Alpine turf exfoliation pans in Lesotho, southern Africa: Climate–process–morphological linkages

Abstract

Although alpine pans have been reported from a variety of mountain regions, these have received limited research attention and are thus amongst the least known of ‘closed basins’. This study investigates macro- and fine-scale morphological attributes and the process dynamics of alpine turf exfoliation pans in the high Drakensberg of Lesotho, southern Africa. Climate data (temperature, precipitation and wind) are used to better ascertain climate–surface process linkages and how these may be associated with the observed morphological phenomena. Thirty pans were assessed for both macro- and fine-scale aspect-controlled morphological attributes. During 2005, Tinytag™ temperature loggers recorded ground temperatures on various pan riser aspects to establish the potential for cryogenic activity. Similarly, wind speed and direction data from 2001 were used to identify the potential role of wind as an erosion and transportation mechanism. The number and total weight of detached turf clumps accumulated at the base of various pan riser aspects were determined in July 1999, September 2001 and September 2004. It is suggested that the pans originated through initial turf disruption by animal trampling and turf burning, followed by the cryogenic up-heave of sediments. The pans typically elongate towards the southeast as they enlarge, demonstrate most active erosion on eastern and southeastern riser aspects, and host seasonal micro-echo and micro-climbing dunes along such riser aspects. It is concluded that an annual cyclic (seasonal) pattern of contemporary climate-driven geomorphic processes, dominated by strong northwesterly winds, control the fine-scale morphological evolution of alpine turf exfoliation pans in the Lesotho highlands.