
- Queenstown. A Geological Guide. Turnbull and Forsyth. 1988. Geological Society of New Zealand Guidebook No.9.
- Central Rocks. A guide to the geology and landscapes of Central Otago. Lee and Forsyth. Geological Society of New Zealand Guidebook No.14 2008.