4.3.2 The Greywacke Hill and Mountain Country
a. The broad soil-landscape patterns of the South Island
high country
The Mesozoic (c. 252 million to 66 million year old) greywacke basement rocks of New Zealand are exposed in our
rapidly uplifting and eroding main mountain ranges,
including the Southern Alps, forming the main soil parent
materials. Greywacke is hard, resistant sandstone comprising
roughly equal amounts of quartz, feldspar, and fragments of
pre-existing rocks cemented together in a mud-rich matrix.
The resulting landscapes tend to be cut through with faults
and comprise steep hills with sharp, linear ridges, planar side
slopes, and well drained hollows.
Generally, there is an east to west trend in the South
Island greywacke ranges approaching the Alpine Fault.
Nearer the Alpine Fault there is higher rainfall, greater rates
of tectonic uplift, steeper slopes, a more rugged landscape,
weaker expression of soil weathering, stronger expression of
glacially carved topography, and fewer pockets of weathered
rock, than in the more distant areas. Thus the broad scale
soil-landscape of the central spine of the South Island can be
divided into three regions: the eastern hills and mountains,
the main divide, and the western alpine region.
In the eastern hill and mountain region the soils are
dominated by Orthic and Allophanic Brown Soils. The main
divide region has a higher rainfall and the land is steeper and
more rugged than the eastern area. The dominant soils in the
main divide region are Acid Brown Soils, Allophanic Brown
Soils, Recent Soils, and Orthic Podzol Soils. The active
uplift and erosion mean that there is often soil disturbance in
the main divide area and hence soils of varying ages are
common. Soil profiles frequently include buried features and
composite soil profiles—for example, a Brown Soil buried
by younger sediment in which a Recent Soil is formed. In
the western alpine region, where rainfall is considerable, the
soils are dominantly Gley Soils, Gley Podzol Soils, Recent
Soils, and Raw Soils.
b. The Gibbs-Raeside line between Allophanic Brown Soils
and Orthic Brown Soils
In the 1940s Harry Gibbs and James Raeside embarked on
the challenging task of the first general soil survey of the
South Island high country, partly to map accelerated erosion.
They identified a significant boundary, now referred to as the
Gibbs-Raeside line. The Gibbs-Raeside line marked the
eastern limit of the Allophanic Brown Soils (then known as
the High country yellow brown earths) which are characterised by strong brown colours, low soil dry bulk density,
and a friable crumb (apedal earthy) structure. Orthic Brown
Soils to the east and south (then known as Southern
yellow-brown earths) had paler colours, and pedal or massive structure. In their 1942 publication, Gibbs and Raeside
and others suggested that the boundary (between what were
then called Southern yellow-brown loams (Allophanic
Brown Soils, which they termed Kaikoura series), and what
they called Omarama series (Orthic Brown Soils) in drier
areas, occurred at about 1000–1200 m altitude in the dry
inland basins, but at about 450 m in wetter areas, and
roughly coincided with the lower limit of the winter snowline and the boundary between tall and low tussock grasslands at the time of European settlement. They also noted
that the Allophanic Brown Soils coincided with areas of
beech forest at lower altitudes.
In the twenty-first century, Andre Eger and Peter Almond
studied contrasting soils, similar to those recognised by
Gibbs and Raeside, in the vicinity of Porters Pass. They
identified Allophanic Brown Soils on southern and western
aspects with the contrasting non-allophanic Orthic Brown
Soils on northern and eastern slopes.
The conclusion reached by Eger and others was that the
Allophanic Brown Soils were likely to have developed
beneath acidifying beech forest (Nothofagus sp.) which
generated soil pHs sufficiently strong to mobilise the aluminium (especially) and silicon ions, the precursors of allophane formation (which involves loss of silicon through
leaching and retention of high amounts of aluminium). It
was proposed that the more eastern Orthic Brown Soils were
developed under a shrubby vegetation. If the boundary
between the two soils was in fact related to a former
Nothofagus forest boundary then it may represent a significant biogeographic, natural ecosystem boundary.
c. Loess in the South Island high country
Some of the high country Brown Soils are formed on loess
materials. Loess is often thought of as being associated with
flat to rolling land of the lowlands, usually seen as smooth,
undulating ‘downland’ landscapes, but it is also important in
hill and mountain landscapes. For loess to accumulate there
needs to be a source of fine (silt or fine sand), loose, dry,
sediment that is susceptible to wind erosion. Australian
insights suggest that clay-sized aeolian ‘parna’ may also
behave as loess if it is flocculated into silt, or fine-sand, sized
particles. The bulk of loess in New Zealand is derived from
riverbeds, the ‘dust engines’ in which most of the fine
material that ends up as loess is derived from abrasion and
breakdown of rocks during river transport, especially during
glaciations. Smaller amounts of fines are contributed by
glacial grinding and by freeze-thaw processes. Aeolian loess
transport from the river beds is driven by the belt of strong
westerly/north-westerly winds in the mid-latitude regions.
4.3 Soil-Landscape Relationships
61
a. The broad soil-landscape patterns of the South Island
high country
The Mesozoic (c. 252 million to 66 million year old) greywacke basement rocks of New Zealand are exposed in our
rapidly uplifting and eroding main mountain ranges,
including the Southern Alps, forming the main soil parent
materials. Greywacke is hard, resistant sandstone comprising
roughly equal amounts of quartz, feldspar, and fragments of
pre-existing rocks cemented together in a mud-rich matrix.
The resulting landscapes tend to be cut through with faults
and comprise steep hills with sharp, linear ridges, planar side
slopes, and well drained hollows.
Generally, there is an east to west trend in the South
Island greywacke ranges approaching the Alpine Fault.
Nearer the Alpine Fault there is higher rainfall, greater rates
of tectonic uplift, steeper slopes, a more rugged landscape,
weaker expression of soil weathering, stronger expression of
glacially carved topography, and fewer pockets of weathered
rock, than in the more distant areas. Thus the broad scale
soil-landscape of the central spine of the South Island can be
divided into three regions: the eastern hills and mountains,
the main divide, and the western alpine region.
In the eastern hill and mountain region the soils are
dominated by Orthic and Allophanic Brown Soils. The main
divide region has a higher rainfall and the land is steeper and
more rugged than the eastern area. The dominant soils in the
main divide region are Acid Brown Soils, Allophanic Brown
Soils, Recent Soils, and Orthic Podzol Soils. The active
uplift and erosion mean that there is often soil disturbance in
the main divide area and hence soils of varying ages are
common. Soil profiles frequently include buried features and
composite soil profiles—for example, a Brown Soil buried
by younger sediment in which a Recent Soil is formed. In
the western alpine region, where rainfall is considerable, the
soils are dominantly Gley Soils, Gley Podzol Soils, Recent
Soils, and Raw Soils.
b. The Gibbs-Raeside line between Allophanic Brown Soils
and Orthic Brown Soils
In the 1940s Harry Gibbs and James Raeside embarked on
the challenging task of the first general soil survey of the
South Island high country, partly to map accelerated erosion.
They identified a significant boundary, now referred to as the
Gibbs-Raeside line. The Gibbs-Raeside line marked the
eastern limit of the Allophanic Brown Soils (then known as
the High country yellow brown earths) which are characterised by strong brown colours, low soil dry bulk density,
and a friable crumb (apedal earthy) structure. Orthic Brown
Soils to the east and south (then known as Southern
yellow-brown earths) had paler colours, and pedal or massive structure. In their 1942 publication, Gibbs and Raeside
and others suggested that the boundary (between what were
then called Southern yellow-brown loams (Allophanic
Brown Soils, which they termed Kaikoura series), and what
they called Omarama series (Orthic Brown Soils) in drier
areas, occurred at about 1000–1200 m altitude in the dry
inland basins, but at about 450 m in wetter areas, and
roughly coincided with the lower limit of the winter snowline and the boundary between tall and low tussock grasslands at the time of European settlement. They also noted
that the Allophanic Brown Soils coincided with areas of
beech forest at lower altitudes.
In the twenty-first century, Andre Eger and Peter Almond
studied contrasting soils, similar to those recognised by
Gibbs and Raeside, in the vicinity of Porters Pass. They
identified Allophanic Brown Soils on southern and western
aspects with the contrasting non-allophanic Orthic Brown
Soils on northern and eastern slopes.
The conclusion reached by Eger and others was that the
Allophanic Brown Soils were likely to have developed
beneath acidifying beech forest (Nothofagus sp.) which
generated soil pHs sufficiently strong to mobilise the aluminium (especially) and silicon ions, the precursors of allophane formation (which involves loss of silicon through
leaching and retention of high amounts of aluminium). It
was proposed that the more eastern Orthic Brown Soils were
developed under a shrubby vegetation. If the boundary
between the two soils was in fact related to a former
Nothofagus forest boundary then it may represent a significant biogeographic, natural ecosystem boundary.
c. Loess in the South Island high country
Some of the high country Brown Soils are formed on loess
materials. Loess is often thought of as being associated with
flat to rolling land of the lowlands, usually seen as smooth,
undulating ‘downland’ landscapes, but it is also important in
hill and mountain landscapes. For loess to accumulate there
needs to be a source of fine (silt or fine sand), loose, dry,
sediment that is susceptible to wind erosion. Australian
insights suggest that clay-sized aeolian ‘parna’ may also
behave as loess if it is flocculated into silt, or fine-sand, sized
particles. The bulk of loess in New Zealand is derived from
riverbeds, the ‘dust engines’ in which most of the fine
material that ends up as loess is derived from abrasion and
breakdown of rocks during river transport, especially during
glaciations. Smaller amounts of fines are contributed by
glacial grinding and by freeze-thaw processes. Aeolian loess
transport from the river beds is driven by the belt of strong
westerly/north-westerly winds in the mid-latitude regions.
4.3 Soil-Landscape Relationships
61