Dissection and Redissection of the Wellington Landscape
[Received by the Editor, August 31, 1957.]
In the development of the Wellington landscape feral (unsubdued) redissection has outpaced cryergic subjugation. Apart from some residual flat areas on summits this landscape is largely a feral one of fine-textured mature dissection. This has developed as a sequel to mid-Pleistocene deformation of a land surface previously planed, the dissecting water streams being largely consequent on tectonically made slopes, and scarps, though they have branched insequently and also extended headward. Under the primitive forest dissection and insequent development have taken place rapidly because the greywacke terrain is shattered and chemically decayed to a great depth.
In a periglacial region in which running-water erosion is less potent than at Wellington, or than it is throughout most of New Zealand, cryergic processes may subdue the slopes of such a finely textured land surface or of one prone to similar fine dissection under appropriate climatic conditions, smoothing them and coarsening the texture of dissection perhaps very considerably. Ravines will be filled with geflual debris, and geliflual corrasion will also take part, a general result being the elimination of many gullies, ravines, and minor valleys and of some spurs. Such subjugation of the relief can be rapid, and cryergic processes seem to be the most efficient that have contributed to coarsening of the texture of dissection and to the development of subdued landscape forms.
Cryergic smoothing of slopes has taken effect in parts of the Wellington district in the late Pleistocene; but subsequently to the latest cryergic episode as well as intermittently between earlier interrupted ones feral relief has been re-established (perhaps many times) by renewed dissection. At such times—i.e., whenever vertical water-stream erosion (linear erosion) has become dominant—all slopes earlier subdued have again become seamed by ravines. It is mainly, therefore, because of redissection that the relief is now fine-textured. The proof that much of the existing fine-textured relief has been cut quite recently is found in the occurrence of fossilized (i.e., completely infilled) ravines, the widespread distribution of which shows that an earlier similar relief pattern, or more than one such pattern successively developed, has been plastered over by deposits of geliflual debris in cryergic ages.
In regions with much slower erosion tempo, slower at least as regards erosion by streams of running water, where postcryergic (postglacial) erosion may be negligible as compared with its activity in New Zealand and where probably running water was just as feeble an agent in the intercryergic intervals, cryergically subdued landforms dominate the landscape, surviving to the present day without postcryergic modification. Such surfaces, seen in parts of western Europe, are composed of broadly smooth, partly convex, subdued landforms, with the water courses very widely spaced. The difference between the types of erosional relief prevailing in New Zealand and in western Europe is explicable on the assumption that during the Pleistocene the process of dissection by running water gained and kept the ascendancy in New Zealand, despite cryergic interludes, whereas in western Europe cryergy was dominant throughout the epoch, the total results of running-water erosion being small owing to feebleness of its activity even in the warmer ages.
The Wellington erosional landscape.
Level upland relics of a peneplain.
Feral dissection of slopes.
Semi-subdued forms on the upland north-west of the Hutt Valley.
Comparison with landforms in the humid-temperate zone of the Northern Hemisphere.
The problem of smoothly convex profiles.
Some late-Pleistocene erosion forms at Wellington.
Summits: rounding of knobs.
Slopes: their redissection.
Fossil gullies: conditions for their development.
Fossil gullies and redissection.
Competence and corrasion by running water.
The bearing of feral relief on the theory of downwearing.
Wellington relief probably all of late Pleistocene age.
In Pleistocene landscape development subjugation of landforms to subdued relief by the cryergic* processes gelifraction, solifluxion, and cryergic corrasion has been opposed by the process of redissection by running water, and in New Zealand, at Wellington in particular, the latter has maintained the ascendancy. Not only have the details of the landscape in the Wellington district been reshaped to some extent by running-water erosion subsequently to the latest cryergic (periglacial) age, but fine-textured patterns of dissection similar in a general way to the present one have existed in intercryergic (interglacial) and possibly precryergic (preglacial) ages. This is indicated by the presence of innumerable fossil gullies on hillslopes.
The Wellington Erosional Landscape
Dans les pays au climat humide c'est surtout la présence et les formes des vallées qui décident du caractère morphologique du paysage.
J. B. L. Hol, 1957.
Stages of senescent or possibly “infantile”, and of sharply mature, or feral† (i.e., unsubdued), upland dissection jointly characterize large parts of the hilly landscape around Wellington, but it is the feral sculpture that is dominant in views across the uplands—with the exception of the hills north-west of the Hutt Valley, where it is less conspicuous (infra). Apart from some fault scarps (Cotton, 1957, p. 777), the older of which are also dissected or retrograded, the details of the landscape are of erosional origin and have been produced by dissection of parts of a deformed peneplain that have assumed various attitudes (Cotton, 1957, p. 776). Some small parts of that surface survive with little modification, as they have remained horizontal and have thus escaped consequent dissection; other much larger parts, which have been tilted or warped so as to produce initial slopes in the current major cycle, have in consequence been intricately dissected to full maturity so that they display diversified erosional relief. This is seen especially on the Wellington Peninsula and in the hills north-east of Porirua Harbour, including foothills of the Tararua Range (Fig. 1).
Level Upland Relics of a Peneplain.
The initial form of summit areas seems to have been a very smooth peneplain. Portions of this hypothetical surface exist in a little dissected condition, though in all cases their minor relief has undergone some change. In general they are relics of those small parts of the initial surface (after upheaval) that have remained horizontal (Photo 1; Fig. 2) while other parts have been warped and tilted. The original size of these has been reduced as valley heads have encroached on them. Level upland remnants are all quite small, some of them only a small fraction of a square mile in area. Most of them have the nearly horizontal surface diversified only by broad valley forms of a kind that has sometimes been regarded as infantile on more extensive plateaux, though this may be a mistaken description (Baulig, 1957b, pp. 107–8).
[Footnote] *Cryergic = “periglacial” as commonly, and perhaps in some cases improperly, used (Baulig, 1956, p. 24).
[Footnote] † Cf. Cotton, 1948, p. 215.
Another kind of upland landform development—namely, sculpture by numerous closely spaced, V-section, shallow valleys—is found but is exceptional. It is seen on the Belmont plateau, north-west of the Hutt Valley, where the shallow dissection as well as the survival of that remnant as a whole can be attributed to a local circumstance, the control of erosion by a temporary base level determined by the presence of a body of resistant rock across which the trunk drainage from the plateau passes (Cotton, 1955, p. 1015, Fig. 3; Stevens, 1957b, Fig. 13). More generally in the case of level and nearly level upland remnants, however, reduction of area has depended simply on the rate at which ravines draining the flanks could eat back into the flat summits by headward erosion, in some cases from all sides.
Though both on the peneplain remnants and on their dissected flanks the axes of small valleys and ravines are closely spaced, the heads of ravines that are encroaching on a summit remnant are not as a rule in line with stream channels* on the summit. They sometimes “break joint” (Photo 1, foreground), and though an appearance of continuity may occasionally be observed it is fortuitous; it does not
[Footnote] * The stream channels and courses referred to in this paper are the axes of valleys, ravines, and gullies in which water sometimes flows. They are not all courses of perennial streams, and only a few of them are shown by blue lines on even the largest-scale topographic maps Only by disregarding the blue-line convention, which attempts to differentiate continuously running from wet-weather streams, can the high drainage density be appreciated—“drainage density” (Horton, 1945, p. 283) being, as Horton's definition is interpreted by other authors, the mileage of channels per square mile in which streams flow, though in some cases intermittently and infrequently.
indicate that the pattern of minor flanking streams is inherited from an earlier-cycle pattern that persists on the summits. The flank pattern seems, on the contrary, to have resulted from insequent branching that has been recently in progress. A more or less similar break of joint, causing offsetting of stream courses, is known in some other cases to be a result of recent transcurrent faulting (Cotton, 1956, Figs. 5, 6), but here the offsets in different stream courses do not match, nor are they aligned with one another in a way that suggests the presence of a fault.
In at least some cases the rate of destruction of peneplain remnants by erosion has been slow because the terrain has consisted locally of little-jointed and therefore resistant greywacke; but for the outcropping of such rock it is probable that few of the remnants would survive at all. Its presence is generally indicated by autochthonous block fields on the upland surface. On such uplands there are broadly open shallow valleys between broadly convex interfluves with low relief (Photo 1, centre). Either the shallow hollows were forms diversifying the initial peneplain or else they must be due to shallow early dissection of a more nearly level surface, a dissection that may have followed a slight lowering of the base level with respect to the peneplain before it was deformed. The relief has undergone a certain amount of change due to cryergic processes, however. It can be assumed that there have been several cryergic ages, and it is possible that the surface has undergone some cryergic modification before and some after the deformation that initiated the larger relief forms in this district and led to isolation of some relics of shallow valleys on the flat uplands as dell-like hanging features. (Note the two dells, D, D, in Fig. 2; they hang above the marginal dissecting ravine heads.)
Fig. 2.—View looking SSW across hills in the eastern part of Wellington Peninsula. The broken line suggests the deformation of the K. Surface, flat-lying at left, domed at right (Hawkins dissected dome, 1,636 feet). D, D—dells.
If the present condition of the surface on the level upland remnants (Photo 1) may be correctly attributed to cryergic processes working on a subaerial peneplain that had suffered little from earlier dissection and if these processes have converted existing shallow valleys into dells (Cotton, 1955, p. 1022) the small extent of the change involved may be attributed partly to the faintness of the initial relief, but it may be due in part also to protection of the level uplands by a blanket of snow in glacial ages. (This is reported to have been the case on quite large areas of the Central Massif of France—Beaujeu-Garnier, 1952, p. 209; 1953, p. 268.)
Feral Dissection of Slopes.
Though the Wellington greywacke terrain as described petrographically (Reed, 1957a) consists for the most part of rock types rather resistant to corrasion, the greater part of it is shattered (Reed, 1957b) and is also affected by chemical alteration to a great depth, so that it can in reality be dissected by running water with.
the greatest ease, and hence very rapidly. The probably lenticular bodies (already mentioned) of massive greywacke which are not thus shattered and decayed occur only sporadically; and the hills and upland ridges consist for the most part of either quite rotten or at least partly weathered rocks—as has become increasingly evident as whole hills have been bulldozed away in recent years.
The almost uniform weakness of the decayed rocks nullifies the effect of interbedding of strata which differ potentially in their resistance to erosion, so that the relief is scarcely at all controlled or more than very slightly influenced by adjustment to rock structure. Hence prevalence of insequent and absence of subsequent branching of streams is easily explicable.
Fine-textured feral dissection* is general on the flanks of the level upland remnants and on ridges that have been upheaved as anticlinal and domical surface forms. The first streams to flow on these during and after uplift have taken consequent courses down slopes assumed by a formerly level surface as it was warped, and the main streams are still consequent.
Patterns of consequent and insequent drainage and dissection border peneplain remnants whether these have been isolated technically or by erosion. There are now branching systems of spurs narrowed almost to sharp edges between the consequent valleys and their insequent branches. (See discussion of redissection infra.). The texture of dissection being fine, maturity of dissection has been attained at many places without very deep incision of the valleys and ravines; but the depth of incision depends on the available relief. A number of sharply domical uplifted areas (Cotton, 1957, p. 780 and Fig. 7) have been dissected so deeply that their cores have become pyramidal peaks from which ridges radiate. One of these, the Hawkins dome (Fig. 1), is seen in Fig. 2 and Photo 2, and another, farther west, in Photo 1.
Whereas the shaping of the surface of small level upland remnants dates for the most part from before the deformation of the terminal peneplain, or K. Surface (Cotton, 1957, p. 774), the mature dissection that characterizes most of the district has taken place after strong deformation of the surface, or at least after the first spasm of such deformation. Except close to valley heads there are ill-defined shoulders on the sides of the dissecting valleys, and in parts of the district there are distinct nicks (without any obvious structural control) in the profiles of the streams in these valleys, from which it appears probable that the upheaval initiating the dissection took place intermittently, continuing after consequent dissection had begun. Apart from this not very conclusive evidence, the general aspect of the landscape on the dissected domes suggests dissection in a single cycle. The deformed surface of the peneplain relict from an ancient cycle has been destroyed by dissection; and, though in seaward marginal parts of the district effects may be seen of general, more or less uniform upheavals (Cotton, 1957) as well as of probably eustatic shifts and minor local deformation of former base levels, the rejuvenating waves of newer cyclic dissection thus engendered have not as yet affected the hill cores.
Semi-subdued Forms on the Upland North-West of the Hutt Valley.
As already stated, the upland north-west of the Hutt Valley (Photos 3, 4) is exceptional in that it is not ferally dissected to the same extent as other parts of this district. Stevens (1957b, pp, 303–10, Figs. 4, 7–12, 15) has described the upland in some detail, figuring many of the stream and spur profiles. He has recognized some fault scarps (not very prominent as landforms), some summit areas which are residuals of the K. Surface, and extensive maturely dissected tracts of moderate relief.
[Footnote] * Such dissection is characteristic of early maturity. In some cases no doubt subdued forms develop “normally” from feral forms, as generally assumed in cyclic geomorphology (cf. Cotton, 1948, pp. 214–6); but recognition of the intervention of a cryergic episode in the process of subjugation of many landscapes makes it inadvisable to continue to define feral and subdued reliefs as successive substages of maturity in the normal cycle.
traversed by open valleys. In some of these he finds “graded reaches” of streams which drain to the Hutt River and are rejuvenated in their lower reaches, where they cross the Wellington Fault zone of recent dislocation in gorges. On the uplands above this zone of rejuvenation the relief might be called subdued, for the slopes are not steep and the summits and crests are broadly convex; but the minor streams and spurs are closely spaced. Such relief may be better described as semi-subdued, for it is very different from subdued relief as seen in the Northern Hemisphere in humid-temperate regions, where spurs are fewer and streams are far apart. This upland surface is in a sense an intermediate-cyclic one—developed, however, not between uniform upheavals but between deformations of the land surface that have involved tilting and warping as well as faulting.
In the “graded” upland valleys there are no flood plains, and, apart from a fine-textured minor relief of faint spurs on their side slopes, the valleys resemble dells, being conspicuously catenary in cross section, or en berceau (Photo 4). They are, at least in part, consequent depressions (shallow tectonic corridors) separating minor blocks that were formed in an episode of faulting much earlier than that which provided the Hutt River with its modern fault-angle valley (Cotton, 1957). The upland valley forms are cryergically modified, but details of the modification have not been studied; nor is it known to what extent the land surface as a whole here owes its form to cryergic processes. It is quite possible, however, that the valley forms may be better explained as tectonic features thus reshaped than as former spill-over courses of the Hutt River now occupied by underfit streams. If north-westward distributaries of the Hutt River ever followed these corridors (Stevens, 1957b, pp. 317–20) this was prior to much cryergic modification as well as to the paroxysm of faulting that provided the river with its inner tectonic valley and fixed it in its present course.
Comparison with Landforms in the Humid-Temperate Zone of the Northern Hemisphere
The feral fine-textured type of mature landscape seen on the Wellington hills is found also in many other parts of New Zealand from the extreme north to Stewart Island in the far south. It is especially well developed on greywacke terrains, but it is not confined to these. Nor is it confined to New Zealand. It is common, indeed, in other regions also, especially warm-humid forested and formerly forested regions, where it seems to have been controlled by susceptibility of the terrain to chemical weathering rather than by lithology. Such relief must not be confused with gullying by accelerated erosion, in which the dissection is ultra-fine in texture (or on a miniature scale) and is, moreover, entirely different in origin, developing superficially and generally on the steeper slopes of an already dissected terrain. Such dissection originates commonly on slopes that have been earlier under forest but have more recently been exposed to rainwash, perhaps sometimes by natural deforestation due to change of climate, but in many cases by clearing for cultivation. Confusion has been caused also by inexact use of the description “badlands”, which has been applied by some authors to maturely dissected landscapes with fine but not ultra-fine texture though insequently dissected like Western badlands. An example of this is the quotation of the description “badlands” from Willis in the caption of a plate showing a fine-textured landscape in China (Davis, 1912, Pl. 5).
Not all forested regions undergo feral dissection. According to Tricart and Cailleux (1955, pp. 79, 114) conditions do not favour dissection of this or any other kind in parts at least of the regions clothed with equatorial rain forest, under which chemical weathering, being very rapid, proceeds so far as to reduce the whole.
of the regolith to fine debris and prevent the formation of stream gravel, thus robbing streams of the tools of corrasion.*
Nor need the foregoing remarks be taken to imply that fine-textured dissection is confined to regions that are or have been forested; it is well known that in semi-arid climates also major salient landscape features are finely dissected. In some of these, however, such as Southern California (cf. Davis, 1912, Pl. 7) the dissection may have taken place under forest in the Pleistocene. Davis's thorough appreciation of the close-set pattern (independent of climate) of consequent and insequent valleys and ravines characteristic of maturely dissected slopes is evident from many of his diagrams (e.g., Davis, 1912, Figs. 116–8). His opinion (1912, pp. 276, 281) that drier climates favoured finer textures might not have been expressed had he recognized in the coarse texture of some eastern North American landscapes a legacy from a cryergic age.
The forms of the feral landscapes of many humid-temperate regions may be compared and contrasted with those characteristic of certain other very extensive regions in roughly corresponding latitudes that have very widely spaced water courses, practically all spring-fed (de Martonne, 1935, pp. 607–8; Tricart and Cailleux, 1955, p. 32). The wide spacing of streams gives a very coarse pattern of dissection with which are combined all the features of thoroughly subdued relief. Such landforms are common in Britain and in most non-glaciated parts of western Europe and are found also in eastern North America, but not in the West or in California, where landscape dissection is fine-textured. The type may be called, though for the present purpose only, west-European. Parts of some New Zealand landscapes bear some resemblance to this type, but generally the slopes of these are less broadly smooth, being dimpled to a greater extent by fine-textured (though at the same time shallow) dissection. On the upland surface north-west of the Hutt Valley, for example, slopes of this kind, probably here resulting from degradation of rather ancient low fault scarps, descend gently from upland blocks to corridors and to a bench that overlooks the inner and newer tectonic valley in which the Hutt River flows (Photo 3). The shallow gullies that furrow these slopes may have survived, though with some cryergic modification, from the last interglacial age, but there is also some incipient postcrvergic dissection.
The west-European type of landscape is widely distributed in northern middle latitudes. It is there probably coextensive with periglacial, cryergically modified regions, and it seems to owe its characteristics mainly to shaping by the cryergic processes. Southern England is described by Te Punga (1957, p. 410) as “a typical periglacial landscape”. In his opinion,
Pleistocene denudation took place for the most part during periglacial episodes, for then the whole surface of the landscape was subject to vigorous downwearin as a result of mass wasting, in which solifluxion was particularly active … It seems probable that the effects of successive periglacial episodes were cumulative …: it is unlikely that interperiglacial erosion, seeing that it was restricted essentially to linear processes, was compotent to obscure or obliterate earlier developed periglacial landscape form.
The rapid wasting of the land surface during periglaciation, due to the transportation of enormous quantities of material to lower levels, has produced a landscape of subdued aspect characterized by slopes that are convex near the top and concave near the bottom…. Such slopes…. dominate the present landscape.
Tricart (1951a, pp. 4, 5) has thus described cryergic development of land slopes in France, especially as typified in the eastern part of the Paris Basin:
The upper part of the slope retreats … The steepness of slopes is reduced, cliffs crumble down, and enormous quantities of material descend to lower levels. The landscape form is [not stabilized but] alive … Slope forms [thus developed] being almost immune from attack by the feebler processes of postglacial erosion still dominate present-day landscapes.
[Footnote] * As regards conditions governing dissection and geomorphic ablation the New Zealand “rain forest”, despite its ecological resemblance to equatorial forests, has more in common with the “forest zone of middle latitudes” as described by Tricart and Cailleux (1955, pp. 207–8), who admit the importance of vertical corrasion by running water in this zone.
In places where gelivation and solifluxion operated freely slopes generally have a form convex at the top and concave at the bottom, with an intermediate straight slope. This form is found on limestones as well as on granites, marls, and alluvia with fine matrix. The upper part of the slope is a zone of wastage …: the concave basal part is a slope of accumulation …; on the intermediate part thin coulées of soliflual debris are seen furrowing the rock. Slopes of this form, extremely common in France, cannot be explained as developing in the existing climate.*
In stratified terrains the control of differential erosion by structure may have varied from time to time as one formation or another yielded more readily to denudation with alternation of cold and mild climates (Tricart, 1951b), but the general Pleistocene trend in Europe was towards subjugation of relief in areas not directly affected by river rejuvenation. Quite recently Tricart (1956b, p. 135) has reaffirmed that most French landscapes outside glaciated regions have assumed their present subdued form “in the cold climates of the Quaternary.”
If most examples of very coarse-textured erosional relief in middle-latitude regions must be explained as developed thus under cryergic control, landscapes of this type are not “normal”, as they were called when it was assumed that this was the type produced by the unaided activity of rain and rivers. The history of the development of these landscapes may not be very different, however, from that of some (including those developed under forest in New Zealand) that are fine-textured and may more strictly be claimed as normal according to that definition. The differences, striking as they are and fundamental as they may seem, may be due only to local conditions under which different factors have been dominant in the erosional development in different regions or even in different parts of the same region, though but for this different distribution of emphasis the processes involved in the development may be the same.
Possibly in an early Pleistocene warm age and certainly in long interglacial ages incision of main valleys was going on in both Northern and Southern Hemispheres. In western Europe, for example, a land surface of small relief graded to a high late-Pliocene or Villafranchian base level has been since trenched by large rivers (Tricart, 1949, pp. 168–71). There as well as in New Zealand erosional development of the valley-sides so cut must have been in progress (Tricart, 1949, p. 170) and must at first have been under the control of mild-climate denudation and water work. The latter must have taken part in the dissection of the young valley-sides and the hillslopes generally of all the relatively humid middle-latitude regions. Thus it is reasonable to assume that sculpture of some slopes was going on, probably under forest, and indeed that fine-textured dissection was developing rather widely at least in the wamer parts of interglacial ages; but any fine-textured relief that was in existence in interglacial ages was obliterated (in those regions in which all relief is now subdued) when cryergic denudation supervened in the succeeding glacial ages. Perhaps, however, as seems to be the case now, the tempo of running-water erosion was then also slow in western Europe, at least as compared with the New Zealand tempo, so that there was never a very extensive development of feral relief of the kind common in New Zealand. There is at any rate no clear record of its existence; no extensive cryergic fossilization seems to have taken place of reliefs of this kind that existed either in preglacial or interglacial ages. Tricart (1956a, p. 148) indicates that such fossilization of relief is at least unusual, and he predicts that it will rarely be found.
Evidence regarding intermediate stages being obscure, there is no course open but to resort to speculation to find an explanation of the no doubt complicated developmental history of the present-day contrast between New Zealand feral relief and the west-European subdued landscapes as they have emerged eventually from the Pleistocene with its complex of changing climates. Each landscape it may be
[Footnote] * For a more detailed account of cryergic slope development supported by references to examples from subarctic regions where cryergic processes are now active see Tricart (1950, pp. 193ff.).
assumed has experienced a succession of ages in which cryergic slope-smoothing and non-cryergic, or “normal”, denuding and slope-dissecting processes have alternately taken over the task of shaping landforms. In New Zealand it is obvious that slope-dissecting processes have triumphed; but in western Europe the reverse has been the case, the cryergic processes gaining and keeping the upper hand in this struggle of alternating morphogenetic systems.
There are probably more reasons than one why cryergic smoothing (subjugation) of the landscape might triumph in Europe. For one thing, the potency of Pleistocene cryergy—at least in central Europe—was almost certainly greater than in any part of New Zealand. Measurement of the cubic content of an apron of debris in the Hils district of western Germany has shown, for example, that a structural escarpment 250ft high from which the debris was derived by gelifraction and solifluxion was thereby worn back fully half a mile (Suchel, 1954, pp. 113–4). For another thing, running-water erosion is now, and probably was in past ages also, much more potent in New Zealand than in western Europe. It is known, though the reason is rarely obvious, that the tempo of erosion may be a hundred times faster in one place than in another. In New Zealand erosion by running water, “linear” erosion, which leads to ravining or gully development—to both insequent headward extension and vertical incision by small streams—is now very active, as will be obvious to an observer looking at the turbid runoff on any rainy day from any dissected slope, even one that is forested or well turfed. In the non-mountainous parts of France, on the other hand, Cailleux (1948, 1950) is convinced that erosion of every kind has practically ceased; and Tricart (1951a, p. 5) is in agreement with this when he asserts that relict landscape forms that have been developed many thousands of years ago by cryergic processes are “almost immune from attack by the feebler processes of postglacial erosion”. Assuming, then, that there has been a struggle for survival between normal feral and cryergically subdued forms in landscapes that have been actively developing throughout much of Pleistocene time, the result may have been the production of relief of either the fine-textured kind common in New Zealand or the contrasting west-European type.
It would require a very elaborate diagram to show successive hypothetical stages of such a contest. Fig. 3 might be thought to postulate transition from fully developed.
Fig. 3—A—fine-textured feral dissection of hillslopes on the flanks of peneplain or plateau residuals. B—a subdued landscape of generalized west-European type, in the development of which it may be assumed that cryergic processes have remained dominant throughout most of Pleistocene time notwithstanding climatic changes, inhibiting or neutralizing fine-textured dissection by running water. The drainage density is thus very low.
feral dissection to subdued smooth slopes; but it must be read differently. It shows only the eventual triumph in one case of feral dissection by vertical corrasion and headward gullying and in the other of the cryergic subduing process, with water erosion playing in the latter case a very insignificant part.
The Problem of Smoothly Convex Profiles.
It is not so much the smooth profile, concave at the bottom and convex at the top, as it is the undissected broad extent of the hill slopes that is most characteristic of fully developed subdued landscapes. It has been thought, however, that this form may originate in another way, being produced by normal, i.e., non-cryergic, processes (Baulig, 1950, p. 135; 1957a, p. 917; Cotton, 1952, pp. 198–200). After its development by cryergic processes, if this is possible, it thus seems that the concavo-convex profile may be perpetuated by these normal subaerial processes. This would partly explain the virtual immunity of some hill slopes of this kind in France from postcryergic modification which has been observed by Cailleux and Tricart.
Baulig (1957a, p. 928) remarks: “It does not seem that on the same rocks gelifluxion produces profiles that are radically different, and thus it is difficult to evaluate the part it has played … in shaping the landscape.” He is referring, however, to concave profiles only; he doubts the value of explanations that have been proposed to account for the upper, convex segment of a subdued hillslope profile by the action of cryergic processes. The apparently cryergic erosional development of some small, smoothly domical landforms near Wellington that have been called “round knobs” (infra) may throw some light on the larger problem presented by subdued hillslopes of convex form and more especially a rounding to broad smooth convexity at the margins of plateau remnants which gives them a whaleback form that is commonly seen in periglacial regions.
Some Late Pleistocene Erosion forms at Wellington
Summits: Rounding of Knobs
When the sharp summits and ridges of feral relief are smoothly rounded off, as it would seem they must be eventually under any subaerial erosion system other than pediplanation or glaciation, summits and crestlines must be thereby lowered some-what. If, as is rather generally agreed, accordance of the levels of summits, which generally are of subdued form, can be relied on for the restoration of ancient peneplains that have been maturely dissected, it must, however, be assumed as axiomatic that such summits are not lowered far by erosion even in millions of years.
The convex rounding of summits cannot have been in all cases a purely cryergic process; it may involve sometimes the long-continued operation of soil creep, with which unconcentrated rain wash may co-operate (Baulig, 1950, p. 137). In all periglacial regions cryergic processes (gelifraction and solifluxion) have probably been involved as well, though soil creep would be dominant in interglacial ages. Baulig (1957a, p. 927), however, does not admit the ability of cryergy to round off summits and upper slopes convexly or to continue a rounding begun by soil creep. He deduces instead that a cryergic reshaping of slopes to more concave profiles must take place on the flanks of hills that already have convexly rounded summits so that they will be thereby sharpened and narrowed, eventually developing pinnacles and arêtes if this goes on long enough. Backwearing of slopes, such as is believed to take place in the process of cryergic planation (Tricart, 1950, p. 204, Fig. 69 caption; Te Punga, 1956, p. 335), is here involved, which will narrow and may tend to sharpen crests.
In the development of “round knobs” (Cotton, 1955, p. 1016) in the vicinity or the Hutt Valley (Photo 5; see also Cotton, 1955, Pl. 37: 1) the actual summits do not seem to have been lowered much; their central parts have probably been worn down but little from the K. Surface. This is because they are sited on outcrops
Photo 1.—View looking SSW over hills of the Wellington Peninsula. In the distance at the left, feral dissection of a dome; below centre, a very small remnant of the K Surface at altitude 1,200ft; this is dissected in the foreground. The hills were denuded of forest and grassed a century ago.
Photo by D. W. McKenzie.
Photo 3.—North-west of the Hutt Valley. Semisubdued slopes descend from degraded scarps bounding a small tectonic block to a bench above the young scarp of the Wellington Fault.
Photo by G. R. Stevens; reproduced by permission from the N.Z. Journal of Science and Technology.
Photo 4.—Open valley of the Moonshine Stream above the head of rejuvenation (indicated by an arrow) related to the Wellington Fault scarp. View looking south-east.
Photo by G. R. Stevens; reproduced by permission from the N.Z. Journal of Science and Technology.
Photo 5.—Round-knob remnant of the K Surface, with block field, Eastern Hills, east of Hutt Valley.
Photo by G. R. Stevens; Reproduced by permission from the N. Z. Journal of Science and Technology.
Photo 6.—Frontal view of a coulée of soliflual debris at Belmont, north-west of the Hutt Valley.
Photo by G. R. Stevens; reproduced by permission from the N. Z. Journal of Science and Technology.
Photo 7.—Road-out section at Belmont of soliflual debris forming the coulée shown in Photo 6.
Photo by G. R. Stevens; reproduced by permission from the N. Z. Journal of Science and Technology.
of the resistant (little jointed) greywacke already mentioned, block fields derived from which overspread the summits (Cotton, 1955, Fig. 3). A certain amount of lowering of the surface must be inferred from the fact that these outcrops are weathered to the extent of releasing spheroidal and subangular blocks to make the block fields, and also that numerous blocks of such origin have been conveyed by solifluxion down the convexly rounded flanking slopes, some of them being stranded a considerable distance away, but it is apparently from marginal rather than from central parts of the outcrops that geliflual* debris has for the most part been derived. Thus plateau remnants have dwindled in area to round knobs, the margins being probably worn down by cryergic corrasion and smoothly rounded off under streaming geliflual debris.
This theory (cf. Tricart, 1950, Fig. 68) may be reconciled with the view of Baulig (supra) by supposing that in some cases at least the rounding of a remnant to the knob form has been due to progressive modification of the potentially sharp edges of scarps that have retreated in the direction of the centre of the knob as altiplanation surfaces expanding at a lower level have encroached on it; but any planed surfaces that have thus come into existence have since been dissected. Parts of the Belmont plateau (with shallow dissection) bordering a large round knob figured by Cotton (1955, Pl. 37: 1) possibly have this history. Fine-textured dissection, some of it shallow, as on the Belmont plateau, is ubiquitous in the environment of round knobs, but it has not encroached on their flanks, possibly in part because of the resistance there offered by the bedrock, but largely because the flanks are armoured with a veneer of transported blocks which remain stranded on them and thus afford immunity from dissection such as would impair the smoothly domical form.
Though round knobs, being sited on sporadically scattered outcrops of resistant greywacke, are not very common features of the Wellington landscape, observation of their form serves as a reminder that it is not always necessary to explain broad subdued summits as developed by downwearing from much higher sharp summits as pictured by Davis (1908, exercise 5, Figs. 4, 5; 1912, p. 282; cf. Cotton, 1948, Fig. 179)—who also assumed an unexplained coarsening of the texture (Gliederung) as the relief became subdued (1912, p. 186). Very many thoroughly subdued forms, including whaleback ridges in the subdued landscapes of western Europe, are erosionisolated remnants of either peneplains or structural plateaux and have assumed their present shapes as a result of rounding of the edges to smoothly convex profiles. It must not be lost sight of, however, that in New Zealand and some other mobile regions rather similar domed and arched summits of large extent—in the ranges of Central Otago for example—owe this form not to erosion but to differential upheaval of a surface that was once plane and horizontal (cf. Sharp, 1957, pp. 286–9)
Slopes: Their Redissection.
It may be assumed from the geographical distribution of the landscape type so produced that fine-textured feral dissection of slopes is favoured by warm-humid chemical weathering of rocks; but associated with this there must be either lowlatitude immunity from cryergic interference with running-water erosion, or running water must at least maintain its dominance. Cotton (1955, p. 1016) has thought that cryergic processes have not been sufficiently potent in New Zealand to obliterate fine-textured dissection patterns already in existence—patterns the absence of which would demand special explanation. This view may be modified in the light of the probability that there have been not one only but several episodes of moderately strong cryergy and that land slopes have been redissected by particularly active linear (running-water) erosion both in interglacial ages and in postcryergic time, whereas owing to slower tempo in other regions such redissection has been practically inoperative.
[Footnote] *Cryergic soliflual debris may be more concisely termed “geliflual” (Baulig, 1957a, note 21).
Slopes of the ground, moreover, on considerable parts of the dissected terrain at Wellington (Photo 2) are too steep to permit of lodgment and accumulation of geliflual* debris. As is the case among mountains, many valley-side slopes and valley-floor gradients have here been steep enough to ensure immediate evacuation of any debris that might be brought by solifluxion from less steeply inclined surfaces at higher altitudes, for the flow of water has not ceased altogether in cryergic ages. Where the slopes were very steep also a less water-saturated condition of rock outcrops would partially inhibit the freeze-and-thaw process that was elsewhere vigorously breaking up the superficial layer of the terrain (Tricart, 1956c, p. 311).
On gentler sloping and not too deeply (and steeply) dissected parts of hillslopes, on the other hand, in the Wellington district abundant head has accumulated in gullies so as to obliterate relief due to earlier dissection. As a result the condition of hillslopes on the north-west side of the Hutt Valley fault-angle depression recalls a description of the central Limousin (France), where “the slopes disappeared under coulées; the valleys were filled” (Beaujeu-Garnier, 1953, p. 251).
Most of the gullies of finely dissected hillslope reliefs of possibly precryergic date, and more certainly gullies eroded in intercryergic ages, have been thus eliminated throughout the Wellington district. Innumerable hillslope gullies have been completely infilled and so are concealed, as may be confidently inferred from the fact that such fossil gullies are exposed in section wherever cuts are made for road formation. The conclusion is inevitable that in all places where the average hillside slope is not very steep feral relief that is now seen exists not because any large part of it has survived despite the plastering of slopes with geliflual debris in glacial (cryergic) ages, but rather as a result of redissection in later times subsequently to such smothering of the pre-existing minor relief. After being mantled by voluminous stagnated coulées as a result of probably the latest cryergic plastering (Photos 6, 7) some of the slopes on the north-west side of the Hutt Valley have become submaturely dissected again, convex coulées and all, mainly in the last few thousand years (Stevens, 1957a, p. 287).
At places, or in particular climates, where, in contrast with Wellington, the land surface has for the most part escaped redissection in the intercryergic intervals every broad slope must have become progressively smoother until eventually, in the last cryergic age, a smooth sheet of head has mantled it rather than one with a corrurgated surface made up of confluent convex coulées. The first gathering of streaming geliflual debris into coulées is due to some initial unevenness of the slope, even though it may not be seamed by sharply cut ravines. Re-entering angles between early formed convex coulées are infilled by other coulées during prolonged cryergy or in successive cryergic ages, so that a slope not too uneven to begin with may become quite smooth (Tricart, 1950, p. 196). At Wellington, however, as redissection has taken place already in more than one intercryergic interval, and rather for this reason than because cryergic activity has been less intense, the smoothing of slopes can at no stage have been nearly as perfect as in Europe. On imperfectly subdued slopes even small irregularities will concentrate any runoff there is into consequent runnels, which are likely to be numerous and will thus begin to develop or redevelop fine-textured dissection immediately water flows.
Study of late Pleistocene superficial deposits has shown that the Wellington landscape has experienced many and frequent changes of form. Thus, Brodie (1957, p. 640) observes: “A major modification of the topography has taken place since 20,000 years ago. A vast amount of erosion has occurred and a micro-pattern of drainage developed which has truncated the pre-existing streams and cut down deeply below their former courses.”
[Footnote] * This material when it eventually accumulates and stagnates is commonly termed “head” by geomorphologists, especially in France.
Besides geliflual filling of gullies and plastering of hillslopes with head, account must be taken also of cryergic corrasion, or “shaving” (Te Punga, 1955a, p. 1008; 1957, pp. 405–6 and Fig. 5), on hillslopes. Slopes that have been shaved beneath a cover of geliflual debris are seen in various photographs reproduced by Te Punga (1955a, Pl. 33, 34) and Stevens (1957a, Figs. 10, 11, 13, 14). Some of these are the side slopes of gullies afterwards fossilized under stagnated streams of the debris that has accumulated in and perhaps also flowed through them. At no place where land slopes were gentle enough to permit temporary lodgment of geliflual debris but at the same time steep enough to facilitate its movement downhill does the bedrock surface under the moving sheet seem to have escaped corrasion (Te Punga, 1957, pp. 405–6). Some small spurs must thus have been completely shaved off. Along with plastering of the surface with stagnated debris this has contributed to the elimination of minor relief—i.e., to the general smoothing of slopes in cryergic ages (cf. Tricart, 1950, p. 197; 1951a, p. 4).
Fossil Gullies: Conditions for their Development
The fossil gullies of the Wellington hills (Cotton and Te Punga, 1955b; Stevens, 1957a, p. 288) owe their origin to the presence of erosional relief on a surface that became mantled with cryergic debris; while their eventual exposure in section has followed a renewed dissection by water streams that are largely consequent on the slopes of cryergically plastered and smoothed hillsides. Such persistence, or more correctly such renewal, of relief due to running-water erosion has been foreign to the European regime; it is thus easily understood why, as Tricart (1956a, p. 148) has pointed out, evidence of comparable fossilization of gullies or ravines (with inversion of relief) is rarely seen in Europe. Where there is no fossilization at all of precryergic relief its absence must be due to inefficacy of small-scale dissection immediately prior to the cryergic age in which solifluxion became widespread. It is possible, however, that at some places head-filled valleys do exist which have escaped discovery owing to immunity of the landscape from dissection since their fossilization took place. Meynier (1957) has recently described an isolated example of fossilization and inversion of relief in Brittany.
In Champagne, in a district that has been almost planed by the cryergic processes, Tricart (1950, Fig. 69a) has recorded partial filling of a small valley with geliflual debris; but if valleys available for such infilling are only those of an early Pleistocene precryergic relief much of the evidence of such fossilization may either be removed by later (intercryergic) erosion or “shaved” away in later cryergic ages. Further, any valleys, rather than gullies, that were thus infilled in Europe would most probably be wider and part of a coarser network than Wellington gullies, owing to a tendency, perhaps controlled by lithology, to develop coarser dissection. Having thus greater holding capacity, they might not generally be completely filled or overfilled so as to be occupied by coulées that would later become divides because of convexity. Without such inversion of relief little of the fill could escape later evacuation by running water except in low-lying situations where, being buried, it remained below local base level. Where thus preserved its discovery is unlikely unless it is revealed by deep artificial excavation.
Fossil Gullies and Redissection
The mechanism of primary fine-textured dissection and of redissection after partial (or perfect) cryergic subjugation of the relief cannot be very different. For the most part the small streams initiating dissection in each case must be consequent, but the dendritic patterns assumed by the drainage indicate that there are also many insequent headward extensions and branchings. The mechanism of insequent head-ward extension of ravines thus calls for examination.
In discussion of the inversion of relief that has followed the filling of gullies with head at Wellington Tricart (1956a, p. 149) has described how, especially in
semi-arid regions, ravines that dissect slopes are commonly subsequent or pseudo-subsequent, being closely adjusted to structure, flowing and cutting ravines only on outcrops of weak rock and leaving more resistant ones in relief. Such a case implies fresh rock and a definite type of structure, however. Tricart deduces that the mechanism of redissection at Wellington may be explained in another way: it may be assumed that the burial of a ravined hillslope under debris produces downslope stripes of permeable fill over the sites of ravines, these being separated by stripes of less permeable material sited on the former interfluves. Loss of water by infiltration will prevent new streams gathering on the permeable stripes, but streams will run down and erode the less permeable ones. This explanation is not inconsistent with the view that slope-dissecting streams are mainly consequent. The mechanism proposed in this hypothesis may have operated in some cases; but the fill in Wellington fossil gullies is not generally permeable. Being angular and of assorted sizes with a large proportion of fine material it is usually packed very closely and firmly; Stevens (1957a, p. 288) reports it “firmly consolidated”. Thus the susceptibility of the land surface to erosion has not generally differed to an appreciable extent on the shattered and weathered bedrock of spurs and on the gully-filling debris between spurs, which has been derived from similar bedrock material by cryergic processes without sorting. This is of little account, however, as long as it is not claimed that the redissecting streams are subsequent in origin, working up the hillslope by head-ward erosion along certain more easily eroded stripes.
If the cryergically subdued hillslope was quite smooth consequent redissecting streams might be sporadically placed and spaced; but if the slope was corrugated owing to its being covered by parallel and perhaps confluent convex coulées the consequent streams would be guided by furrows or re-entering angles between these.
Competence and Corrosion by Running Water
Tricart (1956a) has deduced also that small streams cannot, because of lack of competence, corrade vertically as large rivers do—that their beds must become armoured with a pavement of blocks too large to be moved, so that vertical incision will be inhibited. Implications of this deduction must be examined, as it rules out the only mechanism that seems capable of explaining the fine texture of dissection on the Wellington hills and indeed fine-textured dissection almost everywhere—namely, incision of ravines by small consequent streams and by others that develop by headwind extension of and by insequent branching from these consequents.
Gilbert (1877, p. 110), the originator of the concept, does not discuss competence with reference to large rivers, but deals first with running water at the point “where it begins to gather itself into rills.” He postulates its flow over “material that has been disintegrated by weathering”, a description that fits the Wellington greywacke terrain. Flowing over this, at first with low velocity, a water stream “discriminates and leaves the coarser particles”. This will happen at the head of a ravine that is eating back insequently into a Wellington hillside slope; in the funnel-shaped gully of the ravine head the larger stones thus isolated will now usually roll down the surface slope instead of remaining in place so as to armour the ground, but when the forest cover was intact roots might hold them. Eventually, however, as the ravine extends headward they are evacuated by a stream of running water. A short distance downstream the competence of this gathering stream increases rapidly as incision steepens the declivity, thus increasing the velocity, and as a greater flow of seepage water is tapped, thus increasing the volume of the stream even between rains and independently of the runoff. Gilbert has stated the case thus: “As the deepening and concentration of the water progresses … its competence increases and larger ones [fragments] are lifted.” (It is obvious that still larger ones will be rolled down the declivity of the stream bed.) Vertical corrasion being thus in progress, the head of the ravine is steepened, so that it must extend farther headward. At Wellington
once a ravine has developed there is an ample flow of water in it, especially during rains, to evacuate blocks of considerable size, which are seen in the water courses as partly rounded cobbles and boulders. This is just one illustration of the well known fact that coarse bouldery waste is found in but does not choke the channels of small streams with steep gradients. It is waste in transit.
The postulate of an impenetrable payement of residual blocks in the small stream course being thus disposed of, the theory that vertical corrasion is the exclusive prerogative of large rivers may be dismissed as unfounded. Yet no one would claim that streams as small as those in the innumerable ravines on Wellington hillslopes would be capable of cutting down through unfissured and unweathered greywacke as rapidly as a great river like the Colorado of the West incises a canyon in granite. The rate of cutting might be too slow in the case of such streams, notwithstanding their steep declivities, to allow of the incision of gorges, for it is conceivable that interfluves might be worn down by atmospheric denudation at a rate that would almost keep pace with the slow incision of ravines. In the dissection of Wellington hillslopes, however, it is not a case of corrasion in resistant rocks. The shattered and partly decayed condition of the greywacke terrain more than compensates for the small size of the streams dissecting it. That they corrade vertically and vigorously is obvious from the sharp V section of the ravines in which they flow (Photos 1, 2).
In developing a theory of evolution of relief a geomorphologist, especially one accustomed to the coarse texture of dissection on the subdued landscapes of western Europe, might easily fall into error if he failed to recognize the difference between dissection which, as described above, can proceed vigorously under forest, at any rate in a climate like that of New Zealand, and accelerated rill erosion, which a forest cover must prevent, as it protects the ground from raindrop impact and rill development. The claim of Tricart and Cailleux (1955, p. 12) that because a forest cover favours infiltration it must retard and perhaps almost inhibit dissection is an argument applicable only to rillwash dissection such as may occur under conditions of accelerated erosion when forested land is cleared. Vertical corrasion by running streams is quite a different matter, however, depending as it does not on the gathering of rainwash into consequent rills but on the headward extension of ravines which tap ground water.
The Bearing of Feral Relief on the Theory of Downwearing
The prevalence of fine-textured feral relief in forested regions like New Zealand clearly implies that vigorous dissection is in progress, and such dissection cannot but be associated with rapid wearing down of the whole surface of the land. (While the relief remains feral it is not yet a question of wearing back valley sides to gentler declivities, however.) This raises the question whether forest is as protective as some have thought, whether a cover of vegetation necessarily “immunizes” the surface from “intense mechanical erosion” (Tricart and Cailleux, 1952, p. 398) so as to make downwearing quite ineffective. However protective forest may be against raindrop impact and even if it may impede mass movement, it offers little if any protection against vertical stream corrasion, provided streams carry gravel and there is some available relief (or “difference in altitude between base level and the highest parts of the initial surface”—de Martonne, 1935, p. 607), and against insequent development and headward extension of small valleys, which, in a climate where they are associated with progressive though incomplete decay of the rocks, are of the greatest importance in the process of ablation (cf. Tricart and Cailleux, 1955, p. 114).
In the hottest climates, on the other hand, if they are also wet, the case may be different, for chemical weathering may be too thorough to allow residual rock fragments to survive long enough to supply streams with gravel, so that they are without the tools they require for incision of valleys and ravines. This, as Baulig 1950, p. 63) has pointed out, citing remarks of de Martonne in support, is an
acceptable explanation of the broken profiles and ungraded condition of even large rivers tributary to the Congo and Amazon. Tricart and Cailleux (1955, pp. 17–21; also Tricart, 1956d, pp. 305–6) have described a very slowly evolving landscape at Fouta Djalon, Guinea, where considerable relief has developed, with broad and steep slopes, while quite large streams that flow down the slopes fail to make even the smallest incision in them. These streams are without gravel and carry very little sand; there is no abrasive. “Being without any solid load, the current however swift is powerless” to erode (Tricart). Büdel (1954, p. 143; 1957, p. 219) also has found persistently ungraded rivers characteristic of tropical Africa.
Obviously the rivers in a landscape like that of Fouta Djalon can never “age”, must simulate “youth” for all eternity, unless a change of climate supervene that will slow down chemical decay of rocks. The attainment of graded profiles breaks down in the humid equatorial zone as a test of transition from youth to maturity of rivers, whether these are regarded as “cycle” stages or merely conditions, though the test remains valid in middle latitudes throughout very extensive forested and also semi-arid regions. Some would discard the distinction between young and mature rivers, considering the attainment of a graded profile merely a change of condition and one indeed that is liable to reversal by rejuvenations that may not make any great changes in the general appearance of the landscape.
The equatorial forest is the milieu of soil laterization. When this process, which is an indicator of cessation or of extreme slowness of mechanical (though not chemical) ablation, is active Tricart (1957, p. 425) attributes such cessation to the presence of a dense forest cover. Though various factors certainly interact, it is rather the absence of gravel, or the prevention of formation of gravel, that determines retardation of erosion. (“The quantity of coarse waste transported is much reduced, or may be nil”—Tricart, 1957, p. 430.) It seems thus unjustifiable to maintain that a forest cover of itself can prevent dissection (as distinguished from gullying due to accelerated erosion) and the accompanying ablation of the whole land surface.
Wellington Relief Probably all of Late Pleistocene Age
Tectonic development of the larger ridge and valley forms, when the regional peneplain was deformed, has taken place rather recently at Wellington, certainly in the Pleistocene and probably as late as mid-Pleistocene (Cotton, 1957, p. 787). Thus it is unlikely that early Pleistocene cryergic episodes have influenced the forms of the existing landscape. This remark is not applicable, however, to all parts of New Zealand.
Baulig, H., 1950. Essais de géomorphologie, Paris: Les Belles Lettres.
—— 1956. Vocabulaire de géomorphologie, Paris: Les Belles Lettres.
—— 1957a. Peneplains and pediplains. Geol. Soc. America, Bull., 68, pp. 913–29.
—— 1957b. Les méthodes de la géomorphologie d'après M. Pierre Birot, Ann. Géoer, 66, pp. 97–124, 221–36.
Beaujeu-Garnier, J., 1952. Massif Central francçais (Comm. Periglac Morphol), Proc. Int. Geogr Cong. Washington, pp. 209–12.
—— 1953. Modelé périglaciaire dans le Massif Central français, Rev. Géom. dyn., 4, pp. 251–81.
Brodie, J., 1957. Late Pleistocene beds, Wellington Peninsula, N.Z. J. Sci. Tech., B 38, pp. 623–43.
Budel., J., 1954. Klima-morphologische Arbeiten in Aethiopien im Frühjahr 1953, Erdkunde, 8, pp. 139–56.
—— 1957. Die “Doppelten Einebnungsflachen” in den feuchten Tropen, Zeits. Geomorph., 1, pp. 201–28.
Cailleux, A., 1948. Le ruissellement en pays tempéré non-montagneux, Ann. Géogr, 57, pp. 21–29.
—— 1950. Ecoulements liquides en nappe et aplanissements, Rev. Géom. dyn, 1 pp. 245–70.
Cotton, C. A., 1948. Landscape, 2nd ed., Cambridge: Univ. Press.
—— 1952. The erosional grading of convex and concave slopes, Geogr. J., 118, pp. 197–204.
—— 1955. See Cotton and Te Punga, 1955a.
—— 1956. Geomechanics of New Zealand mountain-building, N.Z. J. Sci. Tech., B 38, pp. 187–200.
Cotton, C. A., 1957. Tectonic features in a coastal setting at Wellington, Trans. Roy. Soc. N.Z., 84, pp. 761–90.
Cotton, C. A., and Te Punga, M. T., 1955a. Solifluxion and periglacially modified landforms at Wellington, New Zealand, Trans. Roy. Soc. N.Z., 82, pp. 1001–31.
—— and Te Punga, M. T., 1955b. Fossil gullies in the Wellington landscape, N.Z., Geographer, 11, pp. 72–5.
Davis, W. M., 1908. Practical Exercises in Physical Geography (with atlas), Boston: Ginn and Company.
—— 1912. Die erklärende Beschreibung der Landformen, Leipzig: Teubner.
Gilbert, G. K.,1877. Report on the Geology of the Henry Mountains, Washington.
Horton, R. E., 1945. Erosional development of streams and their drainage basins, Geol. Soc. America, Bull., 56, pp. 275–370.
Martonne, E. De, 1935. Traité de géographie physique, Vol. 2, 5th ed., Paris: A. Colin.
Meynier, A., 1957. Une vallée fossile à Ouessant, Norois, 15, pp. 369–71.
Reed, J. J., 1957a. Petrology of the Lower Mesozoic rocks of the Wellington district, N. Z. Geol. Surv. Bull., 57.
—— 1957b. Fault zones in part of the Rimutaka Range, N.Z.J. Sci. Tech., B 38, pp. 686–7.
Sharp, R. P., 1957. Geomorphology of the Cima Dome, Mojave Desert, California, Geol. Soc. America, Bull., 68, pp. 273–90.
Stevens, G. R., 1957a. Solifluxion phenomena in the Lower Hutt area, N. Z. J. Sci. Tech., B 38, pp. 279–96.
—— 1957b. Geomorphology of the Hutt Valley, New Zealand, N. Z. J. Sci. Tech., B 38, pp. 297–327.
Suchel, A., 1954. Studien zur quartaren Morphologie des Hilsgebietes, Gottinger Geogr. Abh., Heft 17, 148 pp.
Te Punga, M. T., 1955. See Cotton and Te Punga, 1955a.
—— 1956. Altiplanation terraces in southern England, Biul. periglac., 4, pp. 331–8.
—— 1957. Periglaciation in southern England, Tijds. K. Ned. Aardrijksk. Gen. Amsterdam, 74, pp. 401–12.
Tricart, J., 1949. La partie orientale du Bassin de Paris, Tome 1, Paris: Soc. Ed. Ens. Sup.
—— 1950. Le modelé périglaciaire, Cours de géomorphologie, 2e partie, fasc. I (1), Paris: Centre Doc Univ., 272 pp.
—— 1951a. Le système d'érosion périglaciaire, Information Géographique, 5, 7 pp.
—— 1951b. Die Entstehungsbedingungen des Schichtstufenreliefs im Pariser Becken, Peterm. Geogr. Mitt., 2, pp. 98–104.
—— 1956a. In Cotton, C. A., et Tricart, J., Discussions à propos de l'analyse d'un ouvrage. Rev. Géom. dyn., 7, pp. 148–9.
—— 1956b. France, in “Rap. Com Morphologie Périglaciaire U. G.I., Cong. Internat. Rio de Janeiro,” Biul. periglac, 4, pp. 118–38.
—— 1956c. Etude expérimental du probléme de la gélivation, Biul periglac., 4, pp. 285–318.
—— 1956d. Types de fleuves et systèmes morphogénetiques en Afrique occidentale, Bull. Sec. Géogr. Com. Trav. hist. et scient., 1955, pp. 303–42.
—— 1957. Applications du concept du zonalité à la géomorphologie, Tijds. K. Ned. Aardrijksk Gen Amsterdam, 74, pp. 422–34.
Tricart, J, et Cailleux, A., n.d., Conditions anciennes et actuelles de la genése des pénéplaines, Proc. Int. Geogr. Cong. Washington 1952, pp. 396–9.
——1955. Introduction a la géomorphologie climatique, Cours de géomorphologie, Paris: Centre Doc. Univ., pp. 228.
Professor C. A. Cotton, 2 Manuka Avenue, Lower Hutt, N.Z.