Sir,
Reference Andrews,Andrews’ (1971) comments do not invalidate my generalization that there is a considerable “contrast between the englacial debris load of temperate and polar glaciers”. Although I admit that Baffin Island glaciers may transport less englacial debris than many glaciers in Spitsbergen (some in Spitsbergen are clearly similar to those in Baffin Island), I believe that their englacial debris content still contrasts markedly with that of temperate glaciers. Reference Goldthwait,Goldthwait’s (1951) description of a 32 km stretch of the margin of the Barnes Ice Cap bears this out. He described the ice up to 30.5 or 61 m above the glacier toe as “layered with myriads of dirt-filled thin fractures striking roughly parallel to the ice edge. . . the dirt filling. .. is from 1/100 inch to 1 inch [0.25 to 25.4 mm] thick. . . small pebbles and even large boulders are occasionally seen”. This description, together with his plates lB and 2A, and figures 2 and 3, and other descriptions such as those of Reference Ward,Ward (1952) and even Andrews’ comments, are sufficient to convince me of the difference between the mode of debris transport in Baffin Island and that of temperate glaciers that I have studied in the Alps, Norway and Iceland. These latter generally transport sub-glacially derived debris in a basal layer rarely more than 5-10 cm thick (exceptionally 1 m thick). Above this there is a virtual absence of subglacially derived debris at levels comparable to those in which such debris appears to be carried in, for instance, the Barnes Ice Cap (apart from relatively rare wedges of basal debris which have been thrust up along undoubted fault planes). Andrews’ assessment of 5% englacial debris content for ice-cored moraines in Baffin Island itself serves to accentuate this contrast. I would estimate this is at least one or two orders of magnitude greater than the subglacially derived debris content of temperate ice above the basal zone. The thickness of supraglacial till overlying dead ice in Baffin Island is estimated on average as 0.5 m by Andrews, and 1 m by Goldthwait and Ward for the south-east part of the Barnes Ice Cap. This till has presumably formed as a result of melting of immediately underlying ice and thus indicates that it contained a substantial englacial load.
Andrews’ observation that he has only seen sharp till/ice contacts in Baffin Island, and that there was no incorporation at these contacts, is irrelevant. The source of the englacial debris may be far from the margin, as might be expected from the basal freezing hypothesis. But as far as the Barnes Ice Cap is concerned, basal incorporation must be clearly responsible for its englacial debris load as there are no nunataks to provide a supraglacial source.
It is hoped shortly to present a comprehensive theory to account for the contrasts between the debris loads of different types of glaciers.
