Lateroglacial landforms in subtropical high mountains: A case study from the Karakoram and Hindukush Mountains

Lateroglacial landforms are located at the lateral margins of the glaciers and are part of the glacial landscape systems (Iturrizaga 2001). The term "lateroglacial" defines the ice-marginal areas at the lateral sides of the glacier. The lateroglacial environments pass over glacier downwards into the latero-frontal and finally the proglacial environments. In principal, they can be found along any valley glacier, but they occur preferentially at valley glaciers in high mountain regions with sufficient debris supply areas and their associated forelands.

Lateroglacial landforms were investigated systematically as a specific geomorphological unit along 53 glaciers in the Karakoram and Hindukush (72°-79°E; 35°-36°N) in regard to their distribution, evolution and morphodynamics (Iturrizaga 2001, 2003, 2007). They are best-developed in these mountain regions, which show the highest concentration of large valley glaciers outside of the pole regions with glacier lengths of up to 72 km in combination with a surrounding high mountain relief of up to over 8000 m a.s.l.. Lateroglacial sediment complexes may attain a length of up to several to tens of kilometres and are referred to as "lateroglacial valleys". Lateroglacial valleys are ice-marginal depressions between the glacier and the valley flank and sometimes even show a drainage system over short distances. They are not true valleys in sensu stricto, but rather glacier-marginal discontinuous depressions. Where mountain spurs stand out against the glaciated main valley, the lateroglacial valleys may be interrupted. The topographical depression can be filled up with different types of sediments or the deposition of the sediments itself may even be the trigger for the formation of the lateroglacial valleys. These landform assemblages are classified as a specific geomorphologic unit of the glacial environment. One of their main characteristics of the lateroglacial sediment regime is the damming effect by the glacier, which results in a large-scale sediment storage along the glacier margins. The lateroglacial sediments are polygenetic landforms consisting of a variety of different debris source areas.

Besides their large horizontal distribution, they are spread over a considerable vertical range in the Karakoram and occur between an altitude of about 2500–5000 m a.s.l.. The upper limit of the occurrence of lateroglacial sediments deriving from processes of glacial deposition is theoretically given by the altitude of the equilibrium line altitude (ELA). Due to the fact that a major part of the glaciers are avalanche-fed glaciers and therefore show steep inclined head walls of up to 3000 m in height, the distribution of lateroglacial sediments starts usually 1000–1500 m lower than the ELA.

The transition from the valley flank to the glacier has in different morphological variations. One of the most well-known and prominent landform is the lateral moraine valley. It is a mostly linear depression or sediment assemblage between the lateral moraine and the valley flank. It had been often confused with an ablation depression (see below) or commonly referred to as "ablation valley". Visser (1938) carried out the first systematic investigation on these landforms and postulated an insolation-controlled distribution of all lateroglacial landforms. This terminology has been widely criticised by many authors (v. Klebelsberg 1938, Hewitt 1993). Consequently, the non-genetical expression "lateroglacial valleys" has been introduced (Iturrizaga 2001).

The formation of lateral moraine valleys is mainly a result of a) dumping processes of the lateral moraine against the valley flank, especially during times of a comparatively smaller extent of the glacier, and b) different type of debris inputs from the adjacent tributary valleys and valley slopes (fig. 1 & 2). Their width attains a size of up to 1 km. The great dimension already indicates that ablation processes can only play a subordinated role in their formation. The lateral moraine is one of the most distinct depositional landform in lateroglacial environments. They may attain a height of about 250 m. Their large size has been attributed to repetitive glacier advances and accordingly to various deposition processes. The lateral moraine is often composed of an older moraine core, which has been superimposed by younger moraine layers and/or processes of moraine accretion. The time period of their formation goes in general back to the Neoglacial and Little Ice Age. Once the lateral moraine has been built up, it prevents the direct debris transfer from the glacier to the interior of the lateroglacial valleys. Consequently, the sedimentary system in the ice-marginal depression is well protected from glacial activities, unless some over spilling or break through of the lateral moraine takes place. In addition to that, the lateral moraine impedes the drainage of the tributary rivers into the glacier system.

The lateral moraine valleys are distinguished into two principal types: a) The V-shaped lateral moraine valley, in which the lateral moraine is directly connected with the adjacent valley flank (fig. 3). These landforms develop by dumping processes occurring at the glacier margin. b) The lateral moraine valley with a valley bottom floor (fig. 4). The incorporated sediments are composed of heterogeneous debris sources:

1. Primary processes of rock disintegration such as ice avalanches and freeze-thaw processes as well as glaciofluvial sediments from the main and tributary glaciers provide debris for the formation of lateroglacial sediments, especially for the formation of the lateral moraine.

2. A considerable part of the debris supply for the lateroglacial sediment complexes derives from the tributary valleys, in particular at glaciers framed by highly dissected mountain relief (fig. 1). The sediment cones, such as alluvial fans, debris flow cones and avalanche cones drain towards the glacier either into an existent ice-marginal depression or even onto the glacier. Especially large-sized, catastrophical debris flows can even initiate the formation of a lateroglacial valley. Rockslides, debris flows and snow avalanches, deposited into a lateroglacial valley, frequently dam lateroglacial rivers and cause the deposition of lacustrine sediments. As a result a considerable proportion of the lateroglacial sediments is of non-glacial origin. This fact has to be taken into consideration regarding glacier reconstruction in recent non-glaciated mountain valleys. Relict lateroglacial valleys occur as high as 1000 m above the present glacier surfaces and are important landforms for reconstructing the Pleistocene glacier thickness.Dendritic glaciers in which the individual tributary glaciers recede and loose the contact to the main glacier are prone to the formation of lateroglacial sediment complexes.

3. The lateroglacial sediment landscape is built up to a great extent by the secondary debris supply in form of the reworking of older glacigenic deposits mantling the valley flanks (fig. 2). Many glaciers show a close interfingering of Late Glacial slope moraines and younger lateroglacial landforms. After the gradual down-melting of the Pleistocene glacier surface, moraine deposits remain along the lateroglacial margins, partly as terraces, partly as amorphous deposits and are dislocated into the lateroglacial valley by different types of mass movements.

4. In some parts, the sediments are deposited in form of small scaled lateroglacial sandar. These are glaciofluvial deposits which originate directly from the melt water of the main or the tributary glacier. They are located between the glacier and the valley flank or the lateral moraine. After the deglaciation, these sediments are preserved as kame terraces.

The formation of the lateroglacial valleys might be triggered by the formation of an ablation depression as it can provide initial sediment traps (Iturrizaga 2003). The ablation depression is a lateroglacial landform which is a void located between the valley flank and the adjacent glacier (Oestreich 1906, Visser 1938) or between the sediment covering the valley flank and the glacier. Its formation is a direct consequence of insolation and reflection effects by the bedrock or the sediment (fig. 5). Due to the heating of the rock surface and the subsequent emission of long-wave radiation, the glacier ice is melting back at its margins forming a striking void. The width of ablation depressions varies from some meters to decameters. Their optimal distribution area is located along glaciers in subtropical latitudes with high rates of insolation close to the solar constant and low rates of humidity promoting a high transparency of the air. They may also occur in high latitudes, but they are smaller in size. However, the overall distribution of lateroglacial sediments is dominated by topographical factors rather than by insolation.

Iturrizaga, L. (2001): Lateroglacial valleys and landforms in the Karakoram Mountains (Pakistan). In: GeoJournal, Tibet & High Asia (VI), Glacio-geomorphology and prehistoric Glaciation in the Karakorum and Himalaya. Vol. 54 (2-4), 397-428.

Iturrizaga, L. (2003): The distribution and genesis of lateroglacial valleys in the Karakoram Mountains (Pakistan). In: Zeitschrift fuer Geomorphologie N.F., Suppl.Bd. 130, 51-74.

Iturrizaga, L. (2007): Die Eisrandtaeler im Karakorum: Verbreitung, Genese und Morphodynamik des lateroglazialen Sedimentformenschatzes. In: Geography International, Vol. 2, Shaker Verlag, Aachen 389 pp.

Klebelsberg, R. v. (1938): Visser's Karakorum-Glaziologie. In Zeitschrift fuer Gletscherkunde, Band 26, Heft 3/4, 307-320.

Oestreich, K. (1906): Die Taeler des nordwestlichen Himalaya. In: Ergaenzungsheft Nr. 155 zu Petermanns Geographische Mitteilungen, 106 pp. Gotha, Justus Perthes.

Visser, P.C. (1938): Glaziologie. In: Visser, Ph. C. & J. Visser Hooft (Eds.): Wissenschaftliche Ergebnisse der Niederlaendischen Expeditionen in den Karakorum und die angrenzenden Gebiete in den Jahren 1922, 1925 und 1929/30. Band II, 215 pp.