RAINFALL AND TEMPERATURE ATTRIBUTES ON THE LESOTHO–DRAKENSBERG ESCARPMENT EDGE, SOUTHERN AFRICA

Located near the southeastern limit of Africa, the Lesotho‐Drakensberg and associated escarpment is the highest range of African mountains south of the massifs in Tanzania. At the escarpment summit and on the adjacent high peaks, the climate is generally interpreted as marginal periglacial yet few data, specifically rainfall and temperature, exist on record at these altitudes. Climatic data from two temporary field stations on the escarpment edge, one of which is the highest rainfall station yet on record in southern Africa, provide contemporary surface‐climate conditions. Mean annual rainfall recorded between 2001 and 2005 averages 767.8 mm at Sani Pass summit (three complete years), and 753.2 mm on Sentinel Peak (two complete years); these values are less than those recorded for the same period in the mountain foothills. Even though rainfall is slightly below long‐term rainfall averages for the area due to a marginally dry spell, the data show that earlier estimates of between 1000 mm and 2000 mm rainfall per annum on the escarpment are too high. A measured mean air temperature of 5.8°C at Sani Pass, however, falls within the range estimated for the escarpment summit. Frost cycles in air and at the soil surface are frequent in winter, but absent in soil for summer, and no long‐duration surface‐soil freeze was measured. Temperatures thus confirm the marginal periglacial nature as postulated for previously, but precipitation data indicate a dryer environment than anticipated. Palaeoenvironmental scenarios, notably arguments for former glaciation based on extrapolations from somewhat exaggerated contemporary precipitation values, thus require re‐consideration.     

Annually re-forming miniature sorted patterned ground in the high Drakensberg,southern Africa

An analysis of miniature sorted circles and polygons provides the first detailed assessment of sorted patterned ground from a southern African alpine region. Pattern dimensions and particle sorting were determined from two sites in the high Drakensberg. Although the sorted patterns in the high Drakensberg are somewhat polygenetic in developmental origin, they are primarily frost-induced. Miniature sorted patterned ground below 3200 m a.s.l. on the Drakensberg plateau develops annually during the winter months and disintegrates towards summer. The development of miniature sorted patterns within five to six weeks demonstrates the effect of regular freeze–thaw cycles at higher altitudes in the Drakensberg. © 1997 by John Wiley & Sons, Ltd.     

Surface‐climate attributes at Injisuthi Outpost,Drakensberg, and possible ramifications for weathering

Aerial and sub‐aerial climatic data were collected from a station at 1920 m a.s.l. in the Injisuthi region of the South African Drakensberg. Sensors monitored air temperature, soil surface and rock surface temperature, for two rock types, over the summer and winter of 2001/2002. Rainfall was measured from the summer of 2001 to January 2004. These are the first rock and soil surface‐climate data to be collected for an exposed site at this altitude in the area. Rainfall over the two calendar years 2002 and 2003 was found to be below estimates for the region, but patterns imply numerous rock wetting and drying cycles in summer. At the site, air, rock and soil temperatures differ considerably on a diurnal basis with respect to both absolute temperature and daily ranges. Mean rock daily ranges, as conducive to possible thermal fatigue, are found to be similar in the summer and winter periods. Of the two rock types monitored, the darker coloured basalt attained higher maximum and marginally lower minimum temperatures than the sandstone. Soil frost did not occur at 2·5 cm depth, but rock did reach below ?6 °C in winter. Both rock types maintain relatively high rock temperatures in winter (exceeding 25 °C), thus chemical weathering is probably only moisture restricted during this dry period. Findings highlight the importance of directly monitoring rock temperature when attempting to discern the rock weathering environment. Copyright © 2006 John Wiley & Sons, Ltd.     

ROCK-SURFACE TEMPERATURES OF BASALT IN THE DRAKENSBERG ALPINE ENVIRONMENT, LESOTHO

Rock temperature data are presented for a variety of topographic localities at a high Drakensberg site. The objective is to investigate the spatiotemporal variations of surface rock temperatures in high Drakensberg basalt. The temperature results are then used to discuss possible implications for thermal stress fatigue and frost‐induced weathering. Tinytalk? data loggers and probes were used for rock‐surface temperature recording. Long‐term measurements were recorded over 12 months from May 2002 to April 2003, at a 1‐hour logging interval and rock depth of 1 cm for a highaltitude (3300 m a.s.l.) interfluve and fracture site. Whilst the north‐facing rock surface experiences negligible hours below ?3°C, the south‐facing rock surface and interfluve sites are subjected to considerable periods below ?3°C, which falls within the ‘frost cracking window’. It is concluded that the substantial contrasts of recorded rock thermal parameters over small spatial scales between various topographic settings, highlight that site‐specific measurements across the broader scale are required for an adequate evaluation of regional weathering and its associated landform development.     

Rock weathering on the eastern mountains of southern Africa: Review and insights from case studies

The mountains in the eastern region of southern Africa are of significant regional importance, providing for a diverse range of land use including conservation, tourism and subsistence agriculture. The higher regions are comprised of flood basalts and are immediately underlain by predominantly aeolian-origin sandstones. Our understanding of the weathering of these basalts and sandstones is reviewed here, with particular focus on the insights gained from the Lesotho Highlands Water Project and an ongoing study into the deterioration of rock art. While the chemical weathering attributes of the basalts have been substantially investigated, it is evident that the environmental surface conditions of rock moisture and temperature, as affecting weathering processes, remain largely unknown. Within the sandstones, studies pertaining to rock art deterioration present insights into the potential surface weathering processes and highlight the need for detailed field monitoring. Outside of these site-specific studies, however, little is understood of how weathering impacts on landscape development; notably absent, are detail on weathering rates, and potential effects of biological weathering. Some palaeoenvironmental inferences have also been made from weathering products, both within the basalts and the sandstones, but aspects of these remain controversial and further detailed research can still be undertaken.     

Recognition and palaeoclimatic implications of late Quaternary niche glaciation in eastern Lesotho

Geomorphic evidence of former glaciation in the high Drakensberg of southern Africa has proven controversial, with conflicting glacial and non‐glacial interpretations suggested for many landforms. This paper presents new geomorphological, sedimentological and micromorphological data, and glacier mass‐balance modelling for a site in the Leqooa Valley, eastern Lesotho, preserving what are considered to be moraines of a former niche glacier that existed during the Last Glacial Maximum (LGM). The geomorphology and macro‐sedimentology of the deposits display characteristics of both active and passive transport by glacial processes. However, micromorphological analyses indicate a more complex history of glacial deposition and subsequent reworking by mass movement processes. The application of a glacier reconstruction technique to determine whether this site could have supported a glacier indicates a reconstructed glacier equilibrium line altitude (ELA) of 3136 m a.s.l. and palaeoglacier mass balance characteristics comparable with modern analogues, reflecting viable, if marginal glaciation. Radiocarbon dates obtained from organic sediment within the moraines indicate that these are of LGM age. The reconstructed palaeoclimatic conditions during the LGM suggest that snow accumulation in the Drakensberg was significantly higher than considered by other studies, and has substantial relevance for tuning regional climate models for southern Africa during the last glacial cycle. Copyright © 2009 John Wiley & Sons, Ltd.     

Block and Debris Deposits in the High Drakensberg, Lesotho, Southern Africa: Implications for High Altitude Slope Processes

Past evaluation of high altitude slope development in Lesotho, southern Africa, is largely based on hypothetical or macro-scale geomorphic approaches. Consequently, the information pertaining to high altitude southern African Quaternary slope environments has remained rather rudimentary. The present study describes the morphology and discusses the likely palaeogeomorphic processes of blockstreams and debris deposits on the Popple Peak and Njesuthi-Mafadi south-facing-slopes in the Drakensberg. The geomorphic evidence provides much needed information to help improve the understanding of south-facing slope processes during past colder periods. A model for high altitude Drakensberg south facing slopes is presented and used to challenge and expand on recent models and ideas on southern African valley asymmetry. It is found that solifluction and debris flows/avalanches were operative on south-facing slopes during past cold periods and thereby contributed to past slope development at some high altitude sites in Lesotho. However, the geomorphological observations do not support the valley asymmetry hypothesis and it is suggested that greater caution be exercised in valley-form interpretations, particularly where geomorphological ground-truthing has been absent.     

Moraine Formation at an Advancing Temperate Glacier: Brigsdalsbreen, Western Norway

Owing to increased winter balances especially since AD 1988/89, almost all valley outlet glaciers of Jostedalsbreen in western Norway are experiencing the largest advance since that of the early 18th century, the regional "Little Ice Age" maximum. Brigsdalsbreen advanced 441 m between 1987 and 1997. By the end of this period, the glacier had reached the outlet of the proglacial lake Brigsdalsvatnet, ploughing into unfrozen, fine-grained, water-soaked glaciolimnic sediments from the lake bottom and forming frontal moraines. These moraines are characterised by a lack of internal structures and preferred fabric. Owing to the strong advance, the moraine morphology is constantly changing, leaving only temporary moraine ridges.
Observations made along the glacier front suggest that the formation of these moraines can best be described as "bulldozed moraines", since the term push moraine, commonly associated with advancing glaciers, should be restricted to permafront environments. Different processes involved in moraine formation at frontal and lateral glacier margins result from variations in proglacial sediment properties, microrelief and glacier dynamics. Among these processes, large boulders left in the proglacial areas are pushed forward, forming pressure ridges on the distal side. Some of the largest boulders ( c . 80–120 m³) are overturned or rotated by the glacier.     

Turf exfoliation in the high Drakensberg, Southern Africa

Limited research attention has focussed on turf exfoliation as a denudation process in mountain environments. This paper examines some characteristics of turf exfoliation forms identified within particular valley zones in the Drakensberg alpine belt. Morphological and sedimentological data are presented for turf exfoliated sites investigated in the Mashai Valley of eastern Lesotho. It is found that a variety of processes, including needle ice action, biological activity, fluvial processes and deflation, operating synergistically, are responsible for contemporary turf exfoliation in the high Drakensberg. It is apparent that the strong seasonality from mild, wet summers to cold, dry winters has helped induce the annual cycle of dominating processes.