The Forms of Glacial Erosion

Abstract

Glacicl erosion is universally attributed to two processes — abrasion and plucking — and though satisfying explanations in physical terms have yet to be offered of the mechanics of either, their reality and their general characteristics are not in doubt. Abrasion is self-evidently witnessed by polished and striated surfaces, by fluting and streamlining and by such distinctively original forms of ice-moulding as roches moutonnées. Plucking is evidenced in a variety of ways. There is G. K. Gilbert’s (1910) argument that in southern Alaska where there is clearly evidence ‘of an enormous amount of glacial degradation, it was a matter of surprise to find the reduction of the surface to smooth sweeping curves was a somewhat rare phenomenon. By far the greater number of exposed glaciated areas, even where degradation has been profound, abound in low embossments and in more or less angular groins or re-entrant spaces showing little trace of abrasive action.’ This is equally true elsewhere. There is J. P. Dana’s argument (cited in Gilbert, 1910, p. 207) that the blocks and boulders in the moraines of the Laurentide ice sheet, which received no falling stones from mountain peaks, must have been plucked bodily from the underlying bed. In Britain this argument is best exemplified by the blocks of Silurian grit that have been torn from the floor of Crummack Dale and carried uphill to rest on the limestone scar that overlooks the Craven fault. But plucking has its most conspicuous results in the cliffs of conies, though as late as 1896 E. Richter thought that because such cliffs show no signs of abrasion they were not glacially sculptured but fashioned by frost weathering above the surface of the ice.