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Volume 82, 1954-55
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Upland Margins

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

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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.

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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

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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)

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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)

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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.)

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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.)

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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.)

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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.)

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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.

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