On the identification, genesis and palaeo-environmental significance of pronival ramparts

Abstract

A pronival (protalus) rampart is a ridge, series of ridges or ramp of unconsolidated debris formed at the downslope margin of a perennial or semi-permanent snowbed overlooked by an exposed bedrock cliff. These landforms were traditionally regarded as simple and easily understood since the mechanisms of debris transport were intuitively considered to include supranival debris transport whereby clasts dislodged from the exposed cliffs above roll, bounce and slide over a snowbed under the influence of gravity. However, most studies focus on relict examples and few accounts document debris transport, or investigate rampart genesis, at actively-accumulating sites. This has led to circular reasoning and assumptions about rampart morphology, site characteristics, constituent material, genesis and palaeo-environmental significance. A review of existing literature reveals that rampart development was conventionally thought to extend downslope or outward below snowbeds of increasing thickness and extent but not all actively-accumulating ramparts fit this model. Given the over-reliance of research on relict pronival ramparts, this thesis focusses on actively–accumulating examples in order to improve our understanding of their genesis, clarify rampart identification and re-evaluate their palaeo-environmental significance. Rampart genesis is addressed by focussing on active sites on sub-Antarctic Marion Island and at Grunehogna, Western Dronning Maud Land, Antarctica as well as all other actively-accumulating ramparts documented across the globe. An alternative model for genesis in the form of retrogressive (upslope) development under fluctuating, and possibly declining, snowbed volumes is presented and relative-age dating techniques are recognised as being particularly useful in aiding with the assessment of rampart genesis. It is also found that ramparts which exhibit a distal slope at repose do not necessarily develop below snowbeds which are increasing in extent and thickness. The different modes of rampart genesis demonstrate that environmental conditions may change during their development and maintenance. The morphology and position of pronival ramparts on a slope are found to closely resemble glacial moraines, rock-slope failures and other discrete talus-derived landforms such as protalus rock glaciers, protalus lobes, avalanche deposits as well as morphologically similar geological structures. As such, their identification can be difficult. In the past, studies have used the characteristics of relict pronival ramparts to develop diagnostic criteria to distinguish ramparts from glacial moraines, rock-slope failures and other talus-derived landforms. These diagnostic criteria are assessed against information gathered from actively-accumulating pronival ramparts and a focus is placed on site characteristics, rampart morphology and sedimentology. Evidence presented in this thesis shows that several of the previously suggested ‘diagnostic criteria’ are invalid and a new set of criteria, with an emphasis on using a multiple-working hypothesis, are proposed to facilitate the identification of relict (as well as actively-accumulating) ramparts in the field. Classification of several landforms as pronival ramparts in southern Africa has been scrutinised in the past. The proposed diagnostic criteria are used to clarify their identification. Based on the criteria presented here, none of the landforms previously recognised as pronival ramparts in southern Africa should be regarded as pronival ramparts. The most morphologically compelling examples in southern Africa are the landforms at Mount Enterprise in the Eastern Cape Province though the site characteristics and the constituent material of the ridge do not suggest a rampart origin. Alternative origins which should be investigated for these landforms range from scree deposits and rock-slope failures to stone-banked lobes. As is the case in southern Africa, relict pronival ramparts are typically used to infer palaeo-environmental conditions. The absence of pronival ramparts at ideal topographic sites in southern Africa questions the persistence of late-lying snow along and on the Lesotho-Drakensberg escarpment during the Late Quaternary. This observation is in contrast with the notion of niche glaciation in preferential locations above 3000m a.s.l. and demonstrates that, although pronival ramparts can typically only be used to infer more snowy conditions, their presence or absence can, in certain contexts, be useful in palaeo-environmental reconstructions.