exploited. Increased surface sealing by impermeable
surfaces such as roads and roofs leads to lowered
infiltration and more rapid runoff, exacerbating
flooding in strongly urbanised catchments.
14.3.2 Soil Variability in Soft Rock Hill Country
Soft rock (papa or mudstone) hill country occurs predominantly in the extensive, productive, pastoral hilly land of the
east coast from East Cape to the Wairarapa, and in the
Rangitikei/Whanganui areas of southern North Island. The
blueish grey papa/mudstones are sedimentary rocks derived
from clays, silts, and sands that were deposited in the sea
mainly during the Miocene period (between about 24 and 5
million years ago) which have been rapidly uplifted as a
result of the collision of the Pacific and Australia tectonic
plates. Because the rocks are relatively young and were not
deeply buried they are not particularly indurated (cemented
into hard rock). Thus, the papa has low strength compared
with New Zealand’s much older basement greywacke or
schist rocks. Whereas basement rocks will breakdown by
physical processes to form stony colluvium or alluvium, the
soft rock papa readily disaggregates into the primary texture
(sand, silt, or clay) particles. The papa in many subsoils is
sufficiently soft that it can be cut with a spade.
Due to the rapid uplift and erodibility of the papa, the
landscape tends to be hilly with steep gullies cut into the soft
rock by streams and rivers. The soft rock hill country is
susceptible to mass movement, especially landslides (slips),
and mud- or earthflows. A soil slip is initiated when water
accumulates at the contact between the soil and the underlying slowly permeable rock. Soil pores fill with water, and
the weight of water-saturated soil, and consequent lowering
of soil strength, opens crevasses in the soil cover. The mass
of soil may crack across the slope and part company from
the relatively stable ridge material. Once cracks have formed
they provide a conduit for water to enter the interface
between the soil and underlying papa.
During periods of prolonged heavy rain, the soil material
becomes saturated and heavy. On steeper slopes, the slip
material loses its coherency and a slide forms with a
water-lubricated shear plane on the steep rock interface (e.g.
see Fig. 4.7). Movement of the wet soil mass rapidly accelerates
downslope and what is initially a landslide on a shear plane in
the upper slope may transform into a mud-slide (earthflow) with
material flowing on down the hill. Some material will be
deposited on foot slopes where the flow loses momentum, as
the slope declines, and runout deposits from mud/earth-slides
can extend well down valleys. Some material may be carried by
floodwaters into rivers where it may be deposited on river flood
plains or carried out to sea to form the next generation of sedimentary rocks. A scar is often left with a steep exposure of raw
blue papa at the head of the landslide. Deposits of displaced
material, some still coherent with topsoils preserved, and some,
just piles of saturated subsoil, are left on the lower slopes.
In the Taranaki-Whanganui region, Mike Crozier and
Brad Pillans found that following recent deforestation, and
associated slope destabilization, erosion rates increased to
three times their previous rate. The forces involved in the
erosion include earthquakes but the main culprit is rainstorms with a return period of about 10 years which, during
a single event, can remove soil and underlying regolith from
5 to 15% of hillslopes over areas of up to 100 km
2 . Michael
Selby had earlier suggested that intense rainfalls are
responsible for most of the slope erosion in the greywacke
ranges of the lower and middle Waikato Basins. Crozier, in
2005, concluded that landsliding in New Zealand most
commonly occurs in the form of multiple-occurrence landslide events, simultaneously involving thousands to tens of
thousands of landslides over areas extending up to 20,000
km
2 . Agricultural production loss through loss of soils,
damage to road and rail infrastructure, and increased flooding, are the main consequences of these events.
Over time, especially in the years since the native forest
was removed, continuing erosion and recovery in many parts
of New Zealand has left a legacy of high soil variability.
There are some discernible order to the soil variability on
soft rock hill slopes, with a soil mosaic pattern, including
Brown Soils (Chap. 2), Recent Soils, and Raw Soils
(Chap. 13), related to landscape position, erosion slip processes, and time since erosion occurred. A number of different zones are often apparent.
1. Ridges. The ridges are generally relatively stable with erosion
scars rarely extending to the ridge crest. Soils on the ridges are
usually the least variable, often with strongly developed
Brown, Pallic, Ultic, Podzol, or Allophanic Soils.
2. Stable hillslopes. Areas of the hillside that have not been
subject to landslide activity for a considerable time (centuries or millennia) will usually have mature soils that
relate to the climate of the area, ranging from Pallic
through Brown to Podzol and Ultic Soils. In areas, marginal to the central North Island volcanoes, Allophanic
Soils may predominate on the less-steep slopes. However,
on steeper slopes (>*26°), the tephras may have been
largely removed by surface erosion and so Granular, Ultic,
or Brown Soils will be formed in the previously underlying, more strongly weathered, clay-rich materials.
3. Erosion head-scarps. Head-scarps are exposed shear
planes and usually occur in the lower shoulder slope or
upper back slope of a hillside. Head-scarps are generally
14 Recent Soils
223
surfaces such as roads and roofs leads to lowered
infiltration and more rapid runoff, exacerbating
flooding in strongly urbanised catchments.
14.3.2 Soil Variability in Soft Rock Hill Country
Soft rock (papa or mudstone) hill country occurs predominantly in the extensive, productive, pastoral hilly land of the
east coast from East Cape to the Wairarapa, and in the
Rangitikei/Whanganui areas of southern North Island. The
blueish grey papa/mudstones are sedimentary rocks derived
from clays, silts, and sands that were deposited in the sea
mainly during the Miocene period (between about 24 and 5
million years ago) which have been rapidly uplifted as a
result of the collision of the Pacific and Australia tectonic
plates. Because the rocks are relatively young and were not
deeply buried they are not particularly indurated (cemented
into hard rock). Thus, the papa has low strength compared
with New Zealand’s much older basement greywacke or
schist rocks. Whereas basement rocks will breakdown by
physical processes to form stony colluvium or alluvium, the
soft rock papa readily disaggregates into the primary texture
(sand, silt, or clay) particles. The papa in many subsoils is
sufficiently soft that it can be cut with a spade.
Due to the rapid uplift and erodibility of the papa, the
landscape tends to be hilly with steep gullies cut into the soft
rock by streams and rivers. The soft rock hill country is
susceptible to mass movement, especially landslides (slips),
and mud- or earthflows. A soil slip is initiated when water
accumulates at the contact between the soil and the underlying slowly permeable rock. Soil pores fill with water, and
the weight of water-saturated soil, and consequent lowering
of soil strength, opens crevasses in the soil cover. The mass
of soil may crack across the slope and part company from
the relatively stable ridge material. Once cracks have formed
they provide a conduit for water to enter the interface
between the soil and underlying papa.
During periods of prolonged heavy rain, the soil material
becomes saturated and heavy. On steeper slopes, the slip
material loses its coherency and a slide forms with a
water-lubricated shear plane on the steep rock interface (e.g.
see Fig. 4.7). Movement of the wet soil mass rapidly accelerates
downslope and what is initially a landslide on a shear plane in
the upper slope may transform into a mud-slide (earthflow) with
material flowing on down the hill. Some material will be
deposited on foot slopes where the flow loses momentum, as
the slope declines, and runout deposits from mud/earth-slides
can extend well down valleys. Some material may be carried by
floodwaters into rivers where it may be deposited on river flood
plains or carried out to sea to form the next generation of sedimentary rocks. A scar is often left with a steep exposure of raw
blue papa at the head of the landslide. Deposits of displaced
material, some still coherent with topsoils preserved, and some,
just piles of saturated subsoil, are left on the lower slopes.
In the Taranaki-Whanganui region, Mike Crozier and
Brad Pillans found that following recent deforestation, and
associated slope destabilization, erosion rates increased to
three times their previous rate. The forces involved in the
erosion include earthquakes but the main culprit is rainstorms with a return period of about 10 years which, during
a single event, can remove soil and underlying regolith from
5 to 15% of hillslopes over areas of up to 100 km
2 . Michael
Selby had earlier suggested that intense rainfalls are
responsible for most of the slope erosion in the greywacke
ranges of the lower and middle Waikato Basins. Crozier, in
2005, concluded that landsliding in New Zealand most
commonly occurs in the form of multiple-occurrence landslide events, simultaneously involving thousands to tens of
thousands of landslides over areas extending up to 20,000
km
2 . Agricultural production loss through loss of soils,
damage to road and rail infrastructure, and increased flooding, are the main consequences of these events.
Over time, especially in the years since the native forest
was removed, continuing erosion and recovery in many parts
of New Zealand has left a legacy of high soil variability.
There are some discernible order to the soil variability on
soft rock hill slopes, with a soil mosaic pattern, including
Brown Soils (Chap. 2), Recent Soils, and Raw Soils
(Chap. 13), related to landscape position, erosion slip processes, and time since erosion occurred. A number of different zones are often apparent.
1. Ridges. The ridges are generally relatively stable with erosion
scars rarely extending to the ridge crest. Soils on the ridges are
usually the least variable, often with strongly developed
Brown, Pallic, Ultic, Podzol, or Allophanic Soils.
2. Stable hillslopes. Areas of the hillside that have not been
subject to landslide activity for a considerable time (centuries or millennia) will usually have mature soils that
relate to the climate of the area, ranging from Pallic
through Brown to Podzol and Ultic Soils. In areas, marginal to the central North Island volcanoes, Allophanic
Soils may predominate on the less-steep slopes. However,
on steeper slopes (>*26°), the tephras may have been
largely removed by surface erosion and so Granular, Ultic,
or Brown Soils will be formed in the previously underlying, more strongly weathered, clay-rich materials.
3. Erosion head-scarps. Head-scarps are exposed shear
planes and usually occur in the lower shoulder slope or
upper back slope of a hillside. Head-scarps are generally
14 Recent Soils
223