Lake Wairarapa exists because of the convergence of two of the earth’s great crustal plates. This creates a pattern of parallel ridges and depressions on land and under the sea, imparting a ‘grain’ to the whole eastern North Island. The Wairarapa Valley is part of an immense depression that stretches from Palliser Bay to Hawke Bay, interrupted only at Mt Bruce. Lakes Wairarapa and Onoke lie in the lower valley between two ridges, the Aorangi and Remutaka Ranges.
The depression is filled with an enormous amount of sand, silt and gravel eroded from the mountains and deposited by rivers. Like the ﬂat surface of water in a bath, sediment deposited by water makes a ﬂat or nearly ﬂat plain. However, our Wairarapa Valley is one of the most tectonically mobile places on earth, so in places its ﬂoor is already being squeezed into folds.
Lakes Wairarapa and Onoke are parts of the depression not yet filled but are the shallow, shrinking remains of a lake that was much larger.
Separating the depression from the Remutaka Range to the west is the great Wairarapa Fault. This last moved on 23 February 1855, in one of the largest known earthquakes on the planet; a colossal release of energy that moved hundreds of cubic kilometres of rock in a few seconds.
The solid rock ﬂoor of the depression is deeply buried under the gravel and other sediments that form the plain. A gravity survey in the 1970s showed that these are about two kilometres deep under the lake and 3.2 kilometres under Featherston. The surface sediments are all river or lake deposits but deeper layers are likely to be of marine origin because the valley has previously been open to the sea.
Wairarapa Valley was previously an arm of the sea. Onoke Spit is formed by storm waves that sweep gravel oﬀ the shallow bottom of Palliser Bay and heap it up, cutting oﬀ Lake Onoke from the sea. The Spit is a storm beach that traps a lagoon (Lake Onoke) behind it. The Ruamahanga River has filled in the area between Lakes Wairarapa and Onoke with silt, making two lakes instead of one.
Although the Ruamahanga River is the main drainage of the Wairarapa Valley, it enters the lake well to the south. Te Maire Ridge (one of those pushed up by the squeezing crustal plates) has risen in the Ruamahanga’s path. This diverts the river eastward towards Martinborough, so its entry to the lake now lies south of Kahutara – or did until the Wairarapa Catchment Board changed it again in the 1960s. The former course of the Ruamahanga River can be seen as a ribbon of low, swampy ground beside the Greytown-Martinborough road near Morison’s Bush.
While the mountains on both sides of Wairarapa Moana are rising, the lake bed is too, but more slowly. The result is a great bending of the earth’s crust. Looking south from Western Lake Road, the far skyline near Lake Ferry is warped into a broad, open ‘U’ like a saucer in cross-section. In the coming millennia, this ‘saucer’ will continue to bend and rise, making Lake Wairarapa even shallower.
Text John Rhodes
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Content on this page was last updated: 09/03/2017 12:44pm