Soil flowed over the slopes of KwaZulu Natal in South Africa about 30,000 years ago

Several landscape processes cause the movement of soil over slopes. Erosion from overland water flow and sudden landsliding are among the best-known of these processes. Creep, the slow and depth-dependent movement of soil due to gravity is another landscape process (Heimsath et al., 2005). This vignette is concerned with the identification and occurrence of a fourth landscape process: solifluction; the flowing of saturated soil over slopes.

When solifluction occurs over a frozen underlying layer, the process is sometimes also called gelifluction. This is one of the reasons that the word solifluction has a strong connotation of very cold environments. Nevertheless, solifluction can occur everywhere where there is seasonal saturation of soil on slopes.

Recognizing solifluction

On the surface, active or fossil solifluction is easily recognized by the lobate forms that are produced. This also distinguishes the process from the slower and more uniform process of creep, which does not produce lobes.

It is altogether more difficult to identify ancient solifluction deposits that have been partly eroded and/or covered by other deposits that were caused by other processes. Two properties that relate to the flowing movement of solifluction can be used in such cases.

First, solifluction deposits are very badly sorted--i.e. very well mixed. In environments where sediments with different grainsizes are present, this will lead to deposits that contain these different grainsizes in almost random arrangement. These very well mixed deposits are called diamictons.

Second, in solifluction deposits clay minerals are preferentially arranged along the edges of sand grains. This is caused by the fact that the sand grains roll around inside the deposit while it is flowing. The arrangement of clay around grains is increased by repeated wetting and drying. Arrangement of clay around sand grains is called granostriation.

Granostriation can only be observed under a microscope. Fig. 1 shows a microscopic photo of a 20 micrometer aggregate of a solifluction deposit. The aggregate has been displayed such that clay has gold and orange colors and (much larger) sand grains show as gray and black areas. The aggregate displays very good mixing and some preferential arrangement of clay around sand grain. In other words, it is a diamicton with granostriation and thus has solifluction properties.

Solifluction in KwaZulu Natal

A study in the Okhombe Valley in the province of KwaZulu-Natal in South Africa (Figs. 2 and 3) found deposits that had precisely such properties. These deposits were situated in flat positions under slopes of up to 45 percent steep and covered under layers of other deposits. The slopes consisted of mudstone and sandstone, providing both the clay and the sand that was found in random arrangement in the deposits.

The deposits were dated to be between 50 and 30 thousand years old (Temme et al, 2008), a period in which the area was markedly colder and less vegetated than today. It was presumed that occassional soil saturation in the area occurred when snow melted in spring during this period, and the deposits were therefore called solifluction deposits. Although the area had been even colder in the last Ice Age (from 20.000 to 12.000 years ago), it had at that time also supported more vegetation, supposedly preventing soil from flowing.

Relevance for wider geomorphological discussions

Interpreting these deposits as solifluction meant giving a new angle to a lively discussion about landscape processes in the nearby Drakensberg mountains, also in the province of KwaZulu Natal. Although it is generally agreed that no glaciers existed in the Drakensberg in the last Ice Age, this discussion centred around the question whether there had been glacial conditions, and which--if any--processes were caused by those conditions (Boelhouwers and Meiklejohn, 2002; Grab, 1998).

The conclusion of that discussion was that conditions in the last Ice Age had not been cold enough to cause gelifluction. Due to the unclear meaning of solifluction as mentioned above, solifluction was also implicitly and wrongly added to the list of processes that were neither expected nor observed in the Drakensberg and surrounding areas. And by extension, it was not a candidate-process to explain landscape development in an even less cold period--between 30 and 50 thousand years ago.

Nevertheless, solifluction can occur everywhere where there is seasonal saturation of soil--and the deposits that it produces have really been found in Okhombe Valley, close to the Drakensberg. This realisation has re-opened the door for a wider appraisal of the role of solifluction in shaping the landscapes around the Drakensberg--although not anymore in a glacial context.

• Boelhouwers, J.C. and Meiklejohn, K.I., 2002. Quaternary periglacial and glacial geomorphology of southern Africa: review and synthesis. South African Journal of Science, 98(1-2): 47-55.
• Grab, S., 1998. Periglacial research in Africa: past, present and future. Progress in Physical Geography, 22(3): 375-384.
• Heimsath, A.M., Furbish, D.J. and Dietrich, W.E., 2005. The illusion of diffusion: Field evidence for depth-dependent sediment transport. Geology, 33(12): 949-952.
• Temme, A.J.A.M. et al., 2008. Climate controls on late Pleistocene landscape evolution of the Okhombe valley, KwaZulu-Natal, South Africa. Geomorphology, 99(1-2): 280-295.