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Geology

Human settlement of the Riverton-Aparima area began several hundred years ago but the region's geological origins go back more than 250 million years to when the supercontinent of Gondwanaland dominated Earth's southern hemisphere.
The rocky coast of Western Southland is a great place to reach into deep time and appreciate New Zealand's hidden history.





The topographic map above shows a 40 x 30 km area west of Invercargill.
The main towns are Riverton-Aparima (lower right) and Tuatapere (upper left).
State Highway 99 is the "Gondwana Trail" between them.
The aerial photograph image reveals the forest of the Longwood Range (high point 804 m) surrounded by farmland.





A digital terrain model allows the towns, farms and trees to be stripped away and the shape of the solid ground to stand out.
Unlike the actively rising, youthful mountains of Fiordland, the Longwoods are a rounded, worn-down range.
Geologically young plains and terraces surround the Longwoods on all sides and are especially prominent north and east of Riverton-Aparima and around the coast.
The DTM is a canvas on which geology can be painted.
The four colours on the simple geology map show the type and age of rock that lies under a particular spot:

YELLOW = Holocene-Pleistocene (0-2 million year old) gravels and sand of modern and old beaches and rivers, including Ice Age deposits.

ORANGE = Pliocene-Eocene (2-40 million year old) Waiau Group: soft sandstone, mudstone and limestone. Some fossil wood and shells.

PURPLE = Jurassic-Triassic (150-250 million year old) Median Batholith: hard crystalline granite, diorite and gabbro.

GREEN = Permian (250-280 million year old) Brook Street Terrane: hard green pillow lava, basalt dikes, volcanic breccia, sandstone and mudstone.

Each of these four chapters in the local geological history can be represented by its own iconic rock:




Green-coloured Brook Street Terrane rocks can be seen along the coast at Riverton-Aparima, Taramea (Howells Point) and Tihaka.
Coarsely crystalline Median Batholith granites are exposed between Colac Bay-Oraka and Monkey Island.
Softer, chalky Waiau Group formations make up the cliffs from Monkey Island past Orepuki to Waiau River mouth (and also at Clifden Caves, north of Tuatapere).
The youngest rocks in the area are represented by the constantly moving beach pebbles and by the grey andesite lava of the South Island's youngest volcano: Solander Island - Te Niho a Kewa.



On a clear day, the eroded stump of Solander Island can be seen 70 km to the southwest from the clifftops near Orepuki.
When it was erupting 150,000 to 400,000 years ago, it would have been about the size of Mount Taranaki.

More information

Buy the colour brochure A Guide to the Geology of the Riverton-Aparima District from Te Hikoi's Retail Shop. Other geology books and maps are on sale there too.

Map images from GNS Science and Land Information New Zealand. Crown Copyright reserved.