For loess accumulation there needs to be vegetation to
trap the airborne material otherwise it may be rapidly
removed by water runoff. At the site of deposition the
ground surface needs to be stable to accumulate a significant
loess deposit. If the receiving surface is a highly porous
gravel the loess may be washed into the gravel matrix.
Eventually the pores will be plugged, preventing further
loess infiltration.
Loess usually accumulates very slowly—even a dusting
of loess on leaves will, in time, accumulate on the soil
surface to form substantial deposits many metres thick. In
general, the fastest rates of loess accretion were in the cold
glacial periods, when the mean rate was c. 3–10 mm per
century. The fastest rates were 15–25 mm per century, the
slowest < 1 mm per century. Although most loess was
deposited before the Holocene, loess is still being deposited
today albeit at a much reduced scale, especially along the
margins of South Island braided river beds. Wind gusts may
reach high velocities (> 100 km.hr
−1 ). Such wind is erosive
and contributes to transport of loess in the uplands, predominantly from slopes exposed to northerly and westerly
winds, to the relatively sheltered south or east facing slopes.
Thus soils on slopes with southerly or easterly aspects may
have deeper, finer textured, horizons than soils with northerly and westerly aspects. Such loess deposits are always
modified to some degree by pedogenic processes that continue while the loess is accumulating, slowly and with weak
impact during cold periods but faster and more effectively
during warm periods (i.e. by developmental upbuilding pedogenesis, Chap. 10).
d. Soil variation in the high country
Spatial soil variability in hill and mountain land is often high
and falls into two broad types: soil patterns where soil age
(time) is the dominant determinant of differences (temporally
arrayed), and soils of similar age where topography is the
dominant influence (spatially arrayed).
The soils of alluvial terraces, fans, or glacial moraines can
serve as important time markers in mountainous regions.
Where land surfaces can be dated, a relationship may be
discerned between estimated ages and a number of soil and
land development indicators, including soil properties, land
surface morphology, rock weathering rinds, and lichens. Soil
classification may provide a useful indicator of soil development but use of specific chemical, physical, or other
morphological properties is likely to provide more precise
time/soil development correlations. Although tephra deposits
in much of the central and western North Island can provide
marker beds of known age only one tephra, the
Taupo-volcano-derived Kawakawa/Oruanui Tephra (aged c.
25,400 years) provides an important age datum in the South
Island. This tephra has been identified as a thin layer or as a
concentration of tiny glass shards (a cryptotephra) in peats,
loess, and alluvial and glacial deposits in Canterbury, Otago,
Westland, and the north-west Nelson region.
Correlations between time and soil development, or other
indicators, can then be applied in cautious reconstruction of
chronologies of soil patterns in a local terrain. Trevor Chinn
and Ian Whitehouse used rock weathering rinds and lichen
growth correlations with time to estimate the ages of screes
in the Canterbury mountains, and thus the timing of scree
activity. They found evidence that the prominent mountain
screes in the landscape were not caused by human management but had formed long before grazing animals were
introduced. Thus the screes are recognised as geologic features that pre-existed human settlement.
Temporally arrayed soil patterns are generally formed by
erosion-deposition processes. Younger soil profiles result
from relatively recent deposition of eroded soil, or recent
truncation by erosion. Older soil profiles result from comparatively longer periods of stability when the soil has
escaped losses by erosion or burial by sediment. The pattern
of soil maturity and the evidence of erosion or sedimentation
enable inferences to be made about the history, processes,
and nature and severity of erosion, at a site. Erosion and
deposition can be triggered by several causes including
heavy and prolonged rainfall, earthquakes, and heightened
susceptibility of soil materials to erosion due to advanced
weathering or changes in protective vegetation cover.
Instability may occur as a periodic phenomenon that, once
triggered, can lead to extensive erosion within a catchment.
An example is the periodic destruction of patches of forest
by big storm events, leading to a mound-pit microtopography and soil disruption/mixing as a result of tree overturn.
Spatially arrayed soils have similarly mature profile
development, but systematic variations in the soil profile are
related to topography. Phil Tonkin developed a model of the
relative stage of soil development in relation to aspect and
altitude in the South Island high country. At lower altitudes,
in the drier (mean annual rainfall 600–700 mm) environment, there were no differences between soils with northerly
and southerly aspects and vegetation was equally limited by
dryness.
At intermediate altitudes where the mean annual rainfall
is somewhat higher (700–1200 mm) the relative stage of soil
development was more advanced on the wetter, more leached, south aspect compared to soils with a north aspect. As
a result of the higher leaching, the wetter southerly aspect
soils had lower soil fertility but, because of higher water
availability, better vegetation growth than soils with northerly aspects.
In contrast, at the highest altitudes with a mean annual
rainfall of 1200–1400 mm, sufficient rain falls to promote
62
4 Brown Soils
trap the airborne material otherwise it may be rapidly
removed by water runoff. At the site of deposition the
ground surface needs to be stable to accumulate a significant
loess deposit. If the receiving surface is a highly porous
gravel the loess may be washed into the gravel matrix.
Eventually the pores will be plugged, preventing further
loess infiltration.
Loess usually accumulates very slowly—even a dusting
of loess on leaves will, in time, accumulate on the soil
surface to form substantial deposits many metres thick. In
general, the fastest rates of loess accretion were in the cold
glacial periods, when the mean rate was c. 3–10 mm per
century. The fastest rates were 15–25 mm per century, the
slowest < 1 mm per century. Although most loess was
deposited before the Holocene, loess is still being deposited
today albeit at a much reduced scale, especially along the
margins of South Island braided river beds. Wind gusts may
reach high velocities (> 100 km.hr
−1 ). Such wind is erosive
and contributes to transport of loess in the uplands, predominantly from slopes exposed to northerly and westerly
winds, to the relatively sheltered south or east facing slopes.
Thus soils on slopes with southerly or easterly aspects may
have deeper, finer textured, horizons than soils with northerly and westerly aspects. Such loess deposits are always
modified to some degree by pedogenic processes that continue while the loess is accumulating, slowly and with weak
impact during cold periods but faster and more effectively
during warm periods (i.e. by developmental upbuilding pedogenesis, Chap. 10).
d. Soil variation in the high country
Spatial soil variability in hill and mountain land is often high
and falls into two broad types: soil patterns where soil age
(time) is the dominant determinant of differences (temporally
arrayed), and soils of similar age where topography is the
dominant influence (spatially arrayed).
The soils of alluvial terraces, fans, or glacial moraines can
serve as important time markers in mountainous regions.
Where land surfaces can be dated, a relationship may be
discerned between estimated ages and a number of soil and
land development indicators, including soil properties, land
surface morphology, rock weathering rinds, and lichens. Soil
classification may provide a useful indicator of soil development but use of specific chemical, physical, or other
morphological properties is likely to provide more precise
time/soil development correlations. Although tephra deposits
in much of the central and western North Island can provide
marker beds of known age only one tephra, the
Taupo-volcano-derived Kawakawa/Oruanui Tephra (aged c.
25,400 years) provides an important age datum in the South
Island. This tephra has been identified as a thin layer or as a
concentration of tiny glass shards (a cryptotephra) in peats,
loess, and alluvial and glacial deposits in Canterbury, Otago,
Westland, and the north-west Nelson region.
Correlations between time and soil development, or other
indicators, can then be applied in cautious reconstruction of
chronologies of soil patterns in a local terrain. Trevor Chinn
and Ian Whitehouse used rock weathering rinds and lichen
growth correlations with time to estimate the ages of screes
in the Canterbury mountains, and thus the timing of scree
activity. They found evidence that the prominent mountain
screes in the landscape were not caused by human management but had formed long before grazing animals were
introduced. Thus the screes are recognised as geologic features that pre-existed human settlement.
Temporally arrayed soil patterns are generally formed by
erosion-deposition processes. Younger soil profiles result
from relatively recent deposition of eroded soil, or recent
truncation by erosion. Older soil profiles result from comparatively longer periods of stability when the soil has
escaped losses by erosion or burial by sediment. The pattern
of soil maturity and the evidence of erosion or sedimentation
enable inferences to be made about the history, processes,
and nature and severity of erosion, at a site. Erosion and
deposition can be triggered by several causes including
heavy and prolonged rainfall, earthquakes, and heightened
susceptibility of soil materials to erosion due to advanced
weathering or changes in protective vegetation cover.
Instability may occur as a periodic phenomenon that, once
triggered, can lead to extensive erosion within a catchment.
An example is the periodic destruction of patches of forest
by big storm events, leading to a mound-pit microtopography and soil disruption/mixing as a result of tree overturn.
Spatially arrayed soils have similarly mature profile
development, but systematic variations in the soil profile are
related to topography. Phil Tonkin developed a model of the
relative stage of soil development in relation to aspect and
altitude in the South Island high country. At lower altitudes,
in the drier (mean annual rainfall 600–700 mm) environment, there were no differences between soils with northerly
and southerly aspects and vegetation was equally limited by
dryness.
At intermediate altitudes where the mean annual rainfall
is somewhat higher (700–1200 mm) the relative stage of soil
development was more advanced on the wetter, more leached, south aspect compared to soils with a north aspect. As
a result of the higher leaching, the wetter southerly aspect
soils had lower soil fertility but, because of higher water
availability, better vegetation growth than soils with northerly aspects.
In contrast, at the highest altitudes with a mean annual
rainfall of 1200–1400 mm, sufficient rain falls to promote
62
4 Brown Soils