The glacial buzzsaw and its limitations: mountain glaciation in British Columbia and in Britain

Erosion by warm-based glaciers is highly effective and has produced distinctive mountain landscapes. Downward erosion dominates, but headward erosion by recession of cirque headwalls is sufficient to displace drainage divides and lower summits on cirque crests. This ‘glacial buzzsaw’, however, is not universal: it does not apply in the many areas where low-gradient surfaces survive above cirques. It applies patchily in British Columbia, Britain and Romania. Keywords—glacial buzzsaw; mountain glaciation; Bristish Columbia; Britain There is now general agreement that glacial erosion can be faster than fluvial erosion, so long as ice is ‘warm’ and wet-based. In fact, glacial erosion rates increase with the amount of basal water [1] and with the rate of basal sliding: hence in Patagonia there is a maximum at 44° S, where precipitation was greatest during the Last Glacial Maximum [2]. In a simple glacier, ice discharge and rates of basal sliding are greatest at the Equilibrium Line (E.L.). But in a large valley glacier system, tributaries converge to give greater discharges further down the trunk glacier: together with the increasing amount of basal water, this gives greater erosion downstream, where major troughs and rock basins are eroded. Considerable headward erosion is observed also at glacier sources, often on one side of a mountain range. This requires not just glacial abrasion, but also glacial quarrying and/or headwall collapse by rock mass failure. The balance between headward and downward glacial erosion seems to have varied at different stages of Quaternary glacial history [3]. The buzzsaw hypothesis asserts that rapid glacial erosion around the E.L. Altitude (ELA) tends to truncate mountain ranges and control the altitudes that summits can reach [4]. This implies that summits are on the edge of cirques and in course of being lowered as cirques develop. This is most clearly the case where sharp ridges delimit contiguous, intersecting cirques, as for example in the Washington Cascades and the Southern Alps of New Zealand. It can apply also where contiguous cirques on one side of a ridge are extending at the expense of the opposite slope. It cannot apply where gentle summit slopes survive above the cirques, as in the Godeanu and Parîng Mountains [5]. As such gentle summits rise little above cirque headwalls, the general relation between summit altitudes and cirque floor altitudes [6] applies here as well as where ridges are sharp: thus it does not provide support for the buzzsaw hypothesis [7]. Support may be provided, however, by geomorphological mapping and detailed hypsometric analysis [8]. The southern Coast Mountains of British Columbia are deeply dissected by glacial erosion, both by ice sheets and local glaciers. Ridges are sharp, and mainly with intersecting cirques, as in the Bendor Range and farther west. They have clearly been lowered by headward cirque erosion, yet the evidence for a glacial buzzsaw is weaker than to the south in the Washington Cascades. Cirque floors follow the trend of ELA, which rises by 1000 m from the coast to the dry interior, but summit maxima do not. Thus as you go west or southwestward into the mountains, cirque glaciers are replaced by valley glaciers and eventually icefields as mountains rise higher above ELA (Fig.1). This implies that uplift has been rapid enough to take these mountains through the zone of most effective erosion. That is more evidently the case with the largest icefields of middle latitudes: Patagonia, the Karakoram and the St. Elias of Alaska, which are areas of very rapid glacial erosion. It is likely that rapid Quaternary uplift in these three areas carried mountains quickly through the zone of rapid glacial erosion, into that of cold ice frozen to its bed. This produced ‘Teflon peaks’, rapidly uplifted into the zone of cold, non-erosive ice, while ice streams eroded deeply in the valleys between. DOI 10.15551/prgs.2017.47