The third category, an increasingly important site for viticulture, is the gravel terraces of the main rivers that are extensive at elevations of about 60 metres above sea level. Former gravelly riverbeds on lower land have similar soils. In the fourth category the hills that make up most of the region have either clays or limestone underlying them. Loess is the surface material on much of these hills, and it often forms a discrete topsoil, especially over some of the limestones.
The plains were formed through deposition by the colourfully named rivers that drain them – the Esk, Tutaekuri, Ngaruroro, Clive and Tukituki. As a result, underlying much of them are gravels derived from greywacke, the basement rock of New Zealand.
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At lower elevations, where rivers have changed their course, these former riverbeds are exposed, as is the case with the Gimblett Gravels. Overlaying these gravels and interspersed among them is a wide range of finer sandy and silty materials deposited by the rivers as they have flooded and meandered across the plains and formed wetlands mainly closer to the coast, most of which have been drained.
These soils vary greatly in their stage of development but most are youthful and their composition is complicated by recent geological events. Deposits of ash from the volcanic eruptions of the central North Island have fallen in the headwaters of the rivers and on the lowlands. Loess from the period of the Pleistocene glaciations when vegetation was sparse is still present on much of the Hawke’s Bay hill country but elsewhere it has been eroded, re-sorted, redeposited and reconsolidated as was the volcanic ash.
Upstream from where these rivers emerge from the hills are extensive suites of terraces at different elevations, notably along the Ngaruroro River (Figure 6.3) but also the Tutaekuri and the Tukituki. Where the Ngaruroro comes out of the hills at Maraekakaho, for instance, its bed is about 60 metres above sea level and the extensive terraces above it on either side of the river are between 100 and 140 metres above sea level. Fifteen kilometres further upstream, the riverbed is at about 160 metres and the highest parts of the terraces approaching 200 metres above sea level. Tributaries of the Ngaruroro also have suites of terraces associated with them, especially on its southern or right bank. These are at even higher elevations, almost all being above 200 metres.
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