Part II Periglacially Modified Landforms
“The peculiar properties of pe [ unclear: ] glacial morphogeny produced forms of their own which constitute a highly characteristic assemblage.”
—Jan Dylik (1952)
Cryoturbation on the Belmont plateau
Texture of dissection
Relief of the Belmont plateau
The “periglacial cycle”
The flanks of upland remnants
Periglacial processes in relation to topographic details
Possible development of nivation cirques
Periglacial dell hypothesis
Convex headwater profiles
Valley-head deepening hypothesis
The theory of spontaneous mass movement
Corrasion under moving regolith
The question of origin of upland surfaces
Near Wellington there are a number of comparatively level remnants of an upland surface (Plate 36) that seems to have originated as a peneplain. The minor relief on one of these (on which study of sections of the regolith and head deposits indicate very considerable solifluxion recently in progress) suggests that a formerly less accidented surface has been considerably worn down and has at the same time undergone a somewhat sharp and fine-textured dissection to maturity probably due to a combination of the effects of periglacial denudation in episodes of regional (perhaps world-wide) refrigeration and stream work in milder intervals. On this particular upland, however, a structural peculiarity favours slow degradation. On other peneplain remnants in the district presumably subject equally to cryoturbation (though good sections of the regolith are not observable on them) the periglacial processes may have tended to reduce relief.
In addition there is a remarkable dimpling at the margins of upland remnants. Wide-open, shallow, streamless valley heads give place downstream to the young V-shaped ravines of normal water-cut origin that dissect flanking slopes (Plate 36, Fig. 1) and are in process of grading in accordance with a base-level greatly lowered (relatively) because of irregular upheaval of the land.
Unless the shallow valley-heads scalloping the margins of upland remnants are relict forms that originated under a former (though not very ancient) climatic
control, presumably refrigeration, they must be little altered features of an ancient relief developed in a cycle intermediate between a general peneplanation and the more modern dissection of the flanks of the peneplain remnants. The same may be true of some of the interior parts of upland peneplain remnants.* If such cyclic development took place in this district, as a result of a first small lowering of base-level subsequent to peneplanation, its date must be long anterior to that of the much deeper incision of valley systems that accompanied, or was preceded by, the upheaval with some faulting and warping that has led to the stranding of remnants of the formerly planed—or, at any rate, very subdued—land surface on summits and ridge crests. Though it seems possible that some of the minor relief within and at the boundaries of upland remnants originated in such an intermediate cycle, this cannot be true of all of it. For reasons that will be set out below (pp. 1017–8), there must have been considerable changes in the minor relief of some of the uplands in recent times.
Cryoturbation on the Belmont Plateau.
The name Belmont plateau may be applied for convenience of reference to a remnant of the upland surface that has been examined with some care largely because a number of artificial cuts have been made on it that reveal the nature
[Footnote] * Some such weak relief, with deep and ancient weathering under it, seems essential for any conspicuous development of cryoturbation and solifluxion (Beanjeu-Garnier, 1953).
and structure of the regolith. About a square mile in extent (Fig. 2), it is part of a strip of upland, in some parts with stronger relief than this, between the Hutt Valley depression and the broadly tectonic Porirua Harbour basin, from which it is separated by a high scarp, probably at least in part a fault scarp, that is maturely dissected. (Map reference: N.Z.M.S.1/N160.) North-eastward this strip of high surface is interrupted by a transverse tectonic gap that is traversed by the Pauatahanui-Haywards road; and farther south-west it is dissected by the deeply incised Korokoro valley. Between these the Belmont plateau survives with subdued relief (at altitudes from 1,000 to 1,200 feet), because, though underlain by deeply weathered rocks, its drainage towards the south-east from a divide at the crest of the north-western scarp boundary is held up at a local base-level where it crosses a belt of very resistant greywacke, which is little jointed and neither deeply weathered nor, apparently, susceptible to deep weathering (Fig. 3).
Fig. 3.—Survival of the Belmont plateau on the ridge between the Hutt Valley and the Porirua basin. Sketch not to scale.
Under the soil of the upland surface of the Belmont plateau there are arrested streams or sheets (thick in places) of formerly flowing regolithic debris now forming deposits of head on slopes. The flow of such debris must be regarded as solifluxion that has taken place in the most recent episodes of Southern Hemispheré refrigeration (Te Punga, Part I), and some considerable changes in the form and minor relief of the surface must have accompanied it. In general, however, changes due to solifluxional transfer of material must have effected a softening lather than accentuation of relief, a smoothing of asperities, for the regolith† has certainly flowed down from the moderate slopes of low spurs and ridges into
[Footnote] † The abbreviated definition of “regolith” given by Pettijohn (1949, p. 282), “the products of weathering formed in situ.” seems to exclude the possibility of “flowing” regolith; but the use of “regolith” for material that flows is justified by Merrill's (1904, p. 299) original definition: “In places this covering is made up of material originating through rock weathering or plant growth in situ. In other instances it is of fragmental and more or less decomposed matter drifted by wind, water, or ice from other sources. This entre mantle of unconsolidated matter. whatever its nature or origin, it is proposed to call the regolith.” It detracts from the value of the term to allow it to be applied to dune sands, moraines, and flood-plain alluvia, and such layers are excluded in current usage, but a flowing regolith of locally derived material may quite well be envisaged.
hollows between these, where some of it has accumulated as head. There has been some lowering of the crests of the spurs and ridges owing to off-flow of regolith developed in situ on them by active periglacial congelifraction, though perhaps without very deep corrasion of the bedrock even on the flanks, and with none at all on the crests. The localization of such corrasion as takes place under the regolith on the flanks of spurs and isolated summits has no doubt steepened these somewhat, so that the general effect has been a rounding of summit convexity, with production in some cases of typical “round knobs”—taking this name from Round Knob trigonometrical station (N160/442373), which is an example. (Another example is shown in Plate 37. Fig. 1.)
This material has travelled down slopes towards the axes of gullies, and former slopes and some gullies are buried under head which has accumulated to depths of (exceptionally) up to 50 feet (Te Punga, Part I). Some gullies on the Belmont plateau have been blocked, and their axes have been shifted laterally.
Texture of Dissection
The results of solifluxion do not, however, amount in this district to obliteration of the minor relief with substitution for it of anything resembling the conspicuously (and perhaps abnormally) coarse-textured pattern of swales and whale-back forms characteristic of north-western Europe and considerable parts of Britain, which must, apparently, be attributed to the cumulative effects of periglacial processes (with much smothering under head) throughout a succession of Pleistocene ice ages.
In those parts of the heavily affected regions not contemporaneously subjected to sharp dissection by water streams it seems that the surface forms of normal stream-cut origin have been so thoroughly smoothed away by cryoturbation and solifluxion, and so heavily plastered in some parts with head, as completely to eliminate a finer-textured pattern of relief that must be assumed to have diversified the surface in preglacial times—such, that is to say, as is still to be seen very generally in the humid tropical belts, regardless of the nature of the bedrock. Thorough plastering of the land surface with permeable head will of itself favour coarseness of texture in a stream pattern re-developing, after periglacial elimination of streams, in a subsequent interglacial age (or interstadial).
The extent to which New Zealand landscapes have escaped this transformation even in areas—the Wellington district, for example—that are partly head-covered, is brought out by comparison with hilly country in the Fiji Islands (Lat. 18°), such as is shown in oblique photographs reproduced in a paper by Marshall (1953), for these may very easily be mistaken for typical areas in the North Island of New Zealand. The texture of minor relief at Wellington remains fine and is comparable not only to that of the humid tropics but also to those parts of Italy, § California, and other extratropical regions that have escaped a periglacial remodelling of the surface.
Relief of the Belmont Plateau
At Belnont it appears that a close-meshed net of water streams has continued to dissect the surface either contemporaneously with solifluxion or during warmer
[Footnote] § An investigation of ice-age conditions in southern Italy and Sicily made by Budel in 1950 (as reported by Troll, 1951, p. 59) indicates that that region was treeless and affected by solifluxion only at altitudes above 1,000 metres.
interstadial intervals, and the removal of some of the head from slopes by rain wash at such times has made effective and apparent the steepening of the bedrock surface of spur slopes by the “shaving” process in earlier episodes of solifluxion. Such modification of the plateau surface has eventually resulted from this that only the broad outlines are now preserved of an initially more or less tabular major feature, presumably a peneplain remnant. Relief referable to an “intermediate cycle” is also only hypothetical on the Belmont plateau, if this is to be understood as a cycle anterior to an upheaval that involved considerable deformation of the primitive land surface (p. 1014).
Bordering the plateau there is a belt (that is to say, the outcrop of a steeply inclined stratum or lens) of greywacke almost as resistant to weathering as quartzite, because traversed by only widely spaced joints (Te Punga, Part I), which probably preserves the level of an original peneplain but little lowered by denudation (Fig. 3). Striking south-south-west from N160/460365 (altitude 1,180ft.) to 448343 (altitude 1,225ft.), this formation is indicated by the presence on its outcrops of autochthonous block fields consisting of numerous very large, loose, spheroidally weathered boulders (to which Te Punga has referred) projecting through its soil (Plate 37, Fig. 1). On the adjoining belt of contrasting closely jointed, deeply weathering greywacke dissection has taken place to a depth of about 200 feet. This latter rock yields for the most part fine debris, its soil containing only scattered large spheroidal blocks, and frost-riven angular spalls from such blocks, which have been transported by periglacially flowing regolith from higher outcrops of the little-jointed, resistant greywacke (p. 1008).
The fully mature surface form produced by dissection on the thoroughly weathered greywacke, though it probably inherits its stream pattern in part from some intermediate cycle, has been profoundly modified by processes that have operated since the isolation of the upland remnant. The lowering of the surface as a whole (shown in Plate 37, Fig. 1), though in a sense it is slow degradation in response to a gradual lowering of the local base-level, may be attributed to alternation of episodes of dominant periglacial solifluxion with others of dominant stream cutting, an effect of which has been accentuation of dissection—not cryoplanation, but rather the reverse.
Though very slow progressive lowering, over a long period, of the local base-level must, of course, have controlled the whole course of denudation on the plateau, the numerous shallow minor gullies of the stream network graded to the local base-level are quite freshly cut in the soft, deeply weathered rock to a sharp V-shape that can be attributed only to vigorous vertical corrasion by the streams of water that have flowed through them since the close of the last periglacial episode. Such vertical corrasion has ceased abruptly, however, and all the gullies are now infilled at the bottom of the V by strips of peat swamp up to 10 or 20 feet wide (Plate 37, Fig. 2).
This reversal of process, substitution of filling for cutting though unaccompanied by any flow of the regolith comparable to periglacial solifluxion, can scarcely be explained except by some very recent change in the microclimate. It cannot be attributed to deforestation, which may be expected in most cases to accelerate, not to stop, vertical corrasion, unless under the conditions locally prevailing the turf that has recently become established protects the soil and retards the run-off better than the former forest did. Though this seems to be a hypothesis not unworthy of consideration, an alternative to it may be tentatively suggested,
namely that stream corrasion (linear erosion), active at a postglacial thermal maximum, has been halted by a change to the existing cool, moist climate.
The rather strongly accidented surface of the Belmont plateau seems to be exceptional, if it be derived, like flatter summit areas in the same district, from a former peneplain, and must be attributable to the fortuitous insulation of this weak-rock tract at a high level, in the manner already described. It will be of interest to examine any excavations that are made on other upland-surface remnants near Wellington and to discover what parts of their weaker and more gently undulating relief (as shown in Plate 36, Fig. 2) are underlain by head, and to what extent the process of subcutaneous corrasion has operated on them Their relief remains fine-textured. The special conditions affecting the sculpture of the Belmont plateau have allowed an apparently unusual amount of water erosion to take place—certainly since the last periglacial episode and very probably also between (rather than in) this and earlier similar episodes.
The “Periglacial Cycle”
Bryan (1946, p. 640), the author of the cryoplanation concept, and also Peltier (1950) in his concept of stages of periglacial.landscape development that might lead eventually, if the “periglacial cycle” escaped interruption, to planation, assign a rôle to running water. Though streams of water may be somewhat inefficient if they run only in a short summer season over ground that remains frozen at depth, they must at least receive, transport, and evacuate some of the debris brought by solifluxion from higher slopes, meanwhile maintaining gradients accordant with a local base-level, as has clearly been the case on the Belmont plateau.
Any approach to cryoplanation, or “geliplanation” (preferred by Baulig), by such a combination of processes might be expected to require longer duration of the controlling periglacial conditions than is likely in Pleistocene episodes of refrigeration (Baulig, 1952, p. 182). This must be so, at any rate, on terrains of resistant of moderately resistant rocks, though exceptions may be found locally on materials of very low resistance Thus, in central Poland, where the bedrock of the landscape consists entirely of unconsolidated glacial debris, Dylik (1952) finds that minor relief, most of it constructional and dating from the earlier glaciations that must have been in existence prior to the last periglacial episode has completely disappeared, and that the (relict) landforms that are still in a perfect state of preservation on the existing upland surface are almost entirely due to the process of solifluxion accompanied by rather extensive corrasion under debris streams. In the transformation responsible for the production of the present landscape the part played by running water, though not entirely negligible, has been ancillary. On the margins of uplands water work must have been more effeciive, but Dylik remarks (1952, p. 10) that “the problem of periglacial waters is still rather obscure”
On the plateau of the Central Massif of France, where the terrain consists of resistant rocks. Mme. Beaujeu-Garnier (1953, pp. 251–2, 265) notes that widespread solifluxion, phenomena have changed but not obliterated pre-existing landscape forms. They have been altered “especially by smoothing”. Features attributable to soliflual corrasion are not reported, but, as is the case in Poland also (Dylik. 1952), new spurs built of head figure largely in the altered laudscape. Where “there was a landscape with forms of maturity, it was changed to
a landscape very smooth: the slopes disappeared under the coulées, the valleys were filled”. Thus, “periglacial erosion” can “age a mature relief, inducing senility”. She observes, however, that areas of plateau surface with initially very small relief have escaped significant modification, and so also have high surfaces that have been continuously snow-mantled. As in Poland, post-periglacial changes have been insignificant except on the margins of plateaux and where they are intersected by river valleys.
At very high altitudes and in Polar regions, it must be noted, frigid conditions endure long enough for advanced contemporary development of a periglacial cycle on the rocks that are most susceptible to gelifraction and solifluxion. Troll (1944, pp. 600, 630; 1947, pp. 173–4) describes and figures examples of differential denudation of belts of rock to small or moderate relief, so that they now exhibit subdued forms: (a) at altitudes above 15,000 feet in the Bolivian Andes, and (b) in the tundra region of the Taimyr Peninsula, Northern Siberia. The rocks affected in these belts are argillaceous and impermeable, which favours ground ice development, so that frost weathering, cryoturbation, and solifluxion are very active (in the one case daily throughout the year and in the other case seasonally) and no vegetation at all can exist; thus wastage is rapid.
As indicated on p. 1013, landforms of a special kind are characteristically developed, on the margins of upland remnants, at the heads of streams that dissect, flanking scarps. These somewhat resemble nivation cirques, but are more closely analogous to features of European periglacial landscapes that have been named Dellen and “corrasion niches.” In shallow, wide-open valley heads there are neither permanent streams of water nor even any water channels. In contrast with the swampy-floored V-cut minor valleys described earlier, these depressions though nearly flat-floored are turfed from side to side, and thus appear to have been streamless under the former forest cover. Rainfall is disposed of almost entirely by infiltration.
Apart from possible removal of fine particles occasionally by sheetwash during exceptionally heavy rains, accumulating debris that has been contributed to the hollows by creep and also, perhaps much more abundantly in the past, by flow of the regolith from the sides and head might be (and quite possibly has been when periglacial conditions prevailed) evacuated by mass movement of a watersaturated stream of debris following the axis of each cirque-like hollow. This would take place somewhat in the manner pictured by Walther Penck (1925, p. 60; 1953, p. 74), who however envisaged such flow, actuated in his opinion by gravity unassisted by any transporting medium, and therefore termed by him spontaneous mass movement, as taking place not in a periglacially saturated regolith but in a material (a relatively dry rubble) that is much more stubborn in its resistance to gravity-controlled flow.
Unlike those of apparently analogous hollows in Central Europe (Büdel, 1944, p. 494), the floors of these depressions have not become swampy or peat-covered in postglacial time They are now grassed, but the grass cover has been artificially induced within the last hundred years on a surface recently cleared of forest. There can be little doubt that during Pleistocene ice ages the ground has been bare or nearly so, and that its colonization by the forest that was recently cleared off has followed the amelioration of temperature after the last glaciation. It
would therefore be superfluous to debate the question of the extent to which roots of forest trees impede free downhill movement of the regolith, as Penck (1925, pp. 65–68; 1953, pp. 80–83) has done.
The Flanks of Upland Remnants.
It is unnecessary, and would be out of place, to discuss here whether the isolation of peneplain remnants on summits and ridge crests near Wellington has been due in the main to circumdenudation—i.e., to the erosional development of deep valleys such as have been regarded as subsequent in origin (Cotton, 1912) or whether it is due to a predominance of tectonic relief, the land surface having been (very long ago) warped and broken into differentially upheaved blocks bounded by fault scarps that are now considerably degraded (Cotton, 1955). The surrounding slopes are, in any case, dissected by ravine-headed steep-grade valleys, which are numerous and many-branched because of the ubiquitous fine texture of dissection on this greywacke terrain.
In parts of the Wellington district these well dissected slopes now intersect upward in fairly sharp ridges. In some places, however, the ridges are broader, as the steep side slopes fail to intersect but leave on the crests some remnants of an uplan surface that is more or less well preserved. Where such a remnant is tilted there is not a divide between the upland and flank drainage along the lower edge of it. Elsewhere, however, and in particular along a high edge, there is a sharp line of division, and it is here especially that shallow but otherwise cirque-like hollows (Fig. 4) are found above the heads of flank ravines, which are in part fed by seepage of ground water that has infiltrated through the floors of these.
Fig. 4.—Longitudinal profile of a cirque-like hollow. or dell, at the head of a tributary gully that descends about 200 feet very steeply from the Belimont plateau into a deep valley draining north-westward towards the Porirua Harbour basin. Inset is a view down the headwater dell. Locality, N160/445375.
The hollows are separated from the deep and narrow ravines below (which are fed by seepages of ground water collected in them) by very broadly convex profile “nicks” (Fig. 4). It is not impossible that these very pronounced breaks
Fig. 1 —Solifluxion layers (S1 and S2) resting on weathered greywacke bedrock (B), near Belmont; description in text. S1 is spanned by the 3 ft. ruler. Fig. 2.—Neai view of part of Fig. 1. The 3 ft. ruler spans S1 and its base rests on the shaved surface of weathered greywacke bedrock. Note the angularity of the fragments in S1. Fig. 3.—Hand specimen (3in. across) collected from the position marked by the base of the ruler in Fig. 2. Note the sharply defined, smooth, shaved surface of weathered greywacke bedrock on which solifluxion layer S1 rests. (Plate 33. Figs. 1. 2. 3 Photos by M. D. King)
Fig. 1—Solifluxion debris resting on a shaved surface of weathered grevwacke bedrock, near Belmont. Note the sharply defined contact (sloping down from left to right just above the hammer). Fig. 2.—Large flakes and fragments of hard, spheroidally weathered grevwacke boulders embedded in solifluxion laver, near Belmont. The ruler (bottom centre) is 3ft. long. Fig. 3.—Solifluxion laver (above the white broken line) thickening downslope. Monagha Avenue, Karoar, Wellington. The base of the 3ft. ruller (centie) rests on a shaved surface of weathered grevwacke bedrock (below the line). Fig. 4.—Angulai solifluxion debris resting on a shaved surface of weathered grevwacke bedrock, Monaghan Avenue Karori. The pencil on the contact is 6in. long. Fig. 5.—A frost-riven flake (below the hammer) from a spherordally weathered grevwacke bonulder embedded in a solifluxion laver, near Belmont. (Plate 3, Figs. 1, 2, 3, 4, 5 Photos by MD King)
Fig. 1.—Two solifluxion layers near Wadestown, Wellington. Greywacke basement, weathered red, Overlair by 5 feet of coarse. Pinkish-red solifluxion debris (above 3ft. ruler, and thinning from right to left), overlain by 3 feet of fine, grevish-yellow solifluxion debets. (Photo by M. D. King.) Fig. 2.—Small folds in sandy material at the base of a solifluxion layer when overlies a shaved surface of grevwacke bedrock, near Kaitoke (Photo by M. D. King.)
Fig. 1.— Otari peneplain remnant, Wellington. Locality: 3 miles north-north-wet from centre of Wellington city (co-ordinates: N164/315270). Altitude 1,300 feet. View looking north-north-west “Dells” are seen in middle distance at left. (Photo by Victoria University College Department of Geography). Fig. 2.—Detail of Otari surface (Fig. 1). (Photo by C. A. Cotton.)
Fig. 1.—View looking south-east across Belmont plateau at summit 1,180 ft. (N160/460365). A massive outcrop of resistant grevwacke reduced to roun-knob form is crowned by an autochthonus block field. The foreground shows, features of fine-textured small relief developed on a belt of deeply weathering greywacke which has apparently undergone progressive denudation below the summit level. (Photo by M. D. King.) Fig. 2.—Branching streamn-cut valley on the Belmont plateau, showing swampv axial strip. Note fine texture of dissection (Photo by M. D. King.)
Fig. 1.—Valley head of an interupted cycle hanging above a conspicuous nick (at the interlocking spurs) in the profile of a ravine that dissects the scarp bounding the Belmont plateau on the north-west side (view from divide at head). There are swampy axial strips in the gullies, as in Plate 37, Fig. 2. (Photo by M. D. King.) Fig. 2.—The onque-like valley-head hollow, or dell, shown in Text-fig. 4, as seen from the divide at its head. View looking west. (Photo by M. D. King.)
in the longitudinal profile are true heads of rejuvenation—that is to say, that the ravine below is now extending headward along a widely opened valley form developed almost to senility by the same stream when base-level was relatively much higher. This possibility, however, introduces the hypothesis of an “intermediate cycle”, previously mentioned, which it might be necessary to postulate, dating very far back in the history of the development of this landscape.
Periglacial Processes in Relation to Topographic Details
The later history of the Wellington land surface probably includes more than one glacial (locally periglacial) and interglacial age, or, at least, several stadia and interstadia, so that greater modification of the relief may have taken place during the earlier of these than might be inferred from the present distribution and known thickness of head. This is suggested, indeed, by some of the sections that expose such periglacial debris. At a locality on the Belmont plateau Te Puuga (Part I) has found superposed deposits of head with different lithological characteristics and infers that the lower of these may date from a penultimate refrigeration (p 1011) Quite obviously, however, solifluxional sheets and accumulations of head of that and earlier ages must have been largely destroyed in the process of reworking of the regolith during each periglacial episode, as well as being subject to erosion between these—and all such changes have been accompanied and followed, without doubt, by retouching of the minor relief.
Solifluxion phenomena may be traced in numerous sections revealed in artificial cuts on the subdued slopes of spurs on the Belmont plateau, one of which has been described in detail (Te Punga, Part I) and figured (Plate 33, Figs. 1–3). The extent to which, as this section shows, solifluxion has been active indicates that landforms have here been in process of modification by denudation continuously into the very late Pleistocene, though the plateau has been cut off from rapid dissection and degradation to a lowered base-level by a barrier of resistant rock, as already described (p. 1017).
This plateau affords good examples also—in the headwaters of the Korokoro system and of the north-westward drainage—of marginal scalloping by the cirque-like dells already referred to Close proximity of these features to head-mantled slopes at the same altitude suggests strongly that solifluxion may be the process largely responsible for shaping them. Similar valley-heads are seen along the high ridge that extends southward, towards Karori, from the more nearly level Otari upland remnant (shown in Plate 36, Fig. 1).
Possible Development of Nivation Cirques.
The hollows are somewhat like rivation cirques. (For references on nivation. or snow-patch erosion, see Cotton, 1942, pp. 202–5, and Lewis, 1939) Though such a hypothesis of origin must not be overlooked, it seems not to afford a particularly plausible explanation, because of absence of record of true glaciation features such as glacial cirques (into which nivation cirques may develop), even at very much greater altitudes in the axial ranges of the North Island, except at one restricted locality in the Tararua Range (Adkin. 1912). (See Fig. 1.)
Periglacial Dell Hypothesis.
As another alternative explanation of these landscape dimples it may bo suggested that they have been developed, entirely or in part, by periglacial
solifluxion. If so, they may be either (a) modifications, or (b) excavated extensions of the heads of ravines already in existence. In the latter case at least they may be regarded as analogous to streamless valleys, or “dells”, which, according to Büdel (1944, p. 494) are to be found at all valley heads on the upland peneplains of Central Europe. These were described as Dellen by Schmitthenner (1925). In the English translation of W. Penck (1953) Delle is rendered by “dell”, defined in the glossary as used “in the special sense of a depression up-valley from the source of a stream.” Using the word in this way only, it becomes possible to refer to marginal dimples on the Wellington peneplain remnants as dells, though a more accurate description might be “dell-like marginal dimples”. They are developed as definitely smaller-scale features than the analogous forms in Europe; but this may be explained as in accord with the prevailing fine texture of all dissection on New Zealand greywacke terrains. Examples in Poland figured by Dylik (1952) are about half a mile long. Dylik names these “corrasion troughs “(in another publication “corrasion niches”), and assumes without question that they have been produced by corrasion under soliflual debris streams.
The variant of the hypothesis—(a) above—that such dells are pre-existing ravine heads modified simply by infilling with soliflual debris as a result of mere migration, or flow, of the regolith requires that the convex break of slope at the transition down-valley from dell to ravine head shall be explicable as due to the accumulatition of debris towards the lip. step, or front of the dell floor. The floors have not been observed to be underlain by excessively thick masses of head. Though critical sections that would reveal the thickness of such fill have not been found, th over-all form of the land surface does not suggest such voluminous choking of valley heads as the hypothesis requires. The profile figured (Fig. 4) affords an extreme case of convexity of profile at the step because of an exceptionally steep gradient at the head of the lower valley. In this example, however, the lower valley is a steep-grade, V-cut notch which is rock-walled just below the break of gradient, and this makes it impossible to explain the valley step as due to the presence of fill (hypothesis a).
The heads of some flank-dissecting ravines are without true dells, but in a few such a short headwater segment survives of what can only be the valley of an interrupted (“intermediate”) cycle with but little periglacial modification. A valley head of this kind, though insulated, like the dell-form valley heads, from the young ravine below by a very pronounced nick in the stream profile, is V-bottomed and is joined by some short tributary gullies of the same shape (Plato 38, Fig. 1). This is clearly a case in which the marginal drainage in course of adaptation to a much lowered base-level merely intersects the typically incised surface of the upland (locally with sharp though shallow dissection), though on the Belmont plateau (Fig 3) only very small portions of this are found west of the divide. Valley heads formerly of this kind may conceivably have been altered to the dell-like form by streaming-in of solifluxion debris which has filled their sharply cut gullies with head.
It is normal, however, for Central European dells to be floored throughout most of their length by a layer—only moderately thick—of solifluxion debris, which has moved into them, presumably during an episode of refrigeration, but has not piled up to any great extent. Instead it has flowed through them to all appearaace, somewhat like a glacier, as recorded by Schmidle from observations
in flat-floored valleys of the Black Forest, and by Martin and Poser on other German uplands (fide Budel, 1944, p. 494).
The remaining hypothesis (b) is that the dells are headward extensions of the marginal dissecting ravines, which have been pushed back at low gradients into the level uplands, without any base-level control, by a periglacial erosive process at a time (or more than one time) of general refrigeration.
Convex Headwater Profiles
Before looking into the possible or probable value of the last mentioned hypothesis as offering an acceptable explanation of Wellington dells it is as well to recall that a broadly convex nick is usually present very near the head of the normally concave (and perhaps graded) longitudinal profile of a stream-cut valley. This is a well-known feature, though explanations of it that have been offered may not be applicable to all cases. To quote Chamberlin and Salisbury (1906, pp. 66–67): “As a valley lengthens, the larger part of its profile becomes concave, but the extreme upper end still remains convex.” This seems to imply that valley-head convexity of profile is inherited from an earlier condition of the valley—a condition characterized through the whole or much of the length by a possibly smooth convexity unrelated to base-level*—an assumption admissible only if control by base-level, which determines concavity of profile in most streams, may for some reason be ignored, the principle that a river gains power downstream being substituted. According to Birot (1948; 1949, p. 124), a convex profile will be developed when the initial slope down which an extended consequent river flows in its early youth is steep enough to allow it to become a torrent with velocity increasing downstream, and such a convex segment, once established, may migrate headward into the upper valley in advance of normal grading.
Challinor (1930) has drawn attention to the opinion of Bain (1896) that without base-level control—or before the control eventually exerted by base-level is operative—a river profile will normally develop convexity Like Bain, and also Chamberlin and Salisbury, Challinor pictures the convex headwater profile as formerly extending farther downstream than now, having been encroached upon by headward extension of the concave base-level-controlled profile of the lower course. The relic of it still persisting is in an ordinary case “overlooked” (in Bain's words) because of its “relative insignificance”.
It must be kept in mind that in valley-head dells the saucer-shaped, or catenary, transverse profile is very different from the V-notch form commonly found in that part of the same valley occupied by a degrading stream that is thereby engaged in shaping a concave longitudinal profile. Another feature in which Wellington dells differ from the valley-heads described by Bain (unless he overlooked it) is the concavity of longitudinal profile in the dell itself, right at the Valley head, above the convex nick. This looks like a profile characteristic controlled by a local base-level at the nick, though in examples of dells examined there seems to be no hardrock bar that would afford a temporary base-level; and
[Footnote] * Such, it must be noted. can have nothing in common with true graded convexity of profile in a stream of progressively dwindling discharge. which may be due to loss by evaporation or by seepage, or. theoretically. in one fed (from the banks) with gravel, especially if this is progressively coarsei downstream.
we may accept the dictum of Baulig (1939, p. 301) that a “break of slope” in the profile does not itself exert base-level control.
A steepening of profile from that of a headwater reach or course in a hypothetical high-altitude zone of “slow degradation” to that of a lower (down-valley) course presenting the characteristics of youth has been claimed by Birot (1949, p. 90) as a non-cyclic condition—that is, one not implying a change of base-level—in granite terrain under a hot climate with a dry season; and a similar condition has been found by Strahler in valleys dissecting limestone plateaux in Arizona that were formerly regarded as of two-cycle origin (Cotton, 1953, p. 77). Though some strongly convex nicks may be thus accounted for, these cases do not seem to furnish an analogy that would explain all dell-like valley heads.
Valley-head Deepening Hypothesis
To return to the hypothesis that symmetrical dell-like dimples are developed actually as extensions of valley heads back into the upland by a periglacial process independent of base-level control: this involves a concept of some (moderate) deepening by corrasion effected by a broad rubble stream of regolithic origin such as is provided by solifluxion. A diagram by Büdel (1944, Abb. 3, reproduced as Fig. 5) illustrates this. The concave full line under the corrading stream of solifluxion debris—(5) in the diagram—indicates the locus of corrasion in the axis of the dell. The line might be extended a little farther to right and left, if we may judge by the extent of “shaving” of the bedrock of spurs observed by Te Punga (Part I). The peat layer shown as covering the postglacially stagnant rubble stream find the incised water-stream course (of quite modern development, postdating the peat) are not essential features. In most German examples surface water is drained away by infiltration through the very permeable rubble layer under the broad floor in the axis of the dell (Büdel, 1944, p. 495).
Symmetry of transverse profile of the dimple or dell is a critical point. This process of excavation will not normally develop asymmetry; and the explanation Büdel gives of asymmetric valleys in homogeneous material, in so far as these have been cut in a glacial age (and numerous examples of such are cited), is quite different. It involves vertical corrasion by a water stream in association with solifluxion on the valley sides, with a contrasting degree of exposure to snow- or loess-carrying winds on opposite sides of the developing valley. Absence of
Fig. 5.—Transverse profile of a dell. (After Budel.) Explanation: (1) Underlying rocks; (2) resistant layers, producing shode-stones in the regolith: (3) zone of disintegration and hooked effect (Hakenschlagen), as see in outcrop curvature: (4) solifluxion debris on slopes; (5) corradin stream of solifluxion debris; (6) postglacial peat layer; (7) water channel developed by very recent headward erosion.
asymmetry in Wellington dells (Fig. 4 and Plate 38, Fig. 2) may be regarded as lending some support to a theory of their development by a subcutaneous process of corrasion such as has been described.
The question arises, however, whether it is necessary to assume solifluxion under tundra conditions to explain the movement of a corrading stream of regolith. Te Punga (Part I) reports that solifluxional corrasion, or “shaving”, of the bedrock surface has taken place very commonly in the Wellington district under the regolithic cover on the slopes of spurs. Spurs showing such eorraded greywacke surfaces in section on their side slopes are close to and slope similarly to the walls of dells, notably near the western margin of the Belinont plateau (Plate 33). No demonstration has been possible, however, of tho reality of corrasion under the debris that floors the dells themselves.
The Theory of Spontaneous Mass Movement
The tale of alternative hypotheses in explanation of the Wellington dell-like features cannot be regarded as complete without some further reference to Walther Penek's theory of spontaneous mass movement. Setting aside for the moment collateral and corroborative evidence indicating the recent activity of solifluxion in the vicinity of typical dells, together with the fact that the deposits of head on slopes have the appearance of arrested streams, at least the academic possibility must be examined of explaining dell-making by corrasion that takes place beneath a regolith “spontaneously” moving down very gentle slopes “At the present day”, Penck wrote, “there is spontaneous migration of reduced material” (1953, p. 71).
Whereas corrasion by solifluxion as usually understood may be regarded as a rapidly operating process in that its mechanism is gravity-controlled mass movement of a mobile slurry of broken and weathered material down slopes, the somewhat similar movement postulated by Penck as universal, and therefore aclimatic, must be very much slower. The regolithic rubble he pictures as everywhere “spontaneously” moving down even the gentlest slopes (Penck, 1925, p. 60; 1953, p. 74) is, most of it, practically dry, almost its only lubrication being effected by the colloid products of rock decay it contains, and it can have so little mobility that only extremely small velocity can be assumed for currents consisting of it, if such there be at all on gentle slopes. In many places, if not in most, the amplitude of its spontaneous movement even over a vast period may be negligible.†
This concept of spontaneous movement (as distinguished from transport) is the basis, however, on which the whole superstructure of Penck's theory of slope
[Footnote] † It is clear that Strhler (1952) does not recognize any possibility of spontaneous flow affecting “relatively dry” regolith (“lock and soil”), for he regarrs such material as behaving as an elastic solid or clastic continuum until a “limit of internal cohesive strength or intergranular friction” is exceeded, when landsliding or slumping will supervene Only such material “with liberal amounts of water”—the solifluxional milieu—will flow Its behaviour is compared by Strahlei to that of a plastic solid, though not of a fluid It can, therefore, “remain stable on a slope.” ceasing to move perhaps because “the yield limit has increased, by desiccation of the mass, above the previous yield limit.” It is thus that a solifluxional sheet or stream (mobile only under tundia conditions,) dries out and is arrested to become a deposit of head when it no longer has frozen ground under it. As it dries out still further, becoming then “relatively dry” material, it takes on the properties of an “elastic continuum”, that is to say, in plain language its presentday stable condition.
retreat has been built. The validity of his deduction that slopes retreat always strictly and consistently parallel to themselves stands or falls by it; and, as his geomorphic analysis hinges on this one tenet, if the theory of spontaneous mass movement down even the gentlest slopes is unacceptable, all his carefully argued conclusions are discredited.
Movement of the regolith as a sheet over the bedrock, such as apparently takes place in solifluxion, and even such movement as is postulated in Penck's theory as spontaneous, is a different matter from soil creep, the velocity of which must diminish downward, becoming zero above or at the base of the regolith. Even granting that, as Penck would concede, the velocity of movement of material assumed to be spontaneously migrating downhill will diminish downward, if its basal layer is moving—though ever so slowly—it must theoretically be capable, as Penck recognized, of corrading the bedrock. This is obviously true also of the well authanticated process of solifluxion, which is a relatively very rapid flux, but little impeded by friction and so not necessarily slowed down very much at the bottom of the moving layer. Thus many of the observations Penck has cited and deductions he has drawn from them in favour of the theory of spontaneous mass movement seem, apart from considerations of tempo, eminently applicable to solifluxion.
As regards the mobility of material “reduced” (as the translators have it, in the sense of Aufbereitung), that is to say, prepared and thus made to some extent mobile by some process, or various processes, of weathering: (quotations from Penck, 1924; 1953: page reference to 1924—also, in brackets, to 1953) “Great inequality in the size of the fragments … is of decisive importance for its mobility. For the coarser heavy components exert upon the underlying fine grains, by means of their weight, a non-uniform pressure which these endeavour to evade; they succeed in doing so because they lack cohesion. and on inclined surfaces they escape downwards” (p. 47 [56–57]).
Because of differential movement within the moving mass which is induced partly as described in the foregoing quotation, rounding of fragments takes place by attrition (in addition to rounding attributable to mechanical and chemical weathering, which are continually in progress). Such rounding “indicates that rolling and rotation processes are to the fore during mass movement” (p. (63 ).
Rolling is attributed in part to the increase in velocity of mass movement within a layer from below upward, so that “this type of movement is a rolling over and over, not pushing or sliding, and its physical analogue is flow” (p. 63 ).
Yet there is a component of sliding as well, which when it takes place locally increases the velocity of the bottom layer, providing an optimum condition for corrasion of bedrock. “In artificial sections through rather thick masses of material that has been transported [by mass movement] a sharp boundary … is to be found between substratum and overlying soil, in place of the ill-defined transitionlal zone found in other cases. This is the rule for the steeper slopes … Considerable movement of solid bodies [makes] a slip plane of the upper surface of the rock” (p. 70 ).
Corrasion Under Moving Regolith
It is at such a “slip-plane” surface between bedrock and moving regolith that Penck recognizes a very important development of corrasion.*
“G. Götzinger has already published quite a number of observations on corrasion over the surface of slopes. Similar features are nearly always to be seen on the side walls (i.e., in the profile) of natural sections in the lower-lying parts of the country, wherever these reach down to solid rock. It is found that corrasion is associated with rocks that are not very resistant to weathering, and on those easily affected by mechanical action.
“Such results depend even more on the inclination of the ground than upon the character of the rock and the weight [of the moving material]. Even under a thick cover corrasion comes to an end on fairly steep slopes, since the pressurev exerted by this material decreases with an increasing angle of inclination.
“… Where the inclination is suitable there are signs of corrasion. These parts of the slope are not only a source of rock waste, but also a region over which material from above travels. Weight therefore increases, and capacity for mechanical work, right down to the foot of the slope. Gradient and rock being suitable, the most pronounced corrasion effects are found there” (pp. 95–96 [116–117]).
Baulig (1950, pp. 133, 135) has objected to Penck's theory of corrasion due to mass movement, claiming that the velocity of movement of the regolith down slopes (as soil creep) is zero at the base of the layer, which excludes the possibility of corrasion. Though the objection is quite probably valid as applied to Penck's concept of mass movement, it seems not to apply to solifluxion.
Allowance being made for the apparent necessity of substituting a theory of rapid solifluxion (acting for a short time) for that of almost infinitely slow, but secular, aclimatic mass movement that still continues, it is of interest to examine the more strictly relevant of Penck's observations on
[Footnote] * Walther Penck's use of the term corrasion (German: Korrasion) for the process here described as corrasion beneath a regolith is in accord with the definition given by Gilbert (1877, pp. 100–101), but Gilbert, along with many others who have adopted his definition, recognizing various corrading agents, found it necessary to specify the kind of corrasion operating—i e., the agent responsible for the process—e.g., “glacial” corrasion. Penck, however, has redefined “corrasion” to suit his own purpose, confining the use of the term to specify this particular process—the one that might perhaps be distinguished as subcutaneous—to the exclusion of mechanical erosion of every other kind. The general use of “corrasion” in German writing apparently began with Richrthofen (1886); but A. Penck (1894), perhaps by a slip of the pen, wrote Korrosion (instead of Korrasion), and this form has been used since by other German writers. Büdel (1944), for example, used Korrosion almost exactly with the restricted meaning given by W. Penck to Korrasion (corrasion). W. Penck, refusing or failing (perhaps inadvertently) to make any distinction between Korrasion and Korrosion, objected to the use of the latter by Grund (1903) for chemical erosion (W Penck's note 165). A distinction became established, however, as Grund probably intended, between “corrosion” with this meaning (compare Salisbury, 1908) and “corrasion”, signifying mechanical erosion. Thus “corrosion” is well established in the writings of Martel (1921) and other speleologists. J. W. Gregory (1911) defined both terms in a special paper, and Grabau (1913), citing Gregory, adopted the usage now generally accepted; but, probably inadvertently, he wrote (or, at any rate, printed) “corrode” for “corrade” in a passage on p. 246. Corrasion and corrosion were clearly distinguished by Tarr and Martin (1914).
“dells” and “corrasion valleys” in general (remembering that author's restricted definition of corrasion). Thus, he wrote: “The removal of material beneath corrading masses is more active, more intense, than under those which are not corrading. Should the corrading material unite in streams, the effect due to increase in weight [note an analogy with glacier corrasion] … comes into play. This means that the substratum [i.e., bedrock] is acted upon more powerfully—that is, corrasion is intensified. Now, increased denudation beneath a corradirg ‘mass stream’ [or concentrated stream of solifluxion debris] must excavate its substratum into an elongated hollow, dig a furrow. I should like to propose the term corrasion valleys [Korrasionstaler] for valleys with such an origin…. In every case [such a valley] contains a recognizable mass stream. The valley bottoms never show any sign of a [water-] stream bed or anything like it….
“Wide, shallow trough-valleys [i.e., with catenary transverse profile], having as a rule no stream bed, are the characteristic form found on peneplanes [Rumpfflächen] and landscapes of gentle average gradient all over the world….
“Analogous valley troughs and furrows are met with in temperate regions, chiefly near the heads of valleys formed by [water-stream] erosion, and are of regular occurrence when these lie in flattish country…. [These are] the valley which [Schmitthenner] calls ‘dells’” (Penck, 1924, pp. 92–93; 1953. pp. 112–113).
The Question of Origin of Upland Surfaces
If upland dells have in fact been scooped out by corrasion under streams of periglacial regolith, while at the same time summits have been broadened to the “round-knob” form, there seems little chance of successfully testing rival planation hypotheses by study of minor forms of relief on peneplain remnants in the hope of finding features explicable by processes of (say) Endrumpf or Primarrumpf development, or those deducible from competing theories of levelling by downwearing and backwearing. In the face of the evidence of considerable reworking and retouching of a land surface of small relief at Wellington, in a district which must have been only on the fringe of a periglacial zone, and in the light of reflection on the immense power and the repeated and long-continued operation of cryoturbation and the associated flow of regolith in middle- to high-latitude regions generally when tundra conditions have prevailed, it is impossible to avoid he impression that more or less level residuals of upheaved and fragmented erosion surfaces can retain little of their primitive form. Whether or not that form was intact, or at least reasonably well preserved, prior to the first Pleistocene glacial age, the preglacial form of the surface must almost certainly be now changed beyond recognition or restoration—unless, indeed, recourse is had to some form of the theory of glacial protection of level uplands at the time when most slopes were being completely remodelled.
The diagram, Fig. 6, has been designed to convey a suggestion as to a possible metamorphosis, with solifluxional development of smoothly convex forms associated with dell-like valleys or valley heads, affecting an upland peneplain, or a remnant of such a surface, which has previously been characterized by the concave profiles attributable to planation by valley widening.
This hypothetical type of pre-periglacial peneplain surface is selected merely as an example and without assuming that valley-widening is necessarily the only
Fig. 6.—Suggested periglacial metamorphosis of a peneplaned surface. The middle strip represents a surface developed to peneplanation by valley widening from an immaturely dissected landscape such as that shown at the rear. The foreground strip depiets possible periglacial modification such as might be found on a remnant that has as yet escaped dissection in a new cycle; but some marginal relief shown in front of this is attributable to incipient dissection now in progress and due to a lowering of base-level.
Fig. 7.— Slope profiles (with vertical scale exaggerated) from a divide (at left) to a valley axis (at right), as figured by Birot for a stage of development approaching planation of the land (Profiles selected from Birot, 1949, Fig. 8, p. 41.) A. as developed under a cool-humid climate. B, as developed under a hot-humid climate.
or even the usual process of subaerial planation. Such a surface may be developed rapidly, however, under hot-humid conditions like those of the present-day equatarial rain forests, for “chemical decomposition [of rocks] varies exponentially with the temperature”, and the rate of valley-side weathering and retreat may be ten times as rapid under a hot-humid as under a temperate climate (Birot, 1949, pp. 64, 132).
In late Tertiary times of peneplain development the climate may have been hot in New Zealand, as it apparently was in equivalent and even higher latitudes in Europe, where the peneplanation of massifs of ancient rock is now very generally believed to have taken place comparatively rapidly under the climatic control of processes dominated by retreat of concave valley-side slopes (Fig. 7, B). It may be justifiable to assume that New Zealand participated in the very late Pliocene thermal maximum that conditioned the accumulation of rañas in Spain and Portugal—vast spreads of rapidly produced angular roek debris attributable to hot-arid denudation, which are young enough to have escaped the effects of the quite considerable Pliocene deformation affecting that region but are older than the oldest Pleistocene river terraces (Ribeiro and Feio, 1950, p. 156).
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