Waikato have also experienced a substantial period of drier
conditions with shrub- and grassland dominated cover during cold periods including the last glacial maximum. Thus
the soils in the Pukekohe area have effectively had continuous, wetter, and warmer, conditions which favour the formation of allophane and gibbsite through enhanced silicon
leaching. In contrast, the drier conditions in the Waikato
area, indicated by grassland/shrubland vegetation during the
coldest periods, may have favoured halloysite formation
because of reduced silica leaching.
6.3 Soil-Landscape Relationships
In the Hamilton Basin, Granular Soils are formed on the
mid-Quaternary-aged rolling hills, such as the low hill in the
suburb of Hillcrest where the University of Waikato is situated in Hamilton, and northward and north-eastward of
Hamilton including in the vicinity of Morrinsville. The hills
in the Hamilton Basin are old, Mid-Quaternary landforms
that represent a paleo-topography that was formed before c.
340,000 years ago.
To the west of the Waikato Basin, and in Northland,
Granular Soils form mainly on basaltic and andesitic lava
flows that form rolling land to hill country, such as those of the
extensive basalts of the Tutumoe Plateau north of Dargaville
where Granular Soils merge with Oxidic Soils. The old
basaltic landscapes of South Auckland around Pukekohe and
Bombay (developed on volcanic landforms dating from c. 1.6
to 0.5 Ma) are for the most part draped with between c. 1 and
3.5 m of altered Hamilton Ash beds. Thus, most of the highly
productive soils in the area are not formed on the basaltic
deposits, as is commonly assumed, because the pre-Hamilton
basaltic soils (now paleosols) are buried up to 3.5 m below the
land surface by Hamilton Ash beds.
In steeper landscape positions, such as on the upper
slopes of lava flows around Bombay and Pukekohe East
where the land steepens towards ridge crests, the modern
surface soils are developed in the reddish brown-coloured
exhumed paleosols on the basaltic materials, the mantling
brown-coloured Hamilton Ash beds having been removed
by erosion. Thus the landscape in the Pukehohe-Bombay
area comprises a mosaic dominated by brownish soils
(Granular Soils on Hamilton Ash) and much smaller areas,
near steep crests, of markedly reddish (paleo)soils (Oxidic
Soils on exhumed paleosols on basaltic lavas and associated
deposits).
The occurrence of Granular Soils in the northern Waikato
is spatially patchy and dependent on the upper weathered
Hamilton Ash beds being at the land surface, with the
cover-bed of young ( c. 45,000 years) tephras c. 25 cm
(Naike soil). Where the soils have a thicker cover-bed of
young tephras (c. 60 cm or so) that are dominated by halloysite (not allophane), the soils are classed as a special
two-storeyed Ultic Soil (Chap. 16), classified as a
Buried-granular Yellow Ultic Soil (Kainui soil). The classification recognises the subsurface ‘granular’ character
associated with the weathered Hamilton Ash in the lower
profile. In places, both the ‘Buried-granular’ Ultic Soils are
visible directly alongside Granular Soils over very short
Fig. 6.4 Surface erosion has led
to development of a series of soils
(exhumed paleosols) formed on
sequences of strongly weathered
tephra beds and colluvium at
Waiterimu in the north Waikato.
A Granular Soil on Hamilton Ash
Formation. B The pale-coloured
H1 bed at the base of the
Hamilton Ash Formation
(Rangitawa Tephra, 0.34 Ma).
C Extremely weathered dark
reddish brown Granular Soil on
Kauroa Ash Formation
(>0.8 Ma), one of the oldest soils
in New Zealand. D Ultic Soil on
weathered greywacke colluvium
(?). Photo: H. S. Gibbs
6.2 Soil Profile Genesis
91
conditions with shrub- and grassland dominated cover during cold periods including the last glacial maximum. Thus
the soils in the Pukekohe area have effectively had continuous, wetter, and warmer, conditions which favour the formation of allophane and gibbsite through enhanced silicon
leaching. In contrast, the drier conditions in the Waikato
area, indicated by grassland/shrubland vegetation during the
coldest periods, may have favoured halloysite formation
because of reduced silica leaching.
6.3 Soil-Landscape Relationships
In the Hamilton Basin, Granular Soils are formed on the
mid-Quaternary-aged rolling hills, such as the low hill in the
suburb of Hillcrest where the University of Waikato is situated in Hamilton, and northward and north-eastward of
Hamilton including in the vicinity of Morrinsville. The hills
in the Hamilton Basin are old, Mid-Quaternary landforms
that represent a paleo-topography that was formed before c.
340,000 years ago.
To the west of the Waikato Basin, and in Northland,
Granular Soils form mainly on basaltic and andesitic lava
flows that form rolling land to hill country, such as those of the
extensive basalts of the Tutumoe Plateau north of Dargaville
where Granular Soils merge with Oxidic Soils. The old
basaltic landscapes of South Auckland around Pukekohe and
Bombay (developed on volcanic landforms dating from c. 1.6
to 0.5 Ma) are for the most part draped with between c. 1 and
3.5 m of altered Hamilton Ash beds. Thus, most of the highly
productive soils in the area are not formed on the basaltic
deposits, as is commonly assumed, because the pre-Hamilton
basaltic soils (now paleosols) are buried up to 3.5 m below the
land surface by Hamilton Ash beds.
In steeper landscape positions, such as on the upper
slopes of lava flows around Bombay and Pukekohe East
where the land steepens towards ridge crests, the modern
surface soils are developed in the reddish brown-coloured
exhumed paleosols on the basaltic materials, the mantling
brown-coloured Hamilton Ash beds having been removed
by erosion. Thus the landscape in the Pukehohe-Bombay
area comprises a mosaic dominated by brownish soils
(Granular Soils on Hamilton Ash) and much smaller areas,
near steep crests, of markedly reddish (paleo)soils (Oxidic
Soils on exhumed paleosols on basaltic lavas and associated
deposits).
The occurrence of Granular Soils in the northern Waikato
is spatially patchy and dependent on the upper weathered
Hamilton Ash beds being at the land surface, with the
cover-bed of young ( c. 45,000 years) tephras c. 25 cm
(Naike soil). Where the soils have a thicker cover-bed of
young tephras (c. 60 cm or so) that are dominated by halloysite (not allophane), the soils are classed as a special
two-storeyed Ultic Soil (Chap. 16), classified as a
Buried-granular Yellow Ultic Soil (Kainui soil). The classification recognises the subsurface ‘granular’ character
associated with the weathered Hamilton Ash in the lower
profile. In places, both the ‘Buried-granular’ Ultic Soils are
visible directly alongside Granular Soils over very short
Fig. 6.4 Surface erosion has led
to development of a series of soils
(exhumed paleosols) formed on
sequences of strongly weathered
tephra beds and colluvium at
Waiterimu in the north Waikato.
A Granular Soil on Hamilton Ash
Formation. B The pale-coloured
H1 bed at the base of the
Hamilton Ash Formation
(Rangitawa Tephra, 0.34 Ma).
C Extremely weathered dark
reddish brown Granular Soil on
Kauroa Ash Formation
(>0.8 Ma), one of the oldest soils
in New Zealand. D Ultic Soil on
weathered greywacke colluvium
(?). Photo: H. S. Gibbs
6.2 Soil Profile Genesis
91