leaching on both aspects. However, the northerly aspect was
favoured by higher temperature with consequently greater
soil development and lower fertility.
The degree of leaching can be estimated from the soil pH,
total phosphorus, and exchangeable (sulphuric-acid-soluble)
phosphorus, whereby more-leached soils have lower pHs
and lower total and exchangeable P.
4.3.3 The Mudstone Hill Country
The mudstones, interspersed with some sandstones and
limestones, are mainly weak and poorly indurated, thus
weather readily to form soil. In calcareous mudstone the
main cementing agent is calcium carbonate which is easily
dissolved and leached in high rainfall environments. Sandstone and limestone layers are generally more indurated, and
resistant to erosion, than the mudstones and so tend to stand
out as steeper cliff faces within hill slopes. The softer
mudstones are readily eroded and soils are in a continual
state of development as drainage lines incise, steepening side
slopes, thus causing continued slope instability. Such areas
have potential for severe landslide erosion and deposition in
storm or earthquake events.
The less steep areas, such as rounded ridge crests and
rolling land, tend to be stable, thereby allowing Brown Soils
to develop. However, in more strongly dissected, actively
eroding, land, such as much of the land between Whanganui
and Raetihi, and between Gisborne and Wairoa, many areas
have narrow ridge summits with steep valley sides and river
flood plains on which mainly Recent or Raw Soils will occur.
In the North Island (some distance from the active volcanoes) where shallow (< *35 cm) blanketing of tephra
occurs, the soil pattern often comprises Allophanic Brown
Soils on stable ridge tops, Orthic Brown Soils on more stable
side (back) slopes that lack tephra accumulation, and Raw or
Recent Soils on eroded slopes as well as on valley floors
where eroded material has been redeposited. A slope angle
of about 26°, along with vegetation cover, seems to be
critical in determining if tephra deposits remain in place or
not, generally being lost (eroded) from slopes > *26°. Gley
Soils form in hollows where drainage is poor and on valley
floors where the water table is high. Where tephra deposits
are absent the stable, flatter, areas generally support Orthic
Brown Soils.
A feature of the soft rock hill country is a general lack of
catenary development (a regular, predictable, pattern of soil
development down a slope). Because of the relatively frequent erosion events, which lead to soil material moving
down-hill, leaving scars from which material was removed
and areas of redeposited soil downslope, the soil pattern
relates more strongly to the history of erosion events than to
position on the slope.
4.3.4 Otago Schist Landscapes
In Otago, Brown Soils often occur at higher altitudes where
evapotranspiration is lower, and rainfall is higher, than in the
lowland semiarid basins. The hard schist rock tends to form
landscapes with broad rounded crests and convex to concave
side (back) slopes. The drainage patterns and slopes are
often related to the strike and dip of the underlying schist
rock with steeper slopes tending to form where the schistosity is steeply dipping (as can be observed in Skippers
Canyon and the Cromwell Gorge).
On stable moister slopes, where leaching is sufficient,
Brown Soils form alongside Pallic Soils on drier slopes and
at lower altitudes, and with Gley Soils in poorly drained
shady hollows. The interaction between slope and aspect
provides an environment of high soil variability, as well as
high biodiversity. Northburn Station on the west side of the
Dunstan Range provides an example of the landscape relationship between the Semiarid, Pallic, and Brown Soils
(Fig. 4.4) resulting from more effective precipitation at
higher altitudes. Also evident is a strong topographic effect
with sparse vegetation, enhanced erosion, and shallow soils,
on the drier, windward, north and north-westerly facing,
slopes, and deeper, moister, more strongly vegetated soils on
the cooler, leeward, south-facing, slopes.
Particularly at higher altitude on the Central Otago ranges,
such as the Old Man Range and the Pisa Range, there is some
slope movement. Periglacial activity (still continuing slowly
today) is evident with solifluction lobes and ploughing
boulders visible on some higher slopes together with rock
glaciers and permafrost at some alpine sites. Large-scale
earth-slide features are also visible in the landscape. Where
the boundary between the debris mantle and the underlying
bedrock is ‘rough’ then it is likely that the materials are
locked together and surfaces have been stable for long periods with subsequent stable soil development. However, it is
likely that many of the landslides were triggered by earthquakes (potentially also associated with intense rainfall
events) and could again be remobilised by such events.
A series of large, long-term, landslides on the margins of the
Cromwell Gorge became the subject of much study and work
to stabilise them while the Clyde dam was being constructed
in the 1980s. The fear was that as the dam filled, the rising
groundwaters could mobilise some of the large landslides
with potential for huge volumes of material to fall into the
4.3 Soil-Landscape Relationships
63
favoured by higher temperature with consequently greater
soil development and lower fertility.
The degree of leaching can be estimated from the soil pH,
total phosphorus, and exchangeable (sulphuric-acid-soluble)
phosphorus, whereby more-leached soils have lower pHs
and lower total and exchangeable P.
4.3.3 The Mudstone Hill Country
The mudstones, interspersed with some sandstones and
limestones, are mainly weak and poorly indurated, thus
weather readily to form soil. In calcareous mudstone the
main cementing agent is calcium carbonate which is easily
dissolved and leached in high rainfall environments. Sandstone and limestone layers are generally more indurated, and
resistant to erosion, than the mudstones and so tend to stand
out as steeper cliff faces within hill slopes. The softer
mudstones are readily eroded and soils are in a continual
state of development as drainage lines incise, steepening side
slopes, thus causing continued slope instability. Such areas
have potential for severe landslide erosion and deposition in
storm or earthquake events.
The less steep areas, such as rounded ridge crests and
rolling land, tend to be stable, thereby allowing Brown Soils
to develop. However, in more strongly dissected, actively
eroding, land, such as much of the land between Whanganui
and Raetihi, and between Gisborne and Wairoa, many areas
have narrow ridge summits with steep valley sides and river
flood plains on which mainly Recent or Raw Soils will occur.
In the North Island (some distance from the active volcanoes) where shallow (< *35 cm) blanketing of tephra
occurs, the soil pattern often comprises Allophanic Brown
Soils on stable ridge tops, Orthic Brown Soils on more stable
side (back) slopes that lack tephra accumulation, and Raw or
Recent Soils on eroded slopes as well as on valley floors
where eroded material has been redeposited. A slope angle
of about 26°, along with vegetation cover, seems to be
critical in determining if tephra deposits remain in place or
not, generally being lost (eroded) from slopes > *26°. Gley
Soils form in hollows where drainage is poor and on valley
floors where the water table is high. Where tephra deposits
are absent the stable, flatter, areas generally support Orthic
Brown Soils.
A feature of the soft rock hill country is a general lack of
catenary development (a regular, predictable, pattern of soil
development down a slope). Because of the relatively frequent erosion events, which lead to soil material moving
down-hill, leaving scars from which material was removed
and areas of redeposited soil downslope, the soil pattern
relates more strongly to the history of erosion events than to
position on the slope.
4.3.4 Otago Schist Landscapes
In Otago, Brown Soils often occur at higher altitudes where
evapotranspiration is lower, and rainfall is higher, than in the
lowland semiarid basins. The hard schist rock tends to form
landscapes with broad rounded crests and convex to concave
side (back) slopes. The drainage patterns and slopes are
often related to the strike and dip of the underlying schist
rock with steeper slopes tending to form where the schistosity is steeply dipping (as can be observed in Skippers
Canyon and the Cromwell Gorge).
On stable moister slopes, where leaching is sufficient,
Brown Soils form alongside Pallic Soils on drier slopes and
at lower altitudes, and with Gley Soils in poorly drained
shady hollows. The interaction between slope and aspect
provides an environment of high soil variability, as well as
high biodiversity. Northburn Station on the west side of the
Dunstan Range provides an example of the landscape relationship between the Semiarid, Pallic, and Brown Soils
(Fig. 4.4) resulting from more effective precipitation at
higher altitudes. Also evident is a strong topographic effect
with sparse vegetation, enhanced erosion, and shallow soils,
on the drier, windward, north and north-westerly facing,
slopes, and deeper, moister, more strongly vegetated soils on
the cooler, leeward, south-facing, slopes.
Particularly at higher altitude on the Central Otago ranges,
such as the Old Man Range and the Pisa Range, there is some
slope movement. Periglacial activity (still continuing slowly
today) is evident with solifluction lobes and ploughing
boulders visible on some higher slopes together with rock
glaciers and permafrost at some alpine sites. Large-scale
earth-slide features are also visible in the landscape. Where
the boundary between the debris mantle and the underlying
bedrock is ‘rough’ then it is likely that the materials are
locked together and surfaces have been stable for long periods with subsequent stable soil development. However, it is
likely that many of the landslides were triggered by earthquakes (potentially also associated with intense rainfall
events) and could again be remobilised by such events.
A series of large, long-term, landslides on the margins of the
Cromwell Gorge became the subject of much study and work
to stabilise them while the Clyde dam was being constructed
in the 1980s. The fear was that as the dam filled, the rising
groundwaters could mobilise some of the large landslides
with potential for huge volumes of material to fall into the
4.3 Soil-Landscape Relationships
63