A ridge too far

Equifinality, or convergence of form, refers to the concept in geomorphology where a similar landform can result from different processes. For example, a ridge can form in many different ways in nature. Equifinality can make it difficult to interpret a landscape to determine what happened in the past. It is possible to confuse landforms and come up with an interpretation invoking conditions that never occurred. However, in most cases, steps can be taken to make observations of a landform that identify the processes that formed it and avoid mis-identification. In two previous posts here and here, I talked about landforms in the “Dragon Mountains” of South Africa used to support an argument that permanent ice was present during the last ice age. In this third and final post I will present the last part of that story.

The surficial geology of Africa south of the Sahara is almost entirely composed of one unit called the Karoo Supergroup. The supergroup comprises a sequence of near-continuously deposited rocks, mostly sedimentary, formed over a period of 120 million years. The period of time covers the glaciation of the continent of Gondwana and ends with a gradual period of desertification and then an outpouring of basalt lava that forms the Drakensberg mountains. This termination corresponds to the same time period that dinosaurs walked in Scotland and ammonites swum in southern England. The geological units at the young end of the Karro Supergroup are exposed as layers where millions of years of erosion has cut deep valleys in the Eastern Cape of South Africa.

In the Bokspruit valley of the Eastern Cape, the youngest sedimentary unit of the Karroo Supergroup, the white Clarens Sandstone, lies underneath the lava flows of the Drakensberg Formation. The basalt is flat-lying in distinct layers that vary in mechanical strength. In the upper part of the valley there lies a rock bench that extends around the valley head (see model above). On this bench are accumulations of debris, some linear for short lengths, below a series of steep slopes. The ridge on the east-facing slope has been called the Killmore ridge by Lewis in 1994. Lewis claims that the ridge constitutes a protalus rampart or even a moraine. A protalus rampart, or more correctly a pronival rampart, is a ridge of debris that accumulates in front of a permanent snowpatch. The ridge builds up from debris sliding across the snowpatch’s surface. Therefore the snowpatch must be steep and overshadowed by cliffs.

In 2012 I visited the Killmore ridge with colleague Stephanie Mills to assess whether the ridge is of periglacial or glacial origin. At ~2100 m, the Killmore ridge is at odds with observations by Mills that the ice age snowline laid at much higher elevations in nearby Lesotho (>3200 m). Because a perennial snowpatch means that snow lies all year round, the snowline, or where glaciers form, is usually no more than a few hundred metres higher in elevation. In the British isles, these landforms often occur adjacent to moraines, such as on the Brecon Beacons, on slopes less favourable for snow accumulation. A low snowline must mean that much of the Drakensberg would have held permanent snow or even glaciers and there is no evidence for this.

There are a number of problems with this ridge being a pronival rampart1. First, it is a long way from the wall (about 200 m in places). Ice must be thick to reach the cliff and be steep enough for debris to slide down it to reach the ridge. For snow to reach the cliff, the maximum slope is only 15-20º, less steep than the 30º on the well-described ramparts in the Brecon Beacons. A shallow slope means blocks will not slide under gravity. Any steeper and the snow would be thick enough to form glacial ice, for which there is no evidence. The ridge is up to 17 m thick which is a red flag and indicates that snow behind the ridge would have been thick enough to start flowing under gravity. Second, the blocks on the ridge are oriented. Walking over the most distinct ridge, I noticed that many basalt columns had vesicles (bubbles from when the lava flowed), often infilled with zeolites. I made a makeshift plumb-bob using a boot lace and a rock and quickly saw that most of the blocks on the ridge were tilted away from the cliff. This is explained by a large section of cliff toppling and forming a ridge. Third, the ridge is fine-grained and up to 70% silt, with no loess in the area. By their nature, pronival ramparts are coarse and snow melt transports away fines into fans. Also, there is no evidence of glaciation (or snow patches for that matter) in more favourable higher elevation locations such as Ben Macdhui to the north (the highest peak in South Africa at 3001 m and home to an ephemeral ski field). Lastly, the bench on which the debris resides is geologically defined, not by environmental processes. It follows the boundary between layers of different competence and extends onto the opposite slope of the valley, which is unfavourable to snow accumulation.

It is hard to tell how old the bench or the ridge is. There was no point exposure dating here because the degree of weathering was great and there was no way this formed at the end of the last ice age. The blocks were deeply weathered compared to similar basalt we exposure dated as less than 20,000 years old at Mt Enterprise. However, it is clear that the debris has been accumulating for a long time along the bench and parts of it are ridge-like because of the way the cliff has contributed rock fall, landslides and topples. If an extraordinary explanation is sought for the formation of a feature it needs to have extraordinary supporting evidence. In this case, seeking evidence for low altitude ice went a ridge too far.

1. Mills, S.C., Barrows, T.T., Telfer, M.W., Fifield, L.K., 2017. The cold climate geomorphology of the Eastern Cape Drakensberg: A reevaluation of past climatic conditions during the last glacial cycle in Southern Africa. Geomorphology, 278, 184-194.

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