Article

Earth hummocks (thúfur): New insights to their thermal characteristics and development in eastern Lesotho, southern Africa

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Abstract

The aspect-controlled variations in soil freezing within earth hummocks of eastern Lesotho (southern Africa) are analysed. Ground thermal data were measured for an earth hummock from late autumn to early spring in 1995 and 1996, using Tinytalk™ data loggers. During 1995, ground temperatures were recorded at 15 and 20 cm depth on the hummock north, east, south and west aspects, whilst in 1996 temperatures were recorded at 1 cm, 5 cm and 10 cm on the north and south aspects. The data from 1995 indicate that soil freezing commences on the hummock southern aspects and gradually progresses towards the western and northern aspects, whilst the eastern aspect remained unfrozen throughout winter. The data from 1996 indicate that a thick snow cover almost nullifies the temperature differences between the hummock northern and southern aspects. However, given the relative absence of snow during contemporary winters, freeze intensity and duration is longest on the hummock southern and western aspects, which helps explain earth hummock deformation (elongation and coalescence) in a southwesterly direction on slope gradients ≤3°. Copyright © 2005 John Wiley & Sons, Ltd.

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... Since the duration of snow cover rises with elevation (lower air temperature and higher precipitation), on the elevations of about 1,400-1,500 m the period with snow is expanded and the number of days with snow is estimated to 90-100 (Milovanović, 2010). These circumstances are followed by the decrease of soil temperature, as well as by the reduction of freeze penetration depending on aspects (Grab, 2005b). All climatological data are expected to be more extreme, as Ponor is the depression with temperature inversion. ...
... The lowest inclination is present in the DA area (average 3.5°), while the highest (average 6°) is found in DC. The inclinations are in accordance with the previous studies (Grab, 2005b;Marcu, 2011;Milošević et al., 2015). Regarding the vertical dissection, thufur are developed on the areas dissected up to 10 m/2,500 m 2 (Figure 3). ...
... Regarding the morphology, it is significant to point to the presence of elongated thufur, which is the first observed example in Serbia. On the global level, this morphological type is described and explained by coalescence of neighboring thufur (Grab, 2005b). In Ponor depression, such forms have been determined in the areas DD (20 examples) and DE (8 examples), in the cold air overflow zones. ...
Article
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Thufur (earth hummocks) are small periglacial landforms typical for subpolar latitudes, as well as for the high alpine areas at lower latitudes. Their presence in the mountains of the Balkan Peninsula was spotted during the mid-20th century. In this paper we analyze morphometry and morphology of thufur in the context of physio-geographical conditions for their formation. The main aims are to inventorize the thufur in the study area and to determine the physio-geographical factors which enabled their formation at non-zonal elevations. Statistical analysis was performed on the sample of 305 thufur mapped in the field, measuring their circumference, height, and delineating their areas. Classification of the results revealed morphological varieties in terms of horizontal and vertical development. The elevation of the sampling location Ponor is 1,410 m a.s.l., which is considerably lower than the zonal periglaciation in Serbia, at approx. 1,900 m. Therefore, the role of relief as a climate modifier is analyzed in the context of conditions for the azonal development of periglaciation process. Topographical conditions for thufur formation were analyzed through slope inclinations and vertical dissection, determined using the Digital Elevation Model over Europe with 25 m resolution.
... The occurrence of snow frequently invokes a process suite for landforms such as 'nivation' hollows and cryoplanation terraces, which is said to include freeze-thaw weathering and solifluction; yet such processes are not always adequately identified in the field (Thorn and Hall 2002). In addition, snow is known to influence preexisting landform evolution through its thermal and moisture controls, such as has been reported for palsas and earth hummocks (Seppälä 1994;Grab 2005a). ...
... The primary scientific debate has focused on issues concerning the presence of glacial ice and permafrost during the late Pleistocene. In this paper we consider the spatial distribution of a variety of active and inactive cold region landforms reported from the high Drakensberg, including earth hummocks (Grab 2005a), stone-banked lobes (Grab 2000), solifluction lobes (pers. obs), large sorted patterned ground (Grab 2002;Sumner 2004), block deposits (Grab 1999; and debris (glacial) deposits (Grab 1996a;Mills and Grab 2005;Mills et al. in press). ...
... DEM indicating mapped longer-lasting snow patch distributions (dated 19 August 2000) in relation to the spatial occurrence of thufur (earth hummocks) and block deposits in the southern Drakensberg region rence above 2860 m a.s.l. where contemporary winter frost penetration is still conducive to their development (Grab 2005a). However, the median altitude is relatively low (3040 m a.s.l.) and corresponds with an altitudinal zonation of reduced snow longevity (Figs 2 and 6). ...
Article
Although snow is known to influence landform genesis and distribution, the spatial associations between snow and landforms within particular cold regions has received limited research attention. We present a case study from the high Drakensberg of southern Africa, comparing the contemporary spatial pattern of longest-lasting cold-season snow patches with the distribution patterns of active and relic cold region landforms. Two 30 m resolution sets of TM images dated 3 and 19 August 1990 and a DEM were used to demonstrate the geographic trends of snow patch depletion during late winter. Geomorphological phenomena with known coordinates were then incorporated into the GIS. The spatial distribution of several periglacial land-forms (earth hummocks, stone-/turf-banked lobes, block deposits, large sorted patterned ground) coincides with topographic positions that limit snow accumulation. However, the strong spatial association between longest-lasting snow patches and palaeo-moraines implies substantial snow accumulation at some high altitude south-facing sites during the last glacial cycle.
... volume and larger surface area to volume ratio, and thus respond faster to temperature changes; ii) crest soils have a greater fraction of organic matter, resulting in increased evaporative cooling; and iii) crests are more exposed to freezing temperatures due to less snow cover. Additionally, the pattern of freezing and temperature gradients in the hummocks of eastern Lesotho are aspect controlled, a likely mechanism for the evolution of stripes from hummocks (Grab, 2005b). Mark (1994) and Grab (1997) ascribed contemporary maintenance of patterned ground topography to the movement of soil moisture towards the freezing front (cryosuction). ...
... Further, water and water vapour freeze upon reaching the freezing front, causing accretion of ice in crests and differential frost heaving of the soil (Mackay, 1980;Hohmann, 1997). Cryosuction is prevalent in finegrained soils and has been identified as a potentially important process in the maintenance of thúfur (which are synonymous with the landform we here refer to as 'hummocks' (Grab, 2005a(Grab, , 2005b), as well as other larger periglacial landforms such as palsas. ...
... There was a consistent variation in the pattern and duration of freezing between similar microsites (Figure 4), suggesting that it is the result of clear factors, likely associated with aspect and topography. Grab (2005b) found north-facing parts of hummocks to be frozen for shorter periods than south-facing parts, which is similar to the observations on stripes presented in Figure 3. The z-shaped isotherm (Figure 3) indicates freezing originating from southfacing sides, and this seems to be typical of the NE-SW oriented stripes. ...
Article
The mechanisms of maintenance of earth hummocks and non-sorted stripes in seasonally frozen ground on the Old Man Range, Otago, New Zealand were investigated. These landforms are hypothesised to be active periglacial landforms maintained by seasonal movement of moisture down an energy gradient. We tested three hypotheses: 1) freezing should occur predominantly in the crests of the stripes and hummocks; 2) differential freezing patterns should be consistent between years; and 3) in the presence of a temperature gradient (i.e. in winter), soil moisture content should be greater in crests than in furrows. An array of 39 thermistors at each of two sites was used to monitor soil temperature gradients during three winters, and replicate soil cores were taken in autumn and winter to determine soil moisture gradients. Freezing occurred mainly in the crests and sides (heavily influenced by aspect), and patterns of freezing showed strong interannual consistency, supporting our first two hypotheses. Soil in crests had a higher moisture content in both freezing and thawed seasons, which was inconclusive relative to the expected water gradient. Copyright © 2008 John Wiley & Sons, Ltd.
... This term was first used by Sharp (1942) to define dome-shaped periglacial features typical of poorly-drained environments (French, 2007). Several mechanisms are involved in their formation, including cryoturbation, cryostatic and hydrostatic pressure, differential frost heave and the cellular circulation model (Grab, 2005a(Grab, , 2005b. As a result, the internal structure of earth hummocks reveals a polygenic development, with alternating mineral sediments and peat or organic-rich layers affected by frost activity (Van Vliet-Lanoë and . ...
... Research on earth hummocks has generated interest within the scientific community studying permafrost and periglacial processes. Several studies have been carried out since the mid-1970s focusing mainly on their sedimentary structure (Tarnocai and Zoltai, 1978;Pemberton, 1980;Scotter and Zoltai, 1982;Grab, 2005b;Kokelj et al., 2007;Pintaldi et al., 2016), geochronology of their formation (Ellis, 1983;Van Vliet-Lanoë and Seppälä, 2002), ground thermal regime (Costin and Wimbush, 1973;Tarnocai and Zoltai, 1978;Grab, 2005aGrab, , 2005bScott et al., 2008), hypotheses about their formation, evolution and disintegration (Mackay and MacKay, 1976;Tarnocai and Zoltai, 1978;Mackay, 1980;Grab, 2005aGrab, , 2005bSeppälä, 2005;Killingbeck and Ballantyne, 2012;Gurney and Hayward, 2014), their implications on hillslope drainage (Quinton and Marsh, 1998;Ogata, 2007), and their relationships with vegetation and/or topography (Zoltai and Pettapiece, 1974;Kojima, 1994;Seppälä, 2002, 2003;Ogata, 2005;Pintaldi et al., 2016). The morphometry of earth hummocks has been also subject of research (Pemberton, 1980;Kokelj et al., 2007;Killingbeck and Ballantyne, 2012). ...
... Research on earth hummocks has generated interest within the scientific community studying permafrost and periglacial processes. Several studies have been carried out since the mid-1970s focusing mainly on their sedimentary structure (Tarnocai and Zoltai, 1978;Pemberton, 1980;Scotter and Zoltai, 1982;Grab, 2005b;Kokelj et al., 2007;Pintaldi et al., 2016), geochronology of their formation (Ellis, 1983;Van Vliet-Lanoë and Seppälä, 2002), ground thermal regime (Costin and Wimbush, 1973;Tarnocai and Zoltai, 1978;Grab, 2005aGrab, , 2005bScott et al., 2008), hypotheses about their formation, evolution and disintegration (Mackay and MacKay, 1976;Tarnocai and Zoltai, 1978;Mackay, 1980;Grab, 2005aGrab, , 2005bSeppälä, 2005;Killingbeck and Ballantyne, 2012;Gurney and Hayward, 2014), their implications on hillslope drainage (Quinton and Marsh, 1998;Ogata, 2007), and their relationships with vegetation and/or topography (Zoltai and Pettapiece, 1974;Kojima, 1994;Seppälä, 2002, 2003;Ogata, 2005;Pintaldi et al., 2016). The morphometry of earth hummocks has been also subject of research (Pemberton, 1980;Kokelj et al., 2007;Killingbeck and Ballantyne, 2012). ...
Article
A multi-approach characterization of three earth hummock fields has been conducted to understand the morphometrical characteristics and distribution pattern of these periglacial features in the Zackenberg Valley, NE Greenland. Earth hummocks develop in poorly-drained areas affected by intense cryogenic conditions. An accurate analysis of the morphometrical properties of hundreds of earth hummocks distributed between different Early Holocene moraine systems of the eastern slope of the Zackenberg Valley reveals an important control of microtopography on their distribution. Sedimentological analysis of selected earth hummocks shows evidence of alternating organic-rich layers and mineral units. Radiocarbon dates of the basal organic layers in contact with the permafrost table yielded ages 615 ± 25 and 1755 ± 60 cal yr BP, with lower sedimentation rates over the last centuries when soil formation prevailed. Geochemical analysis of the soils (Glacic Reductaquic Cryosols) showed also significant differences in the properties and composition among the soils of the different fields of hummocks.
... In Arctic, sub-Arctic and alpine wetlands, one of the most widespread landform indicative of cryoturbation is earth hummock (Grab 1994(Grab , 2005Scott et al. 2008;Peterson and Johnson 2018;Milošević et al. 2015;Li et al. 2017). Hummocks are small cryogenic mounds, generally domeshaped (Sharp 1942) and covered by grass. ...
... Hummocks are small cryogenic mounds, generally domeshaped (Sharp 1942) and covered by grass. They are usually closely spaced on flat or gentle slopes (<25°) in wetlands or poorly drained meadows and are formed by the combined action of the frozen soil layer, hydrodynamic forces and growth of plant communities Schunke and Zoltai 1988;Kojima 1994;Grab 2005;Li et al. 2017). Hummocks display different morphological, sedimentological and geochemical characteristics depending on the properties of their environment. ...
... There are many theories about mechanisms of hummock formation, including differential frost heave (Van Vliet-Lanoë 1991;Grab 2005), cryoexpulsion of clasts (Van Vliet-Lanoë and Seppälä 2002), hydrostatic or cryostatic pressure (Lundqvist 1969;Tarnocai and Zoltai 1978) and the cellular circulation model (Mackay 1980). Although the hypotheses of those work for the origin of hummocks have been advanced (Kessler et al. 2003;Peterson et al. 2003), differential frost heave is the most widely accepted model for hummock development (Van Vliet-Lanoë 1991;Grab 2005). ...
Article
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Earth hummocks are widely distributed in arctic, sub-Arctic and alpine regions and have important roles in determining plant diversity and the nutrient content of soils. We investigated the impact of the spatial heterogeneity on soil properties and plant communities caused by the hummocks on the Tibetan Plateau. The results indicated alpine hummocks created higher plant diversity and soil fertile island patterns under Kobresia Genus communities. Vegetation height, cover, above-ground and underground biomass, species richness and diversity at the top of well-developed hummocks were the significantly higher than those in the inter-hummocks and surrounding flat ground. The soil organic carbon content in surface soil layer (0-50cm) at the hummock top was 154.6% and 172.3% higher than those in inter-hummock area and the flat ground, respectively. From the developing to well-developed stage, K. littledalei became the dominant population at hummock top instead of K. humilis, and fertile islands gradually formed with higher soil organic carbon and total nitrogen, lower soil moisture and wider soil temperature variation. RDA analysis further indicated the interactions of plant community succession and soil fertile island could create positive feedbacks to nutrient-rich patches.
... Spaţii libere nu au fost observate în interiorul mușuroaielor, deşi în partea superioară există o structurare proastă a materialelor datorită rădăcinilor plantelor. Textura materialelor care compun mușuroaiele este foarte importantă și pentru evoluția regimului termic (Grab, 2005). Timp de doi ani regimul termic al unui mușuroi înierbat a fost monitorizat la 5 niveluri de adâncime, 10, 20, 30, 40 și 50 cm ( fig. ...
... La toate adâncimile se poate sesiza faptul că, pe durata iernii, temperaturile din mușuroiul periglaciar sunt mai coborâte decât cele înregistrate în cazul terenului plat. Temperaturile mai reduse din interiorul mușuroiului periglaciar sunt semnalate și în alte studii similare (Grab, 1997) și pot fi rezultatul conductivității termice mai ridicate datorate umezelii sporite din mușuroiul periglaciar în timpul iernii (Grab, 1997(Grab, , 2005. ...
... Chiar dacă se admite că mușuroaiele pot deveni alungite și chiar compuse, mecanismul dezvoltării cu preponderență într-o anumită direcție nu este pe deplin lămurit (Grab, 2005). În general, dacă înclinarea versantului este mai mare de 3° (Grab, 2005) mușuroaiele au tendința de a înclina spre aval, datorită proceselor lente de deplasare în masă (ex: creep, solifluxiune etc.). ...
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Studiul prezent abordează problema periglacierii actuale a spațiului alpin carpatic, avându-se în vedere raporturile dintre morfologia de detaliu și structura internă a principalelor fenomene periglaciare respectiv dintre regimul termic și dinamica actuală a formelor periglaciare. Analizând cele mai importante fenomene periglaciare din cuprinsul spațiului analizat, autorul selectează mai multe situri-test din Carpații Meridionali în care utilizând metode geomorfologice și geofizice investighează procesele și formele asociate înghețului substratului care operează în mediul alpin. Datele inedite privind distribuția spațială a permafrostului, regimul termic al stratului activ și factorii top-climatici care condiționează prezervarea substratului înghețat conduc la acceptarea realității incontestabile a existenței permafrostului sporadic în condiții marginale în Carpații Meridionali. Făcând referire la o problematică excesiv simplificată în literatura autohtonă, această cercetare aruncă o nouă lumină asupra proceselor periglaciare din Carpații Meridionali, contribuind prin cantitatea impresionantă de date și prin ipotezele științifice formulate la îmbunătățirea înțelegerii morfogenezei și morfodinamicii periglaciare din România.
... Hummock development under conditions of shallow ground freezing is supported by hummock formation in northern England on low ground since the early 19 th century AD (Pemberton, 1980) and on high ground since AD1950 (Tufnell, 1975), by hummock formation in Iceland within 10-20 years on ground levelled by tillage (Schunke and Zoltai, 1988) and by formation of small hummocks on artificial slopes at 150-300 m asl in Germany (Kelletat, 1969). Hummocks similar to those on Dartmoor occur in areas of shallow (20-40 cm) ground freezing in Lesotho (Grab, 1994(Grab, , 1997(Grab, , 2005b and New Zealand (Mark, 1994;Scott et al., 2008). Collectively, such evidence suggests that hummocks may develop and persist in areas where frost penetrates periodically no deeper than 20-40 cm into the soil. ...
... Though differential frost heave has previously been identified as a cause of hummock initiation (Lewis et al., 1993), the mechanism of subsequent hummock growth from an initial regularly spaced pattern of gentle swells and depressions is less well understood. It has been shown, however, that under conditions of shallow ground freezing, hummock crests and sides freeze first, and that depressions may remain unfrozen during shallow freezing cycles (Mark, 1994;Grab, 1997Grab, , 2005bScott et al., 2008). Assuming that laterally non-uniform frost heave creates an initial pattern of swells and depressions (Figure 9a), these findings suggest that frost heave of swells will be amplified by earlier and deeper freezing (Figure 9b), with ice lens growth within Figure 9 Model of hummock growth by migration of silty soil in advance of inclined freezing planes. ...
Article
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Extensive fields of vegetated, dome-shaped earth hummocks 5–35 cm high and typically 80–200 cm in diameter occur on slopes of up to 15° at elevations of 320–440 m asl on Dartmoor, SW England, but are limited to terrain underlain by metasedimentary rocks and dolerite, and absent from granite areas. Hummocks occur within archaeological sites, implying formation within the last ~ 3000 years and ruling out development over permafrost. The hummocks are composed of frost-susceptible silty soil (modal grain size 10–100 µm) with occasional clasts, but exhibit no evidence for cryoturbation or diapirism. Hummock size and spacing are fairly consistent at particular sites but vary between sites. Hummock age, distribution and characteristics are incompatible with non-frost action origins and most proposed frost action origins (cryostatic pressure, soil circulation, permafrost aggradation, soil injection), but regular spacing and granulometry favour initiation by differential frost heave under conditions of periodic shallow (0.2–0.4 m) seasonal ground freezing. We suggest that hummock growth may reflect migration of silt in advance of inclined freezing planes until equilibrium is achieved with soil loss down hummock sides. Our results confirm that hummock formation by frost action occurs on silty soils in humid cool temperate climates with only limited seasonal frost penetration. Copyright © 2012 John Wiley & Sons, Ltd.
... Wetland vegetation is often dense and characterized by high stands of grasses and sedges. Sedge hummocks are common (Combrink et al., 2021), indicating seasonal temperature shifts resulting in cyclic freeze and thaw of the soils (Grab, 2005). ...
Article
This paper aims to identify chronostratigraphic palaeo-climatic boundaries based on proxy indications from mountain- and coastal wetlands in eastern South Africa and Lesotho. Phase boundaries were identified from timing of climate change inferred by proxies, as well as regime shifts in climate variability. Sometimes magnitude and/or frequency of change was also considered. Summarizing the common palaeo-climatic indications suggest the following chronostratigraphic climate phases: 25 to 18 ka, 18 to 15 ka, 15 to 11.5, 11.5 to 8 ka, 8 to 5.5 ka, 5.5 to 2 ka and 2 to 0 ka. The most robust boundaries were identified at 18 ka, 15 ka and 2 ka, i.e. these boundaries were supported by several proxies/sites. The other boundaries were less clearly detected from available proxies/sites and should be regarded tentative. The timing of a climate shift often coincides at coast and mountain sites. However, the climate conditions within each chronostratigraphic phase sometimes vary between coast and inland sites. The 25 to 18 ka phase was cool and dry with strong and frequent storms, followed by the ca. 18 to 15 ka period when conditions were less severe but still generally cool and dry. At ca. 15 to 11.5 ka several proxies infer warmer climate, with less winter rains. During 11.5 to 8 ka a general increase in wetness is inferred, followed by warming over the 8 to 5.5 ka phase. Between 5.5 and 2 ka a successive change towards wetter is indicated, although timing differ between sites. After 2 ka generally a more variable climate is seen, often with high magnitude shifts between dry and wet. The data resolution, i.e. the number of available wetland records, increases with time from very low during glacial times, to highest resolution during late Holocene. Geographically, sites in the mountain region are overrepresented compared to coastal sites. A comparison with coastal lake records suggests a more variable climate at coastal sites compared to mountain sites during mid- and late Holocene, although different proxy resolution and methodology cannot be ruled out as an explanation. A case study compares multiproxy records from Drakensberg (Sekhokong, Ntsikeni) and the coast (Mfabeni), discussing advantages and problems associated with proxy-comparisons within and between sites.
... Individual snowfalls are usually \5 cm, but falls of as much as 120 cm have been recorded during historical times (e.g., 1843; see Grab and Nash 2010). Mean seasonal temperatures in the mountains range from *10°C (December-February) to *0°C (June-August); ground frost occurs on *180 days/year (Grab 2005). Mean monthly wind speeds recorded in the Sani Valley (in 2001) vary from 3.39 m s -1 (March) to 8.09 m s -1 (August); wind gusts of up to *37 m s -1 have also been measured (Grab 2010). ...
Article
Full-text available
Alpine cryospheric hazards are becoming increasingly prominent under current global/regional climate change scenarios and receiving wide scientific coverage from, in particular, northern hemisphere mountain regions associated with glaciers, permafrost, and extensive seasonal snow cover. However, there is a general paucity of knowledge and attention on cryospheric hazards associated with mountain environments only occasionally/rarely impacted by heavy seasonal snowfalls or severe frost events, particularly those in developing and southern hemisphere regions. Prolonged snow cover in the Lesotho Highlands sometimes carries the consequence of human and livestock deaths owing to isolation and exposure in this developing region. We use daily Moderate Resolution Imaging Spectroradiometer snow cover images for the period 2003–2010, to establish the frequency, extent, and timing of snowfalls across Lesotho. In addition, a digital shape file containing the location, name, and district attributes of 2,016 villages across Lesotho was used to assist in the construction of a village exposure to snow index. A ranking system was applied to each village according to the seasonal duration of snow cover, and the accessibility and proximity to the nearest road. Snowfalls occur on average between 1 and 8 times per annum, with village exposure to snow (potential vulnerability) being generally low, particularly for the lowlands and Senqu River Valley. However, the study identifies that some high-altitude (>2,500 m) villages such as Thoteng, Letseng-la-Terae, and Mabalane are, on occasion, highly exposed to prolonged snow cover, and particularly so during the mid-snow season of July/August. We demonstrate the importance of applying spatiotemporal assessments on infrequent snow occurrences (which carry associated hazards) in developing mountain regions such as Lesotho, with implications to reduce livelihood risks through improved disaster preparedness and a well-informed, focused emergency response.
... This could explain why the majority of the mounds from Muntele Mic site are elongated in a northerly direction, irrespective of slope orientation. According to Grab (2005b) the tendency of hummocks to elongate (Onaca 2013) in a particular direction, could be explained by the coalescence of closely spaced mounds. The elongation is enhanced on the northern aspect of the hummocks where, due to topographic shading, the freeze intensity is higher (Onaca 2013). ...
Chapter
This chapter reviews the existing knowledge concerning the current state of the main frost-driven processes in the Romanian Carpathians. Since permafrost has a patchy distribution and occurs in marginal conditions only in few massifs from the Romanian Carpathians, the key periglacial processes are those associated with seasonal frost, which are widespread above the tree line. The present-day amplitude of solifluction, frost heaving or frost creep is generally lower than in other periglacial environments ranging from few mm to tens of centimetres/year. The results revealed that mass wasting and frost weathering in the alpine environment of the Romanian Carpathians are strongly controlled by ground freezing, which depends on several factors, such as ground materials, topography, vegetation cover, snow cover, water content and incoming solar radiation. Major progress has been achieved in the last few years, when comprehensive monitoring of several periglacial processes (e.g. solifluction, frost heaving, frost creep) has started. The outcomes improved the understanding of current periglaciation in the Romanian Carpathians, but many uncertainties still exist regarding several periglacial processes (e.g. frost weathering) and the role of the environmental controlling factors. The periglacial deposits have a central, but not a defining position within the alpine landscape, occupying a greater extent than glacial or fluvial deposits. Based on the estimated volume of sediments within the alpine sector of a small alpine catchment in the Southern Carpathians, a post-glacial mean denudation rate of 0.26 mm/year was calculated.
... A raised dome ca. 9 m across has formed around the spring, which sits near the centre of the ca. 3 ha wetland. The dome itself is covered with cryogenic earth hummocks (thúfur), ca. 15 cm high and 50-70 cm in diameter, formed as a product of differential frost heave (see Grab, 2005aGrab, , 2005b. The area surrounding the wetland is dry, with indications of gully erosion and alpine turf exfoliation pans (Grab, 2010). ...
Article
The eastern Lesotho highlands are of considerable hydrological importance to southern Africa as a so-called ‘water tower’ for the surrounding region. Here, we contribute proxy-data inferring climate and vegetation changes over the past 1600 years, assessing in parallel inorganic and organic chemical analyses on a sediment core from Ladybird wetland, eastern Lesotho. Several proxies were used to determine changes in local vegetation dynamics, productivity, hydrology (δ¹³ C, δ¹⁵ N, C/N, TOC) and the input and source of the detrital components (Ca/Ti, CIA). The first part of the multi-proxy record (AD 400–800) shows stable terrestrial conditions and low detrital input, followed by higher variability in almost all proxies between ca. AD 900 and 1200. The δ¹³ C record infers a higher proportion of C4 vegetation, tentatively associated with higher temperatures during this phase, coeval with the Medieval Climate Anomaly (MCA). After AD 1200, local conditions change gradually from purely terrestrial, towards the typical wetland environment prevailing today. A higher proportion of C3 plants and possibly an increase in aquatic organisms within the organic matrix corresponds with decreasing detrital input, suggesting locally high available moisture in this part of Lesotho during the Little Ice Age (LIA). Although age-model constraints impedes a robust regional comparison, the inferred climate variability is discussed as a tentative response to enhanced mid-latitude cyclonic activity during LIA, and the variable MCA climate conditions as indirectly dictated by changes in solar activity.
Article
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We present attributes of snow occurrence and dissipation rates (melt and sublimation) for the Lesotho Highlands, based on remotely-sensed MODIS images from 2003–2016. Multi-temporal imagery is used, with SNOMAP and NDSI algorithms applied to MODIS Rapid Response images. The spatial extent of snow loss was determined by daily repeat measurements of snow coverage, which was calculated from each filtered and trimmed MODIS SNOMAP image using the ArcGIS Spatial Analyst tool. Results indicate an average of 11.5 snowfalls per annum for the years 2003–2016; with snow longevity averaging ca. 10 days following individual snowfalls in mid-winter. Snow cover on the highest south-facing slopes persists longest during the months of June to August, in particular along the southern Drakensberg where it averages ca. 58 days during these 3 months. Mean daily melt increases by 1.6% per 1°C rise during the first 5 days post-snowfall, and by 3.3% per 1°C rise for 6 to 10 days post-snowfall. However, snowmelt rates are spatially highly variable given other factors such as wind deflation and wind-induced sublimation. The observed snow trends have important implications for biosystem functioning, regional climate and hydrology, earth surface processes, and rural livelihoods in the Lesotho Highlands. © 2017, South African Water Research Commission. All rights reserved.
Article
Geomorphology plays a pivotal role in linking the traditional subdisciplines of physical geography. This is because geomorphological processes are influenced by climate, and geomorphology in turn strongly controls land surface hydrology and ecosystems. This review assesses the current status of geomorphology in South Africa. Six subfields are identified, viz. fluvial geomorphology, glacial and periglacial geomorphology, rock weathering, arid and semi-arid geomorphology, coastal and estuarine geomorphology, and applied geomorphology. The status of each is critically assessed. Studies in the different subfields reveal progress not only at the site-scale, but that such studies are also commonly set within a wider regional to global context. Regional research is well connected to contemporary global debates. Two of these debates are of particular significance. Firstly, cosmogenic dating techniques can be used to evaluate the age of land surfaces. Geomorphological research in South Africa has begun to utilize this technique to this end. Secondly, geomorphological knowledge can be practically applied. This includes fluvial geomorphology (managing the dynamics of river and wetland systems), rock weathering (preservation of San art and monuments), and coastal and estuarine geomorphology (understanding coastal responses to global sea-level rise) as well as broader environmental management issues. Geomorphology and geomorphological processes are critically important to issues impacting on South Africa in the twenty-first century. These include climate and environmental change, food and water security, and sustainable abiotic resource conservation and management. South African geomorphologists and their international collaborators can, and should, be at the heart of many of these contemporary debates.
Article
Gurney, S.D. & Hayward, S. 2015. Earth hummocks in north-east Okstindan, northern Norway: Morphology, distribution and environmental constraints. Norsk Geografisk Tidsskrift–Norwegian Journal of Geography. ISSN 0029-1951. Earth hummocks (also termed pounus or thúfur) are a common form of periglacial non-sorted patterned ground. The study objectives were to determine the morphology, distribution and development on slopes of earth hummocks in north-east Okstindan, Norway, an area with many hummocks but few documented accounts. The methodology involved detailed geomorphological mapping and precise measurement with a profileometer. The internal structure of the hummocks was investigated through excavations and sediment sample analyses. Fourteen sites with well-developed earth hummocks (accounting for over 650 individual hummock forms) were investigated. The sites have an average altitude of 750 m and occur on slopes with an average gradient of 7°. The hummock heights are in the range 0.11–0.52 m and their diameters 0.7–1.5 m, although coalescent forms are up to 5 m in length. The hummock morphology is characterised by a variable plan form, asymmetry with respect to upslope and downslope forms, downslope elongation, coalescence, and superimposed microtopography. The hummocks’ distribution appeared to have been controlled by the existence of a frost-susceptible ‘host’ sediment, but moisture availability and topographic position played a role. The authors conclude that differential frost heave and vegetation cover stability are critical for the hummocks’ longevity in the studied landscape.
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The paper elaborates on the occurrences of thufur (earth hummocks, pounus) in a sub-alpine periglacial environment of the Vlasina Lake (1210 m a.s.l.) in south-eastern Serbia. Field examination encompassed 8 sites at the southern rim of the lake, at altitudes from 1220 m to 1274 m, where 240 thufur were measured for height (av=32 cm) and diameter (av=51 cm). Parameters of topography (relative heights, gradient, aspect) and overall morphology are analysed as well. The lake basin represents an area of cool air accumulation and temperature inversions. The upper limit of this zone, where 7 out of the 8 studied sites are situated, is the highest closed contour at 1243 m. Analysis of the relation between lithology, morphology and groundwater drainage showed that thufur on poorly drained sites are well developed, whereas the sites with good drainage are characterized by relatively small thufur. Detailed study of the height/diameter relation enabled differentiation between three morphological types of circular thufur: prolate spherical domes, hemispherical domes and oblate spherical domes.
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Subaerial physical, chemical and biological weathering processes are significant contributors to mountain landscape development worldwide, and over long time scales. This paper reports on pseudokarst features formed by different subaerial weathering processes on a high-elevation basalt plateau surface in the Maloti–Drakensberg mountains of eastern Lesotho, southern Africa. Different mesoscale landforms identified on this plateau surface include enclosed depressions (containing seasonal pools) that are morphologically similar to karstic solutional features. Such basalt pseudokarst landforms have not been previously described. This study uses field data on depression morphometry, sediments, water chemistry and rock surface hardness to explore the varied subaerial weathering and erosion processes contributing to pseudokarst depression development. Results show that physical, chemical and biological weathering and erosion processes in combination can impact on the basalt land surface in widening and deepening these depressions. An evolutionary model for pseudokarst depression development proposes that different weathering processes in combination within the ‘critical zone’ cause changes in depression morphology, and that there are significant feedbacks between bedrock, weathering, and the development of distinctive micromorphological forms. It is likely that similar pseudokarst landforms from other locations with non-carbonate substrates have been under-reported in the literature.
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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.
Article
Recent work has indicated that the southern African ice rat (Otomyssloggetti robertsi) is responsible for negative habitat change due to its foraging and burrowing activities in the Lesotho Highlands. Previous work has only focused on short-term ( ≤ 1 year) monitoring and thus the impact of such rodent activity over longer temporal scales remains unknown. To this end, the current study evaluates vegetation cover and O. s. robertsi's burrowing over a 10-year period (1998–2009) across a portion of a wetland fringe/wetland in eastern Lesotho. A 6 m × 11 m plot was established in 1998 and marked out with permanent stakes. Percentage cover and number of tunnels were recorded, with repeat measurements taken in 2001, 2005 and 2009. The findings indicate an overall 69.2% increase in vegetation cover and associated 71% decrease in burrow densities between 1998 and 2009. The area monitored closest to the wetland fringe recorded the most substantial burrow density decrease (by 80%) and vegetation recovery (by 43%) within a three-year period from 1998 to 2001. With a slight shift of burrow density increases towards the wetland centre, particularly between 2001 and 2005, the standard deviation of burrow density (per m) from the wetland periphery towards the wetland centre had decreased from 1.25 to 0.32 over the 10-year period. It is proposed that the environmental influence of O. s. robertsi may be less dramatic at some landscape and longer temporal (i.e. years rather than months) scales due to spatial shifts of colonies, bioengineering and rapid vegetation recovery at abandoned, formerly ‘degraded’ burrow sites.
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Here, we aim to establish possible morphogenetic associations between periglacial phenomena (sorted circles and turf-/stone-banked lobes) by examining (a) rock surface weathering rinds by thin section microscopy and SEM-EDX analysis, and (b) alpine shrub growth patterns of Helichrysum trilineatum supported by AMS 14 C age determination. The study area is the Mafadi-Njesuthi summit zone, one of the highest alpine regions of southern Africa. Rock weathering results indicate that clasts in large sorted circle centres experience high chemical weathering rates on both exposed (upper) and non-exposed (lower) surfaces, whereas clasts in lobes and very large apparently 'relict' sorted circles have been inactive for a prolonged period of time, based on strongly contrasting weathering rates on both exposed and non-exposed clast surfaces. Large sorted circles originating from a previous (possibly Last Glacial Maximum or late-glacial) colder period are still marginally or episodically active, thus restricting plant succession on these landforms. Even where very large sorted circles and turf-/stone-banked lobes are inactive, their legacy continues, as expressed in today's surface morphology, and still influence ground abiotic conditions through the generation of microrelief and microclimate which in turn have impacts on ecosystem patterns including the distribution of H. trilineatum. Such shrub growth patterns are influenced by microscale site morphology, associated abiotic controls, and ongoing seasonal cryogenic activity. Both active and inactive periglacial landforms on highest summits of the Drakensberg represent microscale environments with distinct fine-scale abiotic and geomorphic settings, which in turn are manifested through differences in plant ecology and rock surface weathering, respectively.
Article
Aims Our objectives are to examine the effects of hummock-depression spatial heterogeneity on plant communities and soil properties, and to understand the process of maintaining and adjusting microtopography-mediated hydrological inputs and their spatial fluctuations that produce obvious micro-habitats. Methods We set up 36 plots (1m×1m) and sampled 45 plant and 225 soil samples in flooded and non-flooded hummocks and depressions of the marshy, and the surrounding non-wetland meadows as well as in the Yellow River Source Zone, west China. We evaluated whether the alpine marshy wetland has a fertile-island effect by the comparison method. Important Findings Our results show that hummock presence can increase the spatial heterogeneity of the microhabitat and promote the plant diversity and soil fertility of the Kobresia tibetica community. Plant height, coverage, above-ground biomass, species richness and diversity were significantly higher in the flooded and non-flooded hummock microhabitat than in the areas between hummocks and surrounding non-wetland meadows. Compared with broad alpine meadows, the hummock-depression complex provided a microhabitat favourable to the growth of Cyperaceae. In the 0-50 cm soil layer, the closer the soil layer was to the ground surface, the higher its soil organic carbon and total nitrogen contents. Thus, in deeper layers, the gap between soil nutrients in wetland hummock-depression microhabitat and in the surrounding alpine meadows become smaller. Hence, the wetland hummock-depression microhabitat formed a fertile-island pattern. Therefore, these results contribute towards improving our understanding of ecosystem restoration in alpine marshy meadows.
Article
Research on lowland periglacial landforms and processes is reviewed using papers published from 2003–2007, to identify current interests, trends and directions of advances in knowledge, and to help identify promising avenues for future research. For this period, the total number of identified published research papers was 72. Topics relating to ice wedges, thermokarst, palsas and patterned ground were characterised by high publication activity. Current trends suggest three main research thrusts in the near-future: palaeoenvironmental reconstructions (including Mars) using current process and landform observations as analogues, year-round monitoring of processes using automatic equipment and modelling based on Geographical Information Systems. Copyright © 2008 John Wiley & Sons, Ltd.
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Detailed investigations of peaty earth hummocks called pounus were carried out in northemmost Finnish Lapland in the vicinity of the Kevo Subarctic Research Station. Pounus are complex, often vegetated, cryogenic mounds formed partly of peat. Some have a core of mineral soil or several large stones, or just one frost-heaved boulder covered by peat. They belong to the same group of earth hummocks as thufur and non-sorted circles. The typology of pounus is described and compared with other types of cryogenic earth hummocks. Stratigraphical study of pounus shows how cryoturbation has deformed the previous soil horizons of typical Arctic podzols and has been followed by intense peat growth and later frost reactivation. Microstructures in pounus confirm the frost activity and several phases of peat growth during their formation. Alternating warmer and dryer conditions with water-table rise and renewed peat formation could be inferred. The present stage of development is frost reactivation and peat growth. Pounus are relatively young features. The maximum age for pounu development corresponds with the age of peat formation. Peat of a boulder pounu yielded a data of 330 + 70 BP and a minerogenic pounu gave an age of 710 + 50 BP. The Sphagnum peat formation on peat pounus started intensively about 1000 BP. They are random surficial forms with variable control by frost heave and vegetation growth, independent of the presence or absence of permafrost. They are related to a dynamic snowcover in areas of open birch stands or tundra and their development is polygenetic with alternating frost disturbance and phases of humification or podzolization. Pounus act thermally as a biological refuge in the autumn and spring. At present, perennial frozen cores seem to be common, especially in large peat pounus. Pounu development, as with other earth hummocks, occurs after the main phase of interglacial pedogenesis and can be considered as an indicator of a general cooling trend, developing mostly during the second half of the Holocene. Pounus' relation to climatic change is discussed. Pounus can be considered as an indication of a general cooling trend associated with progressive depletion of summer insolation. Hummock extent is coherent and parallel with trends in permafrost growth, but not directly linked to it.
Article
A brief summary is made of various field studies from Fennoscandia concerned with the origin of palsas, which are peat hummocks with a permafrost core rising above the mire surface. An insulating peat layer is important for preserving the frozen core of the palsa during the summer. Palsa formation needs low winter temperatures to form thick layers of frost, together with water-saturated peat and a thin snow cover. Factors limiting the size of palsas are discussed. Sharp, steep edges, which collect drifting snow, stop the areal growth of palsas. A model for cyclic palsa development is presented.
Article
Ground and air temperature data are presented for the high Drakensberg, southern Africa, representing the period Jan-Dec, 2000. The ground temperatures were measured at depths of 10, 20, 30, 40 and 50 cm through a north-facing sorted circle and a nearby south-facing stone-banked lobe tread, thus providing data for comparative assessments. The primary focus is on determining contemporary cryogenic thermal parameters to 50 cm depth and ascertaining associated environmental implications. Whilst the mean annual ground temperatures are similar for the two sites, considerable variations of summer and winter ground temperatures occur on opposing slopes at ∼ 3,400 m a.s.l. The amplitude of ground freeze varied from -6.5 °C at 10 cm to -0.1 °C at 50 cm through the sorted circle, whilst temperatures dropped to -7.8 °C at 20 cm and - 2.8 °C at 50 cm in the stone-banked lobe. The rate of freeze penetration and freeze duration is considerably more pronounced through the stone-banked lobe, where a maximum freeze duration of 98 days was recorded at 40 cm. The differences in the thermal regimes at the two sites are controlled by site-specific differences in sedimentary properties and ground moisture, microrelief (microclimate) and snowpack characteristics. Given that seasonal freeze may continue into late spring at the stone-banked lobe site, contemporary gelifluction may be possible during periods of snowmelt or spring rainfall. In contrast, the sub-surface of the north-facing sorted circle site is already thawed by the time sufficient moisture is added to the soil profile, thus restricting the potential for contemporary frost heave at this site during years of mild to average climatic conditions. It is suggested that winter ground temperatures are directly affected by the longevity and depth of snow cover in the Drakensberg and thus snow is an important factor determining the distribution of periglacial landforms in the region. © 2004 Gebrüder Borntraeger, D-14129 Berlin · D-70176 Stuttgart.
Article
Strong spatial‘and temporal variations in temperature and effective thermal diffusivity are apparent from short-term thermal records measured, in the peat atop an icecored,palsa in northern Alaska. Intersite differences in near-surface temperature regimes apparently result from topographic influences, vegetative cover, and site wetness. Values of effective thermal diffusivity indicate that ablation of the ice core is inhibited by advection of cold water near the ice-peat contact and by internal evaporation near the surface. These findings support the conclusions of earlier in- vestigators, who emphasized the importance of peat for maintaining palsas near their equatorward limit. Key words: active layer, frost mound, palsa, peat, permafrost, soil temperature, thermal properties, Alaska RÉSUMÉ. Des études de records,thermiques de court terme mesurés dans la tourbe. couvrant une palse ayant un noyau,de glace dans le nord de
Article
This paper presents empirical data on non-sorted patterned ground in the high Drakensberg. Some morphological and sedimentary characteristics for thufur and non-sorted steps are provided. The study also examines process data on ground heave activity at a thufur site. Contemporary cryogenic activity at the non-sorted steps is indicative that the patterns are at least seasonally active. Soil heave measurements at the Mashai Valley thufur site support the contention that some thufur are active in the high Drakensberg.
Article
Recent measurements confirm the current activity of some solifluction terraces in high-alpine cushionfield, under periglacial conditions on the Old Man Range in southern New Zealand. Downslope movement amounts to some 3.5 cm a decade. Extensive areas of fully vegetated earth or soil stripes on gentle slopes (3-7°) and similarly vegetated soil hummocks on adjacent flatter sites are much less obviously active and have been assumed by previous researchers to be relicts of a more glacial climate. Three years of over-winter temperature measurements at up to four depths (5, 20, 40, 60 cm) beneath an adjacent crest and hollow of well-developed stripes (ca. 1.5 m between crests and 30 cm high), and similar measurements over one winter from a nearby hummock site revealed consistent differential temperatures, and usually differential freezing. Crests of both stripes and hummocks remained continuously frozen at least to 20 cm depth over much of the winter while soil in the depressions remained unfrozen at 20 cm depth and only intermittently frozen at -5 cm. Differential freezing would permit water transfer along a free energy gradient from the nonfrozen to frozen regions, where expansion associated with its freezing probably is sufficient to maintain the microtopographic patterns of both stripes and hummocks.
Article
A brief summary is made of various field studies from Fennoscandia concerned with the origin of palsas, which are peat hummocks with a permafrost core rising above the mire surface. An insulating peat layer is important for preserving the frozen core of the palsa during the summer. Palsa formation needs low winter temperatures to form thick layers of frost, together with water-saturated peat and a thin snow cover. Factors limiting the size of palsas are discussed. Sharp, steep edges, which collect drifting snow, stop the areal growth of palsas. A model for cyclic palsa development is presented.
Article
Relationships of vegetation, earth hummocks, and topography were studied in the high arctic environment of Ellesmere Island, N.W.T., Canada. A belt-transect 2 m wide and 114 m long was established on a south-west facing slope of a small knoll, where well developed earth hummocks were recognized. The belt-transect was sectioned in 2 m x 2 m successive quadrats. For each quadrat, cover degree of individual species and total vegetation was assessed and recorded. After earth hummocks were categorized conveniently into three types according to a degree of being covered by vegetation, the number of earth hummocks in each quadrat was counted for each type. A topographical cross-section along the belt-transect was surveyed, and vegetation and earth hummock characteristics were correlated with topographical position. Well developed earth hummocks were recognized mainly at the base of a leeward slope of the knoll where snow possibly drifted and stayed long enough to protect vegetation and earth hummocks from strong wind, especially in winter. Vegetation, too, de-veloped well in places where earth hummocks developed well; it consisted predominantly of Salix arctica, Saxifraga oppositifolia, Poa abbreviata, Dryas integrifolia, and Pedicularis arctica. Development of the earth hummocks was regarded as a synergistic product of such factors as fine soil material, topogra-phy, snow, permafrost, and vegetation. Snow was considered to play a decisive role in developing and maintaining well developed earth hummocks and vegeta-tion by protecting them from adverse wind effects in winter. This is why the majority of the well developed earth hummocks are confined to leeward slope-bases where wind blown snow accumulates and stays relatively long.
Article
Miniature varieties of cryogenic mounds that are capable of forming in seasonally frozen ground are commonly referred to as earth hummocks (e.g., North America), thúfur (e.g., Greenland and Iceland) and pounus (Fennoscandia). Over the past few decades there has been a consistent interest to study earth hummocks from a variety of environmental settings. This review summarizes the current knowledge of earth hummocks, highlighting aspects on the external and internal morphology, and thermal characteristics, which may assist to explain hummock formation. Several hypotheses have been proposed for the genesis of earth hummocks, including the 'cryoexpulsion' of clasts, hydrostatic and cryostatic pressure, cellular circulation, and differential frost heave. These hypotheses are critically evaluated and some research gaps identified. It emerges that considerable advances have been made towards an improved understanding of earth hummock development, modification and disintegration. Much progress has been made in the application of earth hummock studies to a variety of environmental research approaches such as palaeoenvironmental reconstructions and assessing their impact on hillslope drainage.
Article
With global periglacial geomorphology undergoing significant advancements, it is appropriate to review the past and current status of such research in Africa. A brief historical overview of research outputs and approaches is presented for the respective African regions. Potential future quantitative periglacial research needs and approaches identified for Africa include: the examination of active periglacial processes, the identification of landforms and ground-ice forms, the potential for environmental change and the palaeoenvironmental reconstruction, and the application of periglacial studies. It is demonstrated that while periglacial geomorphology has progressed significantly in southern Africa, there has been little or no advancement elsewhere on the continent over the last two decades. None the less, on a more positive note, it is concluded that Africa has considerable potential in future global periglacial research.
Article
Laboratory core freezing data for a Calgary silt are used to validate the secondary frost-heave model of Fowler and Krantz. This model is based on the model of O'Neill and Miller, but incorporates scaling and asymptotic analysis to make the complex physics of the freezing fringe more tractable and employs a more realistic thermal regelation mechanism for ice movement. Although this model does not involve any adjustable parameters, owing to insufficient soil-characterization data, it was necessary to fit one parameter in the equation describing the hydraulic conductivity of the frozen soils. However, this parameter was found to correlate very well with the porosity of the Calgary silt samples. These results were used to develop an equation for predicting the hydraulic conductivity of frozen Calgary silt as a function of soil porosity and unfrozen water volume fraction. These studies strongly support the predictive capability of the Fowler and Krantz model which because of its simplicity can be readily extended to incorporate solutal effects on freezing, soil compressibility, and differential frost heave.
Article
A 12 month study on the thermal regime for a thufa apex and its adjoining depression was undertaken in the Mashai Valley (∽2950 m ASL), Lesotho Highlands, during 1993–94. Pronounced temperature differentials occurred during the winter months when apexes were frozen for several weeks and depressions remained predominantly unfrozen. The data show that existing micro-topography is an important factor controlling the maintenance or further development of thufur in marginal periglacial environments. © 1997 John Wiley & Sons, Ltd. Une étude du régime thermique du sommet d'un thufa et de la dépression qui le borde a été poursuivie dans la vallée de Mashai (2950 m au-dessus du niveau de la mer) dans les hautes terres du Lesotho pendant 12 mois en 1993–94. Des différences de température importantes existent pendant les mois d'hiver quand les sommets des thufurs sont gelés pendant plusieurs semaines et que les dépressions restent non gelées. Les données montrent que la microtopographie existante est un facteur important contrôlant la conservation et le développement des thufurs dans les environnements périglaciaires marginaux.
Article
Zusammenfassung: Die kr yogene Bodendynamik äußert sich in weiten Bereichen der arktischen Tundren Nordamerikas und Nordeuropas in Gestalt von Thufur und Palsas. Diese charakteristischen Formen der Bodenfrostwirkung in den arktischen und subarktischen Tundren, Wie­ sen und Heiden unterscheiden sich außer in Habitus und Dimension vor allem durch ihre Genese und Morphodynamik. Die Aufbeulung der Thufur geht auf kryodynamisch bedingte subkutane Substratumlage rungen zurück; die Aufbeulung der Palsas hingegen beruht auf der Vo­ lumenzunahme des Bodenwassers bei der Bildung von Segregationseis oder, seltener, Massiveis im Palsakern. Die Morphedynamik der Thu­ fur und Palsas wird im einzelnen untersucht, und zwar auch im Hinblick auf ihre aktuelle Intensivierung und geoökologischen Auswirkun­ gen. Summary: The main frost action features in large parts of the North American and Northern European tundras are thufurs and palsas. Thu­ furs (earth hummocks) and palsas as characteristic features of frost action in arctic and subarctic tundras, meadows and heath scrubs are dif­ ferent not only in their appearance and dimensions but also in their origin and morphodynamics. The swelling of thufurs is caused by sub­ surface material transported by cryostatic press ure; the rising of palsas originales in the volume increase of soil water during freezing into se­ gregation ice or sometimes into massive clear ice in the cores of palsas. The author analyzes the development of the thufurs and palsas in de­ tail, with emphasis on their actual dynamics and their geoecological implications. 1. EINLEITUNG Innerhalb der arktischen Periglazialzone werden rund 5 Mio km2 Fläche vom Landschaftstyp der Tundra eingenommen. Das Ökosystem der Tundra mit ihren niedrigen Zwergsträuchern. Gräsern, Moosen und Flechten wird geprägt durch geringe Wärmesummen im meist sehr kurzen Sommer und durch ein schar­ fes Frostregime im überwiegenden Teil des Jahres. Unter der Einwirkung einer besonders starken Frost­ intensität und bei großen Kältegraden, etwa bei Bodentemperaturen von unter -8°C, kommt es im Boden zu Kontraktionserscheinungen, aus denen sich Frostrißsysteme ergeben. Über wasserstauendem Perma­ frostboden sind deren Spalten meistens mit Eis gefüllt, so daß Eiskeilsysteme vorliegen. Diese Formen sind an besonders extreme Bodenfrostbedingungen geknüpft. Viel weiter verbreitet sind jene Formen, die an das saisonale Gefrieren und Auftauen des Bodenwassers und die damit verbundene Volumenverände­ rung des Bodenwassers um 9 Prozent gebunden sind.
Article
Hummocks (nonsorted circles) are widely distributed in arctic and subarctic regions. The hummocks under discussion are composed of fine-grained frost-susceptible soils; the late summer frost table is bowl-shaped; and the hummocks grade from those which are completely vegetated (earth hummocks) to those with bare centres (mud hummocks). The mound form is usually attributed to an upward displacement of material resulting from cryostatic (freeze-back) pressure generated in a confined, wet, unfrozen pocket of the active layer. Theoretically, cryostatic pressure should not develop in a frost-susceptible hummock soil, because ice lensing at the top and (or) bottom of the active layer will desiccate the last unfrozen pocket so that the pore water is under tension, not pressure. Field observations carried out at Garry Island, Northwest Territories, for 1965–1979 and for 1967–1979 at Inuvik, Northwest Territories, involving: summer and winter excavations; the measurement of the deformation of tubes, soil pressure, soil temperature, soil heave, soil moisture migration; and observations on hummock stability provide no field evidence for the cryostatic theory. An alternative model of hummock growth, based upon field observations, is here proposed. The upward displacement of material is believed caused by freeze–thaw of ice lenses at the top and bottom of the active layer with a gravity-induced cell-like movement, because the top and bottom freeze–thaw zones have opposite curvatures. The cell circulation is evident from the grain-size distribution of hummock soils and from upward-moving tongues of saturated soil observable in late summer. The most active period is in late summer. Model experiments in the laboratory have been successful in producing mounds by freeze–thaw of a kaolin slurry in a bowl-shaped container in support of the proposed theory.
Article
Frost heave, ground temperature and moisture were concurrently monitored on two alpine slopes with thin debris mantles. Electrical sensors connected to data loggers permitted automated monitoring of the three variables. Diurnal frost heave of up to 3 cm frequently took place during spring and autumn. Soil moisture contents are regarded as the primary control on the amount of frost heave, while the intensity of freezing is secondary. Heave was usually greatest just after a major rainstorm that produced high moisture contents in the near-surface soil, and decreased with soil desiccation. Rainstorms often interrupted the desiccation process and reactivated frost heaving. Despite reaching a depth of 1 m or more, seasonal frost penetration caused only small amounts to heave, because the lower part of the seasonally frozen layer was not frost-susceptible. These observations imply that the thin debris mantle and periodic precipitation during periods of freeze-thaw combine to result in the predominance of shallow frost creep, which is typical of soil movements in Japanese high mountains.
Article
The Lesotho Highlands contain some of the highest mountains in southern Africa and play an important role in the surface water resources of the region. Several features of the climatology and hydrology of the highlands are reviewed here. The emphasis is on the long-term response and the behaviour on an annual basis. Topics discussed include the variations in rainfall with elevation and location, the relationships between annual rainfall and river flows, and the variations over time in both rainfall and flow records. Results are presented in the form of updated versions of the mean annual precipitation and mean runoff coefficient maps for the region, and examples are presented for typical regression relationships between flows and rainfall data on an annual basis. Also, statistical analyses are described that suggest little evidence for any significant cyclical behaviour or trend during the observational periods for either rainfall or flow data. © 1998 Royal Meteorological Society
Article
When a young palsa has risen sufficiently above a mire (bog) and has developed steep sides, drifting snow collects against these sides and inhibits freezing of adjacent ground. Measurement of the temperature regime on the top surface, and at the foot, of the northern edge of a 2.6 m high palsa in Utsjoki, Finland supports this hypothesis. The minimum temperature recorded in the winter of 1992–93 on the uncovered palsa summit was −23.4°C. After snowfall, the temperature stayed at +0.1°C at the foot of the palsa for over six months, until 20 June 1993. At the edge of the palsa, thick snow cover prevents the establishment of permafrost.Quand une jeune palse s'est élevée assez au dessus d'un marais pour que ses flancs soient en forte pente, la neige remaniée par le vent est accumulée contre ses flancs raides et empěche le gel du sol voisin. Des mesures de température sur la surface et au pied de la bordure septentrionale d'une palse haute de 2,60m à Utsjoki (Finlande) confirment cette hypothèse. La température minimum enregistrée lors de l'hiver 1992–1993 sur le sommet découvert de la palse a été de −23,4°C. Après des chutes de neige, la température est restée a +0,1°C au pied de la palse pendant plus de 6 mois jusqu'au 20 juin 1993. En bordure de la palse, l'épaisse couverture de neige empěche la formation du pergélisol.
Article
Although a considerable amount of periglacial work has concentrated on thufur found in polar and subpolar regions, information pertaining to those in other areas, especially the alpine regions of Africa, is extremely limited. Field observations and measurements provide empirical information on their external and internal morphology in an alpine region of Lesotho, southern Africa. Animal activity, wind action, the effects of surface water and/or ice, and the influence of needle ice are factors which cause thufa breakup. Some thufur in the Mohlesi valley are inactive while others are periodically active under present climatic conditions. Quoique un nombre considérable de recherches aient été consacrées à l'étude des buttes gazonnées des régions polaires et subpolaires, les informations se rapportant à ces formes dans d'autres régions et spécialement dans les régions alpines de l'Afrique sont extrěmement limitées. Des observations de terrain et des mesures ont été effectuées sur la morphologie interne et externe de ces formes dans les régions alpines du Lesotho en Afrique du Sud. L'activité animale, l'activité du vent, les effets de l'eau de surface et de la glace, ainsi que l'action des [pipkrakes] sont des facteurs qui provoquent la rupture des buttes gazonnées. Quelques buttes gazonnées de la vallée Mohlesi sont inactives tandis que d'autres sont au moins périodiquement actives sous les conditions climatiques actuelles.
Article
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.
Article
This paper reviews and discusses the main mechanisms involved in cryoturbation processes and patterned ground. The arguments are based upon field, thermal, moisture and micromorphological data in relation to cryogenic fabric stability and local hydraulic conditions. Differential frost heaving appears to be the main mechanism of cryoturbation, but its action can be helped by cryostatic pressures, by differential swelling and probably also by load casting in poorly drained sites. Differential frost heaving can be emphasized if organics are present. Frost-susceptibility gradients provide an understanding of the geometry of deformations.Ce travail propose une revue et une discussion des principaux mécanismes impliqués dans la constitution des cryoturbations et des sols figurés périglaciaires. Les arguments proposés sont basés sur des observations de terrain, des mesures thermiques et de teneur en eau, et l'étude des microstructures; ces données sont mises en relation avec la stabilité de l'organisation cryogénique du sédiment et les conditions hydriques locales. Le gonflement cryogénique differentiel semble être le mécanisme principal de cryoturbation mais son efficacité peut être accentuée par les pressions cryostatiques, par le gonflement differentiel et probablement aussi par le 'load casting', ce dernier mécanisme ne jouant toutefois que dans les sites engorgés. La présence de matières organiques est susceptible d'augmenter le gonflement cryogénique. Les gradients de gélivité permettent de comprendre la géométrie des déformations observées.
Article
Thesis--University of Natal. Includes bibliographical references (p. 144-146).
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