Sir,
We welcome Hambrey’s comments on our paper, and are interested to find that he was independently coming to many of the same conclusions which we reached. Indeed, some of his earlier observations (Hambrey, 1975, 1976[a], [b] ) helped convince us that our own approach to the origin of foliation was reasonable.
The deformational history of an element of glacier ice is normally very complex, particularly when changes in regime are considered (see, for example, Hudleston (1976) ). Thus we are not disturbed by Hambrey’s list of specific examples of features which do not appear to fit our thesis. In the second paragraph for example, has Hambrey considered the possibility that the coarse debris in marginal areas of glaciers (we presume he is referring to valley glaciers) accumulated by falling from the valley sides in the accumulation a rea, and thus was originally conformable with sedimentary stratification there? Alternatively it could have been eroded from the subglacial parts of the valley sides, presumably by freezing of water in areas where P-T conditions were appropriate. Also, in the second paragraph, has he considered the high probability that the roughly-vertical foliation observed near gently-inclined valley sides took on its vertical orientation some place further up-glacier and at some depth below the surface? In the third paragraph the change in bubble concentration with increasing age can be attributed to displacement of air by percolating melt water, a process which does not imply bubble migration and which is common on the Barnes Ice Cap. Crevasses provide a particularly good access path for such water.
At the end of his fifth paragraph, Hambrey cites several examples of possible shear displacements. A problem with any discussion of su ch displacements in ice is the lack of agreement on what constitutes a discrete “shear plane”. Do the structures to which Hambrey refers involve displacement discontinuities, as in true faults, or do they involve shear zones, a few centimeters in thickness, with continuity of displacement across them? We have observed examples of the latter (Hudleston, 1977) and proposed in our original paper (Hooke and Hudleston, 1978, p. 292) that such structures could be responsible for some cross-cutting foliations. With regard to the former, apparent displacements across discrete planes can result from opening and subsequent closing of tensional fractures (crevasses). Could the structures Hambrey describes be of either of these types? Our main concern is that the term “shear” has been used loosely in the past, and in ways suggesting fault-type displacements. We question whether such displacements occur under appreciable thicknesses of ice, as has been implied in discussions of debris entrainment.
In discussing entrainment of debris by a shear process, Hambrey refers to his paper with Muller (Hambrey and Müller, 1978) in which they discuss movement of dirty ice over clean ice as a result of the fact that ice higher in a glacier moves faster than that below, particularly where the latter is frozen to the bed. We have no problem with this type of process; it is, in fact, central to Hooke’s (1973) model for develop men t of ice-cored moraines. It is not, however, the process usually called to mind by the phrase, “The debris was sheared into the ice”, which is so often heard, and we think it a disservice to equate the two.
We apologize for omitting Hambrey (1976[a] ) from our reference list.
Department of Geology and Geophysics,
University of Minnesota,
Minneapolis, Minnesota 55455, U.S.A.
10 April 1979
R. L. Hooke
P. J. Hudleston
