Moraines and Outwash of the Tasman Glacier.
[Read before the Wellington Branch, July 10, 1941; received by the Editor, July 10, 1941; issued separately, December, 1941.]
In an article recently published, Speight * has stressed the problem of the general absence of conspicuous moraine barriers in the valleys vacated by the Tasman and other New Zealand glaciers, and he suggests that the hypothesis of glacial stagnation and down-wastage must be considered seriously in explanation of this substitution of a more general spreading of glacial debris for the deposit of endmoraine ridges.
The debris, if deposited as an ablation moraine, is either rewashed and reworked, however, or is overspread by fluvioglacial gravel, and, whatever the condition of the glaciers meanwhile, the absence of visible moraines is a result in one way or another of the great volume and strength of the melt-water rivers. These rivers either carry away all the glacial debris from the ice front or they immediately bury beneath gravels such portion of it as is deposited as moraines. The rivers from the glaciers of the western slopes of the New Zealand Alps carry nearly all the debris away; but those of the eastern side have aggraded their valleys so thickly that it may be inferred they have buried extensive morainic deposits.
This raises questions regarding the manner in which outwash is deposited in front of wasting and of retreating glaciers, and also regarding the general conditions governing accumulation and dissection of outwash plains. What is the possibility of the occurrence of such lowering of local base-levels as will permit of the permanent trenching and terracing of valley-train aggraded plains while a glacier still lingers in an upper valley and the supply of fluvioglacial debris is thus maintained? According to Flint's statement of the consequences of ice-front retreat such down-valley trenching is normal. †
Neglecting complications which might arise if conspicuous barriers of end moraine were deposited, it may be permissible to suggest some consequences of retreat on the one hand and of wastage during stagnation on the other, both taking place in the case of a valley glacier which, like the Tasman, carries an enormous debris load. Conditions of deposit may be considered (1) with the ice front stationary and the glacier active; (2) during a subsequent retreat; and (3) during an episode of stagnation and wastage. Strictly speaking, two variants of each of these cases ought to be taken into account: (a) where no moraines are deposited, but only fluvioglacial debris is produced; and (b) where both morainic and fluvioglacial debris are laid down.
[Footnote] * R. Speight—Ice Wasting and Glacier Retreat in New Zealand, Jour. Geomorph., Vol. 3, pp. 131–43, 1940.
[Footnote] † R. F. Flint—The Stagnation and Dissipation of the Last Ice Sheet. Geog. Rev., Vol. 19, Fig. 1, p. 257, 1929; Glacial Geology of Connecticut, Fig. 7, 1930.
In case 1 (a) all glacial debris is removed from the glacier by water streams, which aggrade down-valley and bank gravel against the ice front as fans are built along the face of a mountain. (In designing Fig. 1a, which illustrates this case, it has been assumed that no vertical glacial corrasion is in progress beneath the ice front.) If, later, the whole or a considerable portion of the glacier were to melt away simultaneously, or if it were to retreat rapidly, and if the amount of ablation moraine it deposited were not large, an inward (up-valley) facing scarp leading down to a “fosse” might be left bounding the valley train. *
In case 1 (b) subglacial deposit of ground moraine takes place pari passu with aggradational upbuilding of the valley train, and the glacier snout rides up on the ground moraine (Fig. 1b). That such overriding of moraine is a normal condition in the glacier snout has long been recognised, as may be gathered from the following quotation from W. M. Davis †:
[Footnote] * R. F. Flint—loc cit. (1929), p. 258; (1930), p. 58.
[Footnote] † Glacial Erosion in France, Switzerland, and Norway (1900); reprinted in Geographical Essays, p. 664.
“Toward the end of an ice stream it may well happen that the diminution of its volume and the consequent diminution of its capacity to do work will result in the aggradation of its bed by waste that cannot be carried farther forward. At the same time the outflowing river may be unable to wash away all the waste that is delivered to it, and so… the river may act as an aggrading agent and build up a broad, flat, alluvial fan. Some response to the change thus produced in the altitude of the end of the glacier may be expected far up the channel, whose bed would thus come to be aggraded with till.”
On another page Davis has written: “The deepening of the distal part of the channel accomplished in youth might be followed by a shallowing for a time during maturity, when the accumulation of morainal and washed materials in front of the glacier compelled its end to rise.” *
In case 2 the ice front retreats. Since the supply of glacial debris is kept up, aggradation continues; but rapid retreat may confine aggradation at first to a zone within the “fosse” that has been opened if the former conditions were those of case 1 (a); and during this episode the formerly-deposited valley train may be trenched, as is assumed by Flint. In the case of a glacier like the Tasman, however, there is so much debris supplied that the existence of a “fosse” and the duration of temporary trenching of the down-valley aggraded plain must be very short. For 25 miles down the Tasman valley aggradation is now in progress. Undoubtedly the whole valley train is growing up, and such aggradation will quickly fill a “fosse” and bury the ice-contact scarp and any erosional features temporarily developed in the valley train of stage 1 (a). The tendency to bury the whole ice front will be so great, indeed, that it seems advisable to dismiss case 1 (a) and its consequences as highly improbable.
In case 2 (b), as developed from 1 (b) by retreat as shown in Fig. 2, further deposit of ground moraine may be expected to take place during the retreat of the ice front to the extent of maintaining a graded (and still aggrading) outwash slope.
Next may be considered (3) the effects of rapid down-wastage by ablation during a time when the lower reach of the glacier is almost or quite stagnant. The recent discovery of thick stagnant ice under a supposed end-moraine ridge in front of the Conness Glacier † makes it appear probable that earlier moraine ridges also of that and other glaciers were deposited as ablation moraine. Widespread morainic deposits with nearly smooth to knob-and-kettle surfaces may be expected to result in a similar way from the melting of considerable lengths of stagnant debris-laden ice.
In case 3 (a), developed from 1 (a), a broad “fosse” may be opened, but this will be partly filled with ablation moraine. In case 3 (b), developed from 1 (b), ablation moraine will be deposited without the opening of a conspicuous “fosse” (Fig. 3). If, however,
[Footnote] * Loo cit., p. 668.
[Footnote] † F. E. Matthes—Committee on Glaciers (1939–40), Trans. 1940 Am. Geophys. Union, pp. 396–406, 1940.
the thick ground moraine deposited at stage 1 (b) under the glacier snout slopes up-valley, this may determine an up-valley slope on the newly-deposited ablation moraine also (A). If a glacier still remains in the head of the valley a new outwash apron will advance from it and blanket everything, but meanwhile some trenching of the ablation moraine and of the former outwash plain will occur.
If englacial debris has been very abundant, the stage characterised by an up-valley slope of the surface of ablation moraine and by some trenching of the valley train may be elided (B).
These arguments indicate that progressive retreat is not necessarily, and probably is not generally, accompanied by any temporary lowering of local base-levels such as will lead to dissection of parts of valley trains, but that episodes of down-wastage are perhaps rather more likely to lead to such occurrence. Eventual burial under new valley-train gravels may make all histories end alike, however, producing similar geomorphic results.
In North America there are terraces with pitted treads which are regarded as an indication that dissection of outwash has sometimes taken place before outlying buried ice masses have melted. Thwaites, however, although he makes the somewhat over-generalised remark that “decrease in load is a normal consequence of recession of the ice margin and was accompanied by a decrease of the stream grade,” feels compelled to cite a number of possible accidents and combinations of circumstances which might be called on to explain contemporaneous terracing even in front of a continental ice margin. * In the case of the Tasman and similar glaciers progressive burial of all other valley-floor features beneath a plain of outwash gravel is the prevailing condition as long as a glacier remains in the valley, permanent dissection of the valley trains rarely taking place until after the glacier has disappeared.
The hypothesis of “normal” retreat is capable of explaining the features of the Tasman Valley very satisfactorily. Should it be demonstrated, however, as seems possible, by other evidence that the shrinkage of the Tasman and adjacent glaciers which is now in progress is a case of stagnation and down-wasting, the absence of visible moraines and the undissected condition and continued growth of the valley train can still be satisfactorily explained. †
[Footnote] * F. T. Thwaites—The Origin and Significance of Pitted Outwash, Jour. Geol., Vol. 34, p. 312, 1926.
[Footnote] † The general question of “normal ice retreat or down-wasting?” has very recently been discussed at considerable length in “Current Notes on Geomorphology,” by D. Johnson (Jour. Geomorph., Vol. 4, pp. 85–94, 1941.)