34 Mya
Red Talus along Clarence Drive

Red Talus, Scree Slopes and Boulder Beaches along Clarence Drive

Introduction and Location. The 20 km coastal stretch of the R44 between Gordon’s Bay and Rooi Els, referred to as Clarence Drive, is recognised as one of the world’s scenic coastal drives. Apart from the towering cliffs of Table Mountain Group arenites are the extensive exposures in road cuttings of red-stained talus material being part of the steep scree slopes. South of Kogel Bay a road cut of an earlier raised boulder beach can be compared to the present-day boulder beach at sea level.

Geology and Lithology. The talus deposits contain matrix-supported sub-rounded to angular pebbles, cobbles and boulders set in a red to orange coloured sandy matrix. Present are small and medium size boulders with the longest axes of the largest boulders 500mm to 530mm. The broken rock fragments along the steep bedrock slopes of Table Mountain Sandstones, in the case of the Peninsula Formation, were formed by gravitational falling, rolling and sliding. The matrix of the talus deposits is dominated by medium to coarse sand. The red staining comes from the oxidation of iron minerals that have been washed into the scree material. The talus deposits do not contain material as large as the present-day boulders which lie on top measuring up to 1.5 metre in size. The talus slope at the south end of Clarence Drive at the village of Rooi Els has a +30° angle of repose.

Raised Boulder Beach. Sea level transgressions (rise) and regressions (fall) during cycles of warmer (inter-glacial) and colder (glacial) periods are well described over the past 40 million years in the Cenozoic Era. Evidence of different weathering rates during some of the Cenozoic sea level high and low stands can be surmised from the layered talus deposits along Clarence Drive. In places there are stratified layers of alternating fine and coarser material, probably produced during varying climatic conditions. Exposed in the road cut south of Kogel Bay we see a raised boulder beach located 30 metres above the modern-day boulder beach being shaped and reworked by the present-day sea level. This +30-metre elevation can be correlated to the wave-cut platform seen from The Strand-Gordon’s Bay right through Betty’s Bay, Hermanus, Gansbaai, Struisbaai and Arniston.

Age. The absolute age of the talus deposits is unknown and remains open for debate. The +30m raised boulder beach south of Kogel Bay is seen encroaching into the talus slope and must therefore be younger than the talus deposit. The boulder beach was deposited during an interglacial highstand when the sea level was at least 30 metres higher than the present day. Sea level highs of 30-metre elevations occurred several times in the past e.g., during the Late Miocene to Early Pliocene (5.3 - 3.4 million years ago) and as recently as in the Early Pleistocene (1.8 Ma) hyperthermal (warmer) period.

The talus slopes could have been formed during any of the above events following retreat of the sea level with broken rocks, debris and rubble falling down from the Table Mountain Sandstone sea-cliffs produced by normal processes of erosion. Based on earlier work the talus slopes could have been formed as far back as 34 million years ago when the sea level was a great deal higher. This was followed by the major drop in sea level during the Late Oligocene low stand 30-25 million years ago. Then again, the talus and scree slopes along Clarence Drive could have been formed as recent as 3.4 million years ago or even younger when sea level dropped by 100 to 130 metres with the sea retreating far south close to the shelf edge of the present-day Agulhas Bank.

34 Mya Red Talus along Clarence Drive

Red Talus, Scree Slopes and Boulder Beaches along Clarence Drive

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34 Mya
Red Talus along Clarence Drive