The rock that underlies a vineyard is a major factor, if not the major factor in determining varietal variations in different terroirs.
With several significant exceptions, most of the islands’ vineyards are separated only by thin layers of soil and clay from massive sequences of Jurassic strata that range from 145 to 158 million years in age.
Elevation of the early landform above sea level, and erosion for millions of years, has resulted in vines flourishing where dinosaurs and ammonites once ruled!
The island rock on which we now walk is almost entirely of Jurassic age and our wines derived from these sediments are truly distillations of millions of years of earth’s history.
Though Waiheke is small in area, the age and chemical constitution of the base rock under our vineyards can change markedly and abruptly, even in very small distances.
The multi-faceted hills created by the early upheaval of the land lead to further variation in vineyard aspect vis-à-vis sunlight and wind which, together with rainfall, have a strong influence on the choice of grape variety.
An examination of the geological map of Waiheke Island reveals that the rock strata has been enormously compressed and fractured by tectonic activity causing the landscape to be massively deformed by a series of compression folds whose axes run more or less North and South. Plotting the location of vineyards relative to these formations and their resulting mineral sediments and deposits will reward the wine connoisseur in search of unique varietal expression.
Soil is substantially the product of weathered rock. Most of Waiheke is underlain by weathered, indurated argillite of the Waipapa Group.
During the weathering process, the original rock fabric has been largely converted to clay minerals.
The uppermost levels of the weathered rock (‘rotten rock’) are generally composed of a stiff to hard soil, often criss-crossed with mineralised veins and stained with iron and manganese oxides.
This soil may be relatively thin in places with hard ground close to the surface, or of considerable thickness, with rock being found only at tens of metres of depth.
Overlying the weathered argillite in many places is a mantle of Pleistocene sediment composed of Aeolian (wind-blown) dust; silt-sized particles that had their origins in the ancient continental shelf to the west of the North Island.
A metre-thick layer of this sediment was apparently deposited over Waiheke and now forms a mantle cloaking the underlying bedrock. This sedimentary topsoil shows a propensity to shrink and crack and then swell again with the change of seasons from summer to winter, due to the high proportion of Camontmorillonite clay it contains.
Site variation is enormous, but generally, Waiheke vineyards are highly mineralised but naturally low in pH and phosphate with a high porosity but low permeability.
Exceptions occur in gully floors where richer alluvial soils congregate and in the volcanic ash soils at the eastern end of the island around Stony Batter.
Clay soils are thought to aid ‘mouth feel and structure’ in Merlot, Chardonnay and Syrah, and low-fertility mineralised soils are generally conducive to flavour in wine.
However, the health of the vines on Waiheke requires management of drainage, soil organic matter and pH on an on-going basis.
Waiheke Island’s climate is strongly influenced by the surrounding sea. Being situated in the Hauraki Gulf to the east of Auckland, the island is partially protected from the prevailing colder/wetter west and southwest winds, making it both drier and warmer than the Auckland isthmus.
However, with an area of just 92 square kilometres and a coastline of 133.5 kilometres, it is the proximity of the surrounding ocean that has most bearing on the climate from a viticultural standpoint.
The ocean acts as both a fan and an insulator. Sea breezes moderate rising temperatures in mid-summer. The ocean moderates falling temperatures at night. In simple terms, this means that mean temperatures during the growing season are comparable to much ‘hotter’ regions but without the extremes and, critically, these moderate temperatures extend longer into the early autumn ripening period of March and April, allowing later varieties to ripen fully over an extended period.
This maritime influence is so marked that comparisons with other premium winegrowing regions only serve to highlight the unique differences between each.
Waiheke can grow and ripen a wider range of grape varieties than other regions because of its long, mild season and the significant variations in vineyard site orientation and soil structure. Historically, the wet season begins in April, potentially threatening late varieties such as Cabernet.
However, the standard deviation for rainfall during any particular month is high with rain tending to fall in short-duration deluges. In these circumstances, rapid run-off on sloping sites with hard topsoil becomes advantageous.