Ice in the Dragon Mountains (I)

One of the biggest controversies in southern African geomorphology has been whether the Drakensberg were glaciated. The Drakensberg are the eastern part of the Great Escarpment, a 3500 km long series of cliffs and mountains that separate the inland plateau from the eroded coastal plain. It is the longest such escarpment in the world and produces spectacular scenery. Drakensberg means “Dragon Mountains” in Afrikaans, sounding like it belongs more in Middle Earth than Africa. It has been lost to time as to why the mountains where given that name by the early Dutch. There are several competing theories including the resemblance of the escarpment to spines on a dragon’s back, or because of local legends of dragons living in the mountains fuelled by findings of dinosaur fossils or simply because they were the last area to be mapped (“Here be dragons“).

The Drakensberg typically range 2000-3000 m, the highest mountains in southern Africa. Snow falls on the highest peaks in winter and raises the possibility they were glaciated during the last ice age. This option has been explored by several geographers including Lewis and Illgner in 2001. They described geomorphological evidence for the presence of a glacier on the east side of the escarpment below Mt Enterprise (see map below). The evidence cited includes three moraine ridges, striated clasts and till. Glacial landforms are an excellent insight into climate change, providing ways to estimate both temperature and precipitation in the past. The authors estimated that mean temperature must have been a massive 17 °C colder than present to form a glacier in this location. Very few places on Earth were this much colder during the ice age, restricted only to sites close to the ice sheets or where major warm currents no longer flowed in the ocean. However, the paper is well written, published in a respected international journal and passed peer review, so their findings are worth taking seriously.

In 2012 I travelled to the Eastern Cape with colleague Stephanie Mills to revisit the evidence at Mt Enterprise and provide ages for the landforms using exposure dating. We first turned our attention to the ridges. At the northern end of the innermost ridge at a small quarry I found exposed ignimbrite and tuff, which are pyroclastic rocks (those deposited during explosive volcanic eruptions). At the southern end of the ridge Lewis and Illgner described basalt, but the road floor here cuts through ignimbrite also, but with dipping beds suggesting the ridge was underlain by a giant rotated slap of rock. The road cut through the ridge on its upper side exposing the layers inside it. Surprisingly there were three layers not described in the paper. We agreed it was diamicton (a mixture of very coarse and fine sediments) with angular blocks but couldn’t find any striated clasts or fine-grained till typical of the crushing pressures below a glacier. The bottom layer was ignimbrite which was deeply weathered with concentric layers like onion skin, indicative of great antiquity. The upper two layers represented two distinct layers with at least one buried soil in between. The soil indicates the passage of a large amount of time between the deposition of the upper two units. Most importantly, this mean the ridge’s origin was not a single event but a series of events depositing the various layers. The outermost ridge has a similar number of units, with two distinct diamictons. The published type section for the outer ridge is actually not located on the ridge but outside the mapped glaciation. We had some trouble finding it since the road is in a different place on the article’s map and the scale is off by 50 %.

The authors dismiss the possibility of mass movement as the origin of the ridges. However, a glacial origin does not bear out against the observations. The ridges are underlain by rock and the upper layers are composed of laterally continuous units typical of debris flows. None of the sediments is diagnostic of till or glacial sediments, nor are there any waterlain sediments indicating the presence of meltwater. Rough striations as depicted in the paper can be produced during mass movement transport. There is no cirque or evidence of glacial erosion behind the ridges, identified by us or the authors. There is no evidence for glaciation where snow lies higher on the adjacent peaks, where there are more suitable sites to accumulate snow. Also absent are periglacial landforms, those indicative of snow, frost and ground ice, on the adjacent slopes whose presence would support the temperature drop. The authors attribute the unlikely position of the glacier as being due to snowblow. Marginal glaciers can form where accumulation on the ice is magnified by wind blowing snow off the windward side of a mountain range and redepositing it in the lee. Even with this, the authors estimate that mean temperature must have been unrealistically cold compared to other studies in southern Africa.

We collected samples for exposure dating from boulders embedded in the youngest layer of the surface of all ridges1. The oldest ridge dates to within the last ice age. It seems likely that during this phase of cold, snow would have persisted longer in the landscape and at lower elevations. Higher pore water pressures could have led to a debris flow. The layers under this one must be considerably older. The two younger ridges formed after the ice age: 11,000 years for the flow on the inner ridge and 7,000 years for a ridge composed of blocks inside that. It turns out that this part of the Dragon Mountains is a constantly evolving landslide complex.

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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