Winter-talus ridges, nivation ridges, and pro-talus ramparts

Sir,

In a previous letter to the Journal of Glaciology, D.R. Butler (1986) drew attention to the apparent primacy of R.A. Daly (1912) in describing “winter-talus ridges”, features that are now usually referred to as pro-talus ramparts. Butler also suggested a return to Daly’s original terminology. His letter raises a number of interesting issues.

First, R.A. Daly was apparently not the first person to provide a written description of pro-talus ramparts, though he may well have inferred their mode of formation independently. It is often difficult to establish primacy in scientific explanation, but it is clear from a number of accounts that certain geologists and geographers working in the British Isles were aware that ridges could accumulate at the foot of perennial or even possibly late-lying snow beds several years before Daly’s description was published. For example, in an account of the various types of moraine found in the English Lake District, Ward (1873, p. 426) described a separate category consisting of

“mounds of scree material formed at the base of a slope, by the sliding of fragments over an incline of snow lying at the base of crags”,

and noted that he was indebted for this suggestion to

“Mr Drew, late of Cashmere … he having seen mounds of this kind at the foot of snow slopes among the Himalayas”.

A quarter of a century later, Marr and Adie (1898, p. 56) described a possible example of what would now be termed a pro-talus rampart in Snowdonia, North Wales. On the concentric ridges of the moraine damming Llyn du’r Arddu, a small tarn north-west of the main Snowdon Ridge, they noted that on most of the moraine ridges

“rest sub-angular perched blocks, whilst the innermost crescent of the drift dam consists of angular blocks, as though some at least of this material was rather of the nature of snow-slope detritus than true moraine”.

Even more explicit is a remarkable account of recent protalus rampart development on the north face of Ben Nevis, the highest mountain in Scotland, where Gatty (1906, p. 491) observed in Coire na Ciste

“a little tarn … enclosed by a semi-circular embankment rising some 6 or 8 feet above the water level. This dam is built up stones of all sizes, from pebbles to blocks 4 feet across. Some of these had plainly fallen last spring.… The way in which the dam is forming is this: in the spring a tongue of snow projects over the site of the tarn, and terminates against the dam; blocks wedged off the cliffs by the winter’s frost roll or slide down and come to rest at the foot of the snow-shoot, and so build up the dam”.

Gatty’s account also included two photographs of this feature, and he went on to note that

“Several instances of old semicircular embankments of identical formation are to be found amongst the hills of the English Lake District; these almost certainly all date back to glacial times”.

He also mentioned another recent example in the Alps (p. 492). The above examples reveal that Daly’s (1912) account of pro-talus ramparts was by no means the first to appear, though it may well represent the first documentation of North American examples.

Secondly, it seems debatable as to whether Daly’s term “winter-talus ridge” is more appropriate than “pro-talus rampart”. Not only has the latter term become firmly entrenched in the literature on the topic, but also it seems more accurate, on three counts. First, recent work on actively accumulating ramparts in northern Norway (Ballantyne, in press) has shown that rampart accumulation takes place in summer rather than winter, as these ramparts are entirely covered by snow in winter so that debris from the cliffs up-slope overshoots the rampart crests. In such circumstances, the term “winter-talus ridge” seems inappropriate. Secondly, “rampart” is in some circumstances at least more accurate than “ridge”, as some examples do not possess a proximal slope (e.g. Ballantyne and Kirkbride, 1986, p. 662). Finally, all examples known to the author or documented in the literature lie at or near the foot of a talus slope, which suggests that “pro-talus” is an apt descriptor; that illustrated by Daly (1912, pl. 57) is a fine example of this.

Finally, recent studes of actively accumulating pro-talus ramparts suggest that traditional definitions of such features require revision. Butler (1986) adopted the genetic definition:

“A ‘pro-talus rampart’ is a ridge or ramp of debris that forms where clasts fall from a cliff face, slide, or roll across the surface of a perennial snow bank of somewhat standard dimensions, and accumulate at its base”.

Similar definitions have been proposed by several authors in the past, including the present writer (Ballantyne, 1986; Ballantyne and Kirkbride, 1986). As noted by Johnson (1983), however, the validity of this mode of accumulation tends to have been assumed rather than demonstrated, as most accounts of protalus ramparts concern relict (usually late Pleistocene) examples. Recent work on actively accumulating ramparts in Japan and Norway indicates that other processes such as debris flow (Ono and Watanabe, 1986), reworking of till deposits from up-slope (Harris, 1986), and slush avalanches (Ballantyne, in press) also contribute substantially to the accumulation of at least some ramparts, and account for the presence of abundant fines within these ramparts. A more general definition seems to be required, such as:

“a ridge or ramp of predominantly coarse detritus, usually located at or near the foot of a talus slope, that formed through the accumulation of debris along the down-slope margins of a snow bed”.

Whether perennial snow or firn is necessarily involved seems uncertain, though Gatty’s (1906) account seems to suggest that some examples may have formed in association with seasonal snow beds. Further research is necessary to clarify this point.

I am grateful to D. Unwin of the University of Leicester for drawing my attention to the early accounts of pro-talus ramparts by Ward (1873) and Marr and Adie (1898).