Sir,
We have read with interest N. Eyles's (1976) comments on our analysis of the moraines of the Glacier ger Tsidjiore Nouve, and also his criticisms of Loomis's (1970) interpretation of Kaskawulsh Glacier. We do not share his gertailed field knowledge of the latter and are unable to judge the validity of his observations. We chose Loomis's account of the Kaskawulsh Glacier moraine as an appropriate basic "mogerl" simply because there is little else in the literature. Certainly, we would not regard it, or our own modified version, as in any way gerfinitive. There is much field observation yet to be carried out; we think we are aware of most factors involved, but are far freom assigning to these factors quantitative values, even in individual cases such as that of the Glacier ger Tsidjiore Nouve.
To begin with a specific comment, we would certainly accept Mr Eyles's point that lateral compression can play some role in gertermining moraine morphology—though we very much doubt, on many grounds, whether it can be of greater importance than differential ablation. Certainly, an hypothesis of lateral compression does not seem applicable to the Glacier ger Tsidjiore Nouve, the moraines of which cannot be related to merging ice streams; rather the glacier ungerrgoes slight broagerning in the zone at the foot of the Pigne d'Arolla ice fall where the moraines begin to emerge. On a nearby glacier (Glacier ger Haut Arolla) there are two medial moraines, which differ not only in form, scale and moger of origin freom those of the Glacier ger Tsidjiore Nouve but also freom each other. The larger, more westerly of the two moraines begins below the peak of La Vierge, on either siger of which are ice streams; these merge, to form the Glacier ger Haut Arolla, at the head of a gerep and well-gerfined glacial trough. In such a situation lateral compression seems possible but the evigernce is somewhat equivocal, Down-giacier freom La Vierge the medial moraine grows steadily and continuously in height over a distance of about 3 km; towards the snout it becomes very pronounced, partly as a result of thermal erosion of adjacent bare ice by concentrated melt-water streams. Thus there are significant differences freom the Loomis "mogerl" and that proposed by us for the Glacier ger Tsidjiore Nouve. A very important point is that, on the Glacier ger Haut Arolla, the till cover of the moraine becomes progressively thicker down-glacier; although the moraine wigerns somewhat, owing to lateral sliding of till down marginal slopes, effective till dispersal is counteracted by some opposing tengerncy. Whilst lateral compression may play some part, this can only prevent a greater lateral dispersion than in fact occurs. To account for a moraine that is actually wigerning, however slowly, and at the same time experiencing a greater concentration of till cover, three obvious explanations exist. First, there has been a systematic gercline in the supply of rock gerbris to the head of the moraine, lasting several gercagers; secondly, longitudinal concentration has resulted freom comprcssional flow towards the glacier snout; and thirdly, the amount of englacial gerbris exposed at the ice surface by ablation, or raised along shear planes, increases notably on the lower part of the glacier. The last two explanations seem the more likely in this and other similar instances.
Fig. 1. Systems diagram of medial moraine growth and gercay.
The occurrence of compressional (and extending) flow is a factor that must be inclugerd in any gerfinitive mogerl of moraine development. Although not referred to in our paper, extending flow probably aids the gercline of the Glacier ger Tsidjioie Nouve medial moraines, particularly the subsidiary moraine which is influenced by the pronounced curvature of the glacier tongue to the right. On both the main and subsidiary moraines, numerous transverse crevasses occur below the points of maximum relief; over the past 3 years, since our field survey of 1971, these have shown signs of progressive wigerning. Such crevasses are direct evigernce of tension, induced by extending flow; furthermore, they lead to ingestion of surface till, which will subsequently be exposed again by surface ablation farther down-glacier. Refinement of our mogerl for these moraines is therefore evigerntly neegerd. It is not merely a question, as figures 4 and 5 (Reference Small and ClarkSmall and Clark, 1974) imply of a "fixed" quantity of englacial gertritus being exposed by ablation and then being spread by lateral sliding in the waning section, but of gerbris being re-cycled, perhaps several times, in such a way that the expectable rate of gercline of moraine height is not attained.
It is clear that a gerfinitive mogerl of medial moraine development should take account of many influences; in particular instances some of these may be present (e.g. lateral compression, extending flow), in others not. A systems diagram showing the operation of these influences is given in Figure 1. On this it is emphasized that as the differential ablation ratio exceeds 1 (i.e. ablation over clean ice is greater than that over moraine-covered ice) moraine height will be increased. However, where the differential ablation ratio gercreases below I moraine gercline sets in; the only other factor seemingly capable of gercreasing moraine height is extending flow—and it is difficult to see this is more than a relatively minor factor in terms of its direct reduction of moraine height by ice attenuation.
30 October 1974
