Formation of a pronival rampart on sub-Antarctic Marion Island

Abstract

The formation of a pronival (protalus) rampart on sub-Antarctic Marion Island is investigated. Morphological attributes show debris at the angle of repose on the rampart's proximal slope and at a lower angle on the distal slope. Relative-age dating, based on the percentage moss cover and weathering rind thickness of the clastic component, indicates accumulation mainly on the proximal slope and rampart crest, implying upslope (retrogressive) accumulation. This contrasts with a previously published model for pronival ramparts, which proposes rampart growth by addition of material to the distal slope. Development of the Marion Island rampart is suggested to result from the control exerted by a relatively low-angled surface and a shrinking snowbed. A small debris step formed on the proximal slope appears to be a response to decreased snowfalls due to changing climate over the last c. 50 years. Growth rate of the rampart is considered to be variable during the Holocene in response to changes in climate and debris supply.

Some observations on the formation of an active pronival rampart at Grunehogna Peaks, Western Dronning Maud Land, Antarctica

“…The "diagnostic criteria" suggested by Shakesby (1997) and tabulated by Hedding et al (2007) were used to identify the talus landform under investigation as a pronival rampart as well as identify its origin (Table 1). A (transverse) profile along the rampart firn field boundary and four longitudinal transects were surveyed to determine the width (down-slope) (w), length (across-slope) (l) and general surface morphology of the rampart and firn field (Fig.…”

“…These include the downslope (outward) model of Ballantyne & Kirkbride (1986) and the retrogressive (upslope) model of Hedding et al (2007). The downslope model of rampart development of Ballantyne & Kirkbride (1986) proposes downslope rampart extension at the foot of thickening snowbeds.…”

“…A suggested morphological characteristic of ramparts which extend downslope is that the distal slope is formed at repose (34-38°) by the accumulation of cohesionless cascading debris. However, not all ramparts exhibit a distal slope which is at repose and led Hedding et al (2007) to propose a retrogressive (upslope) model of rampart development which envisages retrogressive development under fluctuating, and possibly declining, snowbed volumes. Thus, depending on which of the existing models of rampart development fit a feature, very different palaeo-environmental conditions can be inferred.…”

The distribution and paleoclimate implications of periglacial landforms in eastern Australia

“…Pronival ramparts, also described as protalus ramparts (Hedding et al 2007, Margold et al 2011) are ridges located on the outer edge of nivation hollows. They are generally composed of angular clast supported boulder deposit lacking fine matrix material, originated from a local source upslope (Derbyshire and Peterson 1977, Ballantyne and Kirkbride 1986, Matthews et al 2011 and commonly display steep distal slopes (Matthews et al 2011).…”

“…Ballantyne 1987, Shakesby 1997) and the feature recognised here is neither particularly large nor from surface exposure, distinctive. The primary formation process for ramparts is the sliding of individual clasts across a steep and icy surface (Hedding et al 2007). However, many modern active pronival ramparts have slopes at considerably less than the angle of repose (e.g.…”