4  Geomorphology

Modified

December 7, 2024

Geomorphology limits the range of soils than can occur at specific locations on the basis of slope, relief, aspect, and drainage. Geomorphology can be described at multiple scales, which break down into progressively simpler shapes. The data points described in this chapter help the pedologist to do that consistently.

4.1 Province

Landscape Provinces identify major geomorphic zones in New Zealand, expressive of large-scale lithological associations, tectonic regime, and climate. Table 4.1 lists their names and Figure 4.1 shows their general distribution.

Table 4.1: Landscape provinces of New Zealand
Northern Provinces Southern Provinces
NN Northland SI South Island axial mountains
NL Northern Lowlands SR Front Ranges and inland basins
NW North-western sedimentary hills SO Otago basin and range
NV Central Volcanic Zone SF Fiordland
NT Stratovolcanoes and ringplains SW West coast piedmont, hills and mountains
NI North Island Axial Mountains SN Northwest Nelson
NS Southern and Eastern Sedimentary Hills SS North Bank and the Marlborough Sounds
NM Marine Terraces, Plains and Dunelands SM Marlborough Mountains
NA Accretionary wedge and old sedimentary hills SP East Coast Peninsulas
NC Chatham Islands SE East Coast Plains and Downs
SL Southland mountains, hills and plains
ST Stewart Island

Northern:

Name Description
Northland
Northern Lowlands
North-western sedimentary hills
Central Volcanic Zone
Stratovolcanoes and ringplains
North Island Axial Mountains
Southern and Eastern Sedimentary Hills
Marine Terraces, Plains and Dunelands
Accretionary wedge and old sedimentary hills
Chatham Islands

Southern:

Name Description
South Island axial mountains: Steep to precipitous, high, glaciated mountains of the South Island Main Divide and associated ranges with strong structural and lithological control of the macro-topography.
Front Ranges and inland basins Steep to very steep dissected mountain ranges with extensive scree, sharp crested peaks and ridges, and deep colluvial and moraine mantled footslopes, separated by wide braided river valleys and fault controlled inland basins with extensive glacial outwash terraces, moraines, and lakes. Predominantly Torlesse greywacke sandstone and mudstone, and tz II schist of acid-intermediate quartz feldspathic composition.
Otago basin and range Prominent steep-sided and flat to rolling broad-crested ranges with tors, separated by wide deformation controlled flat-floored sediment-filled basins and valleys with extensive glacial moraine, outwash terraces, and recent valley fill. Predominately schist (tz IIa – tz IV) with minor Torlesse sedimentary rocks of acid-intermediate quartzo- feldspathic composition. Tertiary aged sedimentary soft rock inliers incorporating minor alkali to basaltic volcanics underlie the floor floors, with remnants present on ridge crests as outliers and fringe the coastline in places. Extensive loess mantles lower elevation slopes.
Fiordland Steep to very steep to precipitous alpine and valley slopes with cirque basins, U-shaped valleys, rolling tops, and coastal fiords developed on hard, massive, coarse-grained, crystalline acid to basic igneous and metamorphic rocks, minor Paleozoic sedimentary, calcareous, and ultrabasic rocks all with very shallow, weakly weathered regoliths. Minor Tertiary sedimentary lithologies and valley floor in-fill.
West coast piedmont, hills and mountains Dissected, extensive fluvioglacial outwash terraces, moraine ridges, and marine beach terraces; Recent broad valley floor floodplains, terraces, and fans infilling abandoned glacial troughs; extensive lowland moderately steep to steep hillscapes; and steep, glaciated and intensely fluvially dissected mountain ranges of granite and gneiss, or ancient sedimentary rocks (greywacke) rocks with a drape of less well indurated Cretaceous-Tertiary cover beds in places.
Northwest Nelson Steep to very steep extensively glaciated mountain ranges with an exhumed peneplain and lithologically controlled topography underlain primarily by hard to very hard heterogenous Paleozoic sedimentary and acid (granitic) igneous and altered volcanics rocks, marble, and the remnants of hard and soft Cretaceous-Cenozoic cover beds. The region includes the deeply weathered Pleistocene gravels of the Moutere depression.
North Bank and the Marlborough Sounds Steep to very steep dissected mountains with angular ridges and narrow valleys dominated by indurated quartofeldspathic sedimentary and lower grade schist rocks [tz IIa, IIb]; a belt of ultramafic and associated volcanic rocks characterised by irregular, hummocky topography; and limestones, volcaniclastics, and basalts to the west. Structural and lithological control of the landscape is evident with thick sandstone sequences forming ridge crests, and valley profiles showing strong asymmetry with prominent dip slopes reflecting the regional schistosity.
Marlborough Mountains Steep to very steep fault splintered structurally controlled intensively dissected sharp crested mountain ranges with extensive bare rock and scree, rock glaciers and minor cirque basins, and extensive steep to very steep dissected lower mountain and colluvial footslopes, separated by long, straight intermittently terraced valleys and minor intermontane basins. Underlain by acid-intermediate quartz feldspathic Torlesse greywacke rocks, with minor basic igneous plutons, dikes, and sills; with remnants of Cretaceous-Pliocene covering strata preserved in fault-angle depressions and along the coastal margin.
East Coast Peninsulas Strongly rolling, moderately steep and steep, dissected collapsed intermediate to basic volcanic peninsulas with bedrock, mixed volcanic bedrock and loess, and loess-dominated colluvium.
East Coast Plains and Downs Extensive, coalescing, broad outwash piedmont fans, aggradational and degradational terraces, lower fan, and low-gradient coastal plain depositional sequences; associated interior basins, and the rolling to strongly rolling to moderately steep hills and downlands underlain by moderately soft and hard young non-calcareous and calcareous rocks, greywacke, and weathered older gravels, with or without a significant loess mantle.
Southland mountains, hills and plains Steep to very steep glaciated mountains shedding volcanic and basic-quartzofeldsphathic colluvium, moderately steep to steep hills with prominent strike ridges developed on andesitic sedimentary rocks, with an intermittent loess mantle; moderately steep to steep hill country developed on deeply weathered plutonic rocks, and the alluvial landscapes sourced from the Fiordland and Wakatipu glaciers; moraines and extensive coalescing and overlapping alluvial aprons, and suites of outwash terraces, fans and floodplains; coastal Recent and older sand dunes, gravel beach ridges, alluvial lagoonal deltaic flats and extensive peat bogs; and encompassed hard and soft rock cored downlands and hills. Deep loess mantles a significant proportion of all topographies but is thickest on the terraces and rolling downlands.
Stewart Island Steep to very steep mountain slopes with minor cirques and moraines; and strongly rolling to moderately steep, dissected, hill and plateau slopes developed on coarse-grained deeply weathered crystalline granitic- to dioritic plutonic rocks; with steep cliffs interrupted by beaches backed by large sand dune complexes extending inland on the western coastline and the drowned valleys of Paterson Inlet, Port Adventure, Lords River and Port Pegasus on the east; extensive sand plain deposits form gently east-dipping flights of terraces throughout the Freshwater River catchment.

Given their very broad extent, region does not need to be recorded in the field. However, the province boundaries are not currently strictly defined so an understanding of their conceptual basis (Note 4.1) is required before a confident allocation can be made.

4.2 Landscape

The landscape around a soil observation comprises a distinct geomorphological pattern with repeating elements. The scale of observation varies but commonly extends from a few hundred to a few thousand meters (Heck 2017). Landscapes nest within Provinces but are not exclusive to one or another.

4.2.1 Parameterising landscapes

A landscape can be described in terms of a short list of parameters: relief, characteristic slope, drainage pattern, and the landforms contained within the landscape.

4.2.1.1 Relief

Relief is the degree of separation between high and low points in a landscape. As described in (Soil and Terrain 2009, p. 45), relief can be visualised by picturing two surfaces roughly parallel to the land surface - one passing through local crests and one through local depressions. The median vertical distance between the two surfaces is the relief.

add a diagram here

Relief is usually best determined from a DEM, but can be estimated in the field. Record in whole meters if estimating, e.g. 55 m. Relief classes may be applied to the recorded data; see Section 23.3.

4.2.1.2 Characteristic slope

The characteristic slope is the most common range of slopes within a regional landscape. These are often controlled by local geology.

Characteristic slope is usually best determined from zonal analysis of a DEM-derived slope dataset, but can be estimated in the field. Estimate in the field as a median and range, in whole degrees, e.g. 25° (20-28). Slope classes may be applied to the recorded data; some options are presented in Section 23.1.

Some landscapes may have two or more distinct characteristic slopes, e.g. the fronts and tops of alluvial terrace elements. In this case the median will describe the dominant surface while the range will capture the steeper areas. Recording multiple characteristic slopes at landscape scale is not considered useful, as adequate context is provided by naming the landscape and its component landforms.

4.2.1.3 Stream channels

Natural stream channel patterns are relevant to understanding landscape development, but are best observed remotely. They do not need to be recorded in the field. If desired, assess the landscape drainage pattern using available data sources and define it using the stream channel attributes given in Soil and Terrain (2023), p. 26-29.

4.2.1.4 Landscape description

Few controlled vocabularies for landscapes exist. The schema given below is provisional and based on terms seen in common use in New Zealand environmental datasets.

Table 4.2: Common Landscape terms
Code Name Description Synonyms and subtypes
Mt Mountains Steep to very steep, high relief (>300 m), fixed and well connected stream channels, often deep Alps, Volcanoes, Mountain country
Hi Hills Gentle to very steep, moderately high to high relief (90-300 m), fixed and well connected stream channels, shallow to deep Hillscapes, hill country, hilly land
Up Uplands Gentle to very steep, low relief (30-90 m) but relatively high elevation, fixed and well connected stream channels, shallow valleys and plateaus, located among hills and mountains Piedmont
Ll Lowlands Gentle to very steep, low relief (30-90 m) and elevation, fixed and well connected stream channels, shallow to deep Downlands
Te Terraces Former plains, uplifted and sometimes covered by loess. Flat to gently undulating upper surfaces bounded by distinctive drop-offs on their downhill end, often backing on to lowlands, hills or mountains at their uphill end. Marine benches
Pl Plains Level to very gentle surfaces created by fluvial and/or airfall tephra deposition. Floodplains, ringplains
Sc Sand country Extensive dune and sandplain areas. Low relief and elevation, gentle to steep slopes, and active stream channels, often with interrupted connectivity Dunefields
Cs Coasts On- and near-shore features created by the interaction of the ocean with the land, including beaches, deltas, lagoons and estuaries Coastlines

4.2.1.5 Composition

A breakdown of the major landforms expected within the landscape should be included (see Section 4.3 below) along with an estimate of their relative dominance (e.g. Cs: Bc 80%, Du 10%, Cl 10% for a coastal landscape comprising steep cliffs fronted by a narrow beach system with some limited dune development in places where the beach is wide enough to allow it).

4.3 Landform

A landform is a component of the landscape, and again can have a variable extent - usually 10’s to 100’s of meters. A landform has a relatively simple shape and is not itself composed of multiple repeating elements in the way that a landscape is. A landform has a characteristic range of slopes and aspects. Landforms also have distinctive composition in terms of geology/parent materials and are usually restricted to particular locations within a landscape.

4.3.1 Naming Landforms

Few controlled vocabularies for landforms exist. The schema given below is provisional and based on terms seen in common use in New Zealand environmental datasets and related publications.

Table 4.3: Common soil-bearing landforms of New Zealand (adapted from Milne et al. (1995), Johnson and Gerbeaux (2004), Tielidze et al. (2021), and Soil and Terrain (2023))
Code Name Description Synonyms and subtypes
Natural
Ba Bar A ridge-like accumulation of sand, gravel, or other alluvial material formed in the channels, along the banks, or at the mouth of a stream where a decrease in velocity induces deposition sandbar, channel bar, meander bar
Bc Beach Low-slope, shorefront accumulation of unconsolidated sediment (usually sand to cobble sized)
Bo Bog Rainfall-fed, peat-accumulating freshwater wetland with low pH and nutrient availability and restricted water flow peat bog, domed bog
Ca Caldera Large hollow in the top of a volcano volcanic crater
Cl Cliff Steep to vertical slope with more than 10 m local relief
Cn Cone Moderate to steep, usually isolated hill built up by volcanic activity
Cr Crest A locally high area, from which the surface slopes downwards in opposite directions Apex, Summit
Dl Delta A fan-shaped accumulation of alluvial sediment, usually with several water channels, at a river or stream mouth. River mouth
Du Dune Accumulation of windblown sand into an elongated ridge or crescent shape
Ew Ephemeral wetland Wetland subject to pronounced seasonal drying. Non-peaty, with neutral pH and moderate nutrient status. Usually in a closed depression
Fl Flat Level area created by deposition Tidal flat, intertidal flat, supratidal flat, infill
Fn Fan Gently sloping, flat to convex depositional slope, often at the foot of a steeper area and built up by a stream or by colluvial action
Fe Fen Rainfall and groundwater-fed peaty freshwater wetland with low to moderate pH and nutrient availability, and slightly fluctuating water table
Go Gorge A valley 10 m or more deep with over three-quarters of its sides sloping at more than 25°, and half or more of its sides formed by cliffs, and less than 5 times as wide as it is deep over half or more of its length, and everywhere less than 10 times as wide as it is deep Ravine
Lv Levee An embankment of flood alluvium built up alongside a river and typically with lower-lying land behind
Ma Marsh Wetlands with large periodic water table fluctuations, relatively high pH and nutrient content, and mainly mineral substrate
Mh Hummocky moraine Low, roughly undulating deposits of glacial till
Mo Mound A small hill with moderate to steep sides, often somewhat isolated Lahar mound, knoll, hillock
Mr Moraine ridge Low ridges comprising glacial till
Pk Pakihi and gumland Wetland in highly leached soils with low pH and nutrient availability. Sometimes periodically dry and usually without peat
Pl Plain Level to gently undulating area built up by alluvial, aeolian, or volcanic deposition Alluvial plain, backplain, sand plain, outwash plain
Pt Plateau Any comparatively flat extensive and elevated area, often dissected and with a steep dropoff on at least one side Tableland
Ri Riser The vertical or steeply sloping surface of one of a series of natural, step-like landforms, as those of a glacial stairway or of successive stream terraces
Rt Tread The flat or gently sloping broadly planar surface of one of a series of natural step-like landforms, as those of a glacial stairway or of successive stream terraces Bench
Sa Saddle A low point (dip) along a ridge or the lowest point between two adjacent elevated areas. In one axis along the ridge or between the elevated areas the land slopes up in both directions and in the other axis, typically at right angles to the first, the land slopes down in both directions
Se Seepage Sloped area carrying a slow but steady flow of groundwater Flushes
Sm Swamp Rainfall and groundwater-fed, somewhat peaty freshwater wetland with high pH and nutrient availability, and seasonally fluctuating water table Backswamp
Sw Swale A low-lying depression between adjacent ridges Interdune, dune slack
Sl Slope Very gentle to very steep inclined surfaces Backslope, midslope, toeslope, footslope, dip slope, flank, headslope, talus slope
St Saltmarsh Wetlands with saline water supply, usually in tidally influenced areas Estuary, mudflat
Vl Valley-bottom Open depression between parallel facing slopes
Human-induced
Cs Cut-surface Flat excavated by human activity
Cf Cut-face Slope excavated by human activity
Em Embankment Ridge or slope built up by human activity
Fi Fill-top Flat built up by human activity

4.3.2 Describing landforms

Where available landforms are unsatisfactory or a more detailed parameterisation of the landform is desired, the landform can optionally be described in terms of its slope, orientation and element.

4.3.2.1 Slope

Record the characteristic slope of the major part of the landform as median and range in whole degrees, e.g. 5° (4-6) for a low-angle fan. Slope classes may be applied to the recorded data; some options are presented in Section 23.1.

4.3.2.2 Orientation

For simpler sloping landforms, record orientation as the characteristic aspect of the major part of the landform. Record median and range in whole degrees, e.g. 315 (300-330) for a roughly northwest direction. For complex landforms with a range of aspects, record the orientation of the landform’s longest axis, looking downslope (e.g. the direction in which a valley bottom is descending). Some landforms may not have an identifiable orientation (e.g. cones, saddles), in which case record NA.

Aspect classes may be applied to the recorded data; some options are presented in Section 23.2.

4.3.2.3 Landform element

The following is adapted from (Soil and Terrain 2009, p. 19–26).

A landform element is the simplest component of a landform, and some landforms are made up of only one element. Landform elements are first assigned to a morphological type - one of a series of simple shapes - at the scale of observation. These are:

Table 4.4: Landform Element Morphologies
Code Name
CR Crest
RI Ridge
SL Slope
FL Flat
DO Open depression
DC Closed depression

Note that these are paired mirror images, e.g. a closed depression is the opposite of a crest. Each morphological type extends to a break of slope or point of 0-curvature.

Slopes can be further refined by including information about their relationship to their neighbouring elements. Up and down slope, four options are available.

Table 4.5: Slope element relative inclination
Code Name Description
X Waxing Element upslope is gentler, element downslope is steeper.
N Waning Element upslope is steeper, element downslope is gentler.
A Maximal Element upslope is gentler, element downslope is gentler.
I Minimal Element upslope is steeper, element downslope is steeper.

Three more are available across the slope.

Table 4.6: Slope element relative enclosure
Code Name Description
L Enclosed Adjacent slope elements face towards each other
E Exposed Adjacent slope elements face away from each other
O Open Adjacent slope elements face the same direction

This combination of properties gives rise to more complex shapes. For example, an open hollow could be described as an enclosed maximal slope (SL A L). A shoulder would be an exposed waxing slope (SL X E). Adding further information about slope and aspect can define a complete element. For instance, for a gently sloping, northeast facing fan - 5 40 SL N E.

TODO add graphics

4.4 Other terrain parameters

The following terrain parameters may be useful in particular contexts.

4.4.1 Slope length

Slope length is used in assessments of erosion vulnerability. Length is measured from the point at which a slope becomes steep enough to lose soil to erosion processes (see Section 11.7), to where it levels out enough to receive soils (or to a drainage channel). This is usually impractical to measure in the field, but can be determined computationally given an appropriately detailed DEM. Alternatively, the slope length can be manually traced in GIS.

4.4.2 Mesoscale topographic index

The Mesoscale Topographic Index (MTI) (Hurst et al. 2022, p. 15) is a measure of how sheltered or exposed a location is within a landscape. To measure in the field, use a compass and clinometer to measure angle to the horizon at the 8 cardinal compass bearings. Record positive values if looking up and negative when looking down. The average of the eight values defines the index, with high values signifying a sheltered area and low values signifying exposure.

The closest automated equivalents of the MTI appear to be the ‘Morphometric Protection Index’ (MPI) as implemented in SAGA-GIS (Yokoyama et al. 2002; Conrad et al. 2015), and the ‘intensity’ output of the ‘Geomorphons’ module as implemented in GRASS-GIS (Jasiewicz and Stepinski 2013; GRASS Development Team 2023). In the case of geomorphon intensity, the output also reports the mean relative elevation in meters of the 8 defining points rather than their average angles above or below the central point.

Field and computer-based measurement of MTI may differ slightly. The in-field version measures against magnetic north rather than ‘map north’, which will vary with spatial data projection.

4.4.3 Terrain shape index

Terrain shape index (TSI, Hurst et al. (2022), p. 15) is a localised version of MTI, where slope angles are measured at cardinal compass points 20 m from the point of observation, rather than to the horizon.

The TSI can also be calculated using the automated tools mentioned above, but will require a DEM with high vertical accuracy and cell size no more than 10 m.