The features are nivation ridges

BehreReference Behre ³ , BirdReference Bird ⁴ , and BretzReference Bretz and Boyd ⁵ have described how a transverse ridge may develop at the base of a talus slope: a snow bank lying against the talus acts as a chute for falling stones, which therefore accumulate at its lower margin. Although seasonal waxing and waning of the snow bank must cause some dispersal of the debris, it is likely that a ridge would eventually be constructed by this process. Both the ridges and their component blocks seen by Bird were quite small, whereas those described by Behre were massive.

Since most of Pangnirtung Pass and Pangnirtung Fiord is lined by talus slopes, it is possible that some of the features here described as moraines originated as nivation ridges. But such ridges would be more likely to develop below the break of slope between hillside and valley bottom than in higher, “perched” locations. Nor can Hypothesis No. 1 explain the “whalebacks” and “mesas” of sand and gravel, the closed depressions, the fresh outer faces, and other peculiarities of the Pangnirtung features.

The features are slumped scree

The constant association of old moraines with talus slopes cannot be too firmly stressed. Not only do both new and old scree fans terminate in the moraines’ inner depressions, but sometimes the moraine crests are actually continuous with, though at right angles to, prominent lines of scree. Since much of the talus is sub-angular or semi-rounded, there is no way of distinguishing it from the nondescript drift of the modern moraines. Moreover the walls of Pangnirtung Pass are generally so steep and smooth that the possibility arises that the “old moraines” may have been piled up by the mass-settling of scree.

SharpeReference Sharpe ¹³ states (p. 68) that “… backward rotation during slip is also very common in unconsolidated soil or coarser debris, as is well known to excavation contractors”. Such movements, known as “slumps”, usually occur slowly and intermittently along one or more slip-planes well below the surface. But Sharpe adds, in a comment unfavourable to the hypothesis, that slumps, including “talus-slumps”, involve only small displacements relative to the size of the moving block.

W. H. Ward’s paper on “The stability of natural slopes”Reference Ward ¹⁷ offers little support to the hypothesis. Assuming that instability is caused primarily by undercutting below and by deposition above, Ward argues that actual slope failure depends on the type and structure of the material involved, and on climatic conditions. The movements resulting from failure include the mass movement of cohesive material and the particle movement of fragmental. The failure of scree involves shearing or rolling of the surface fragments only and is classified by Ward as a “dry fragmental slide”. According to this authority, rotational slipping is essentially confined to cohesive materials, especially clays.

It seems that Hypothesis No. 2—often the most attractive in the field—must be rejected for want of evidence of the processes supposedly involved.

The features are impeded scree

Under this hypothesis the fact that two of the moraines have partial bedrock cores is extended by analogy to all such features. It is assumed that rock knobs obstruct the down-slope migration of scree, causing increasingly large piles to develop. But it is difficult to imagine how upstanding ridges of scree ten or twenty feet high can attain adequate stability, even on long rock benches; nor can most of the complex details of the old moraines be accounted for.

The features are creeping talus and rock glaciers derived from hanging valleys and cirques

In an oft-quoted paper, CappsReference Capps ⁶ described Alaskan “rock glaciers” as being composed of rather coarse and angular debris, and having clear ice in their interstices as far down as he could penetrate. The rock glaciers ended in transverse ridges with steep outer slopes—the former were well lichened and the latter fresh. Although the rock glaciers headed in “cirque-like valleys” and graded into the moraines of ordinary glaciers, Capps believed that slow movement of the debris itself was occurring “in a glacier-like way”. It was to this mass movement that he ascribed the fresh outer slopes.

The whole question was re-examined by KesseliReference Kesseli ⁷ . The features described by him were similar to the old moraines of Pangnirtung Pass, with the following exceptions: all his “rock streams” headed in cirques and similar features; they occurred especially with northern, north-eastern, and eastern orientations; none had bedded or unbedded sands and gravels lying unconformably on top of them; they were not stated to have vegetated upper surfaces and fresh outer slopes; and they had multiple ridges parallel to and at much the same elevation as the outermost ridge.

Because of these differences it is not possible to apply Kesseli’s explanation—that the rock streams resulted solely from past cirque-glacial transport—to all the old moraines of Pangnirtung Pass. Nevertheless there are some features, especially in Weasel Valley, that were undoubtedly the products of former advances of hanging and cirque glaciers, of which some have now vanished.

SharpeReference Sharpe ¹³ believes that rock glaciers are not necessarily of glacial origin, but claims that their parallel “wrinkles” are evidence of slow flowage or rock-glacier creep. As an example of this process he cites movements in a spoil pile near Grand Coulee Dam. But he adds (p. 98) that “creep is common in moraines perched on the sides of glacial troughs and parts of these deposits occasionally come down as landslides or debris-avalanches”. Rock-glacier creep grades into both talus-creep and true glacier flow.

A feature very similar to the old moraines of Pangnirtung Pass was photographed in East Greenland by BretzReference Bretz and Boyd ⁵ . In his caption on page 255 Bretz described the mass of drift as “flowing talus”, though he gave no evidence for this assertion.

Hypothesis No. 4 is a great advance over its predecessors, for old moraines of tributary glaciers, intermingled with scree and flowing downhill, must have accounted for many of the land forms enumerated at the beginning of the chapter. And yet there are long stretches of valley-side which lack cirques or hanging valleys of any significance—it is to explain them that the final hypothesis, strengthened as it is by the evidence of glacial markingsReference Thompson ¹⁵ , is stated and elaborated.

The features are of complex origin, but consist predominantly of moraines deposited by trunk glaciers flowing in Pangnirtung Pass

It is only by this hypothesis that all the morphological details may be explained. The general irregularity of the deposits is in accord with the chaotic state of the modern moraines, subject as they are to differential downwasting and collapseReference Thompson ^{14 ,} Reference Thompson ¹⁶ . In particular, the occurrence of tiered old moraines may be ascribed to successive stages in the downwasting of a trunk glacier. Some of the old drift, particularly that flanking the Great Moraine Gorge (Fig. 5), may be derived from recessional moraines, but most of it must be of lateral, superglacial and subglacial origin. No doubt rockfalls, steady talus-creep and water-washing over many years have increased the predominance of angular and sub-angular blocks on the moraine crests. The patches of fine sand, medium sand and gravel appear as remnants of marginal (kame) terraces, intramorainal terraces, kames, outwash and pond deposits. In some places streams from the hanging glaciers and from unglacierized cirques have likewise contributed sediments.

Rich has described, in a paper on the Catskill MountainsReference Rich ¹⁰ , old lateral moraines similar to those of Pangnirtung Pass. He makes two pertinent comments (p. 25–26) on “inner depressions”:

Although this principle is more relevant to the modern tributary glaciers than to a trunk glacier in Pangnirtung Pass, it is nevertheless a reminder that “inner depressions” are both characteristic and intermittent features of lateral moraines.

It now becomes clear that the better rounding of the boulders on the “green slopes” is due primarily to their origin as ground moraine or coarse outwash and secondarily to their maltreatment by Owl and Weasel Rivers during the dissection of the drift masses. The veneer of bedded sand and gravel upon the “green slopes” has been laid partly by rivulets flowing toward the main rivers from the old moraines and partly, in phases of aggradation, by the main rivers themselves. The “slopes” are now no longer graded to the active floodplains, because of recent downcutting that has left its mark in low, bedded river terraces, very different in appearance and in composition from the older, lichened ones incorporated in the “green slopes”.

The most striking aspect of the old moraines is the freshness of their outer faces, as compared with the rich vegetation of their crests and “inner depressions”. Since the fresh faces are so cornmon we must presumably search for a widely applicable cause—nor is one far to seek. For if the modern moraines have glacial ice as their predominant constituent (Fig. 6)Reference Thompson ¹⁶ , there is no reason to believe that past moraines were different. It is therefore suggested that the cause of the fresh fronts on the old moraines lies in the melting of buried ice. The closed depressions, disrupted drainage systems, and chaotic micro-topography of the moraines may also be explained in this way.

Now, MatthesReference Matthes ⁸ , OdellReference Odell ⁹ , and RichReference Rich ¹¹ have all claimed that ice may be preserved for many years under a blanket of detritus. W. H. WardReference Ward ¹⁸ has pointed out that ice-cores may remain indefinitely, provided the mean annual temperature of the adjacent air is below 0° C. and the annual range of temperature and cover are such that the temperatures of the boundaries of the buried ice masses do not reach the melting point. He considers that three or four feet of rock debris provide adequate insulation in most parts of Baffin Island.

Ward himselfFootnote * saw large masses of glacial ice in old moraines near the snout of Coronation Glacier (Fig. 1). The ice was exposed for a quarter of a mile down Coronation Fiord, beyond which the old moraines had fresh-debris fronts of the usual type. The vegetation cover on top of the moraines was progressively richer away from the ice front and was interrupted by many round ponds. The Coronation example was also instructive in that it illustrated the stages by which a lateral moraine became “old” in appearance. But Ward’s cardinal observation was that the faces of the old moraines were fresh because the buried ice was there least insulated and the debris least stable, because of undercutting and undermelting by waves. Comparable observations were made by the writer in the moraines beside Glacier 25, though here there was no undermining.

Since so many old moraines have fresh faces, without any undermining, it is necessary to assume not only that those moraines have ice cores but that the ice is melting because of increased external temperatures. The important possibility then emerges that the present climatic fluctuation around the North Atlantic OceanReference Ahlmann ¹ has affected Pangnirtung PassFootnote †.

The ice-core hypothesis may be carried further still. In order to explain the very low elevations, the irregular vertical and lateral distributions, the occurrence on top of alluvial fans and the massively lobate appearance of the old moraines, it is necessary to assume not only differential melting and collapse of the ice core but a general downhill migration and spreading of all the materials concernedFootnote ‡.

The processes involved in the downhill migration are physically quite simple. According to Ward a massive ice core might itself flow downhill at a velocity approximately proportional to the fourth power of the ice thickness. Meanwhile the shrouding detritus would tend to slip downward whenever the melting isotherm penetrated far enough to affect the ice. By these two processes—and even by the second one alone—the whole moraine would become flattened and extended downhill. There is evidently something in common between Ward’s ideas and those of Capps, Kesseli and Sharpe, already discussed.