Overberg Geoscientists Group
3,5 Mya
Formation of Wave cut platform
The rise and fall of sea-levels called eustasy have been known over centuries. Either the land rises or falls, because of earth forming process, or the sea rises or falls as glaciers melt or the polar ice caps expand as ocean waters turn into ice. Evidence of this rise and fall of sea-level can be seen all along our coastline, coastal plains and mountain foothills. When sea-level rises above its base level, strong wave action can erode coastal areas. When sea-level drops, pulls back and retreats, higher lying wave-cut platforms, seen as regular steps in the coastal topography, are left behind. By studying these we can get a better understanding of global warming, climate change and present-day sea-level rise.
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The elevated wave-cut platforms present along the Betty’s Bay, Hermanus, Gansbaai, L’Agulhas, Arniston coastline and the Rûens topography tell the interesting history of the evolution of the Southern Cape coastal plain. Over the past 65 million years (Ma) landscape evolution and sea-level fluctuations shaped the Southern Cape coastal plain.
The coastal plain is bordered offshore by the adjacent Agulhas Banks - a broad (~200 km wide), current-swept continental shelf. Offshore of Hermanus various geomorphological features have been identified in Walker Bay that represent remnants of a drowned terrestrial landscape. During most of the past 500 thousand years the sea level has been significantly lower than it is at present. This previously exposed the now submerged terrestrial ecosystem of the current continental shelf, including the now ‘drowned’ Agulhas Banks off the southern-end of the African continent.
Onset of the rhythmic Quaternary glacial-interglacial climate cycles, shorelines have shifted between a maximum depth of 130 meter below and a maximum elevation of 13 meter above present-day sea-level approximately every 100 000 years since 900 000 ago. During most of this time (~90 %) sea-level has been significantly lower than it is at present, exposing a now submerged terrestrial ecosystem of what is now the continental shelf and drastically altering the landscape.
Several coastal towns in the Overstrand settled on flat areas near the sea, in cases with steep mountains on the landward side. These can be seen from The Strand right through Betty’s Bay, Hermanus, Gansbaai, Struisbaai and Arniston. The wave cut-platforms are up to 30 meters above the present sea level. Created when the sea level was at least 30 meters above present.
Along the coast to the east of Betty’s Bay, particularly on the left (east) bank of the Palmiet River Estuary, immediately west of Kleinmond, a +15 to +18 m wave-planed terrace from the last Interglacial 120 000 years ago can be seen. At Kleinmond, an older upper terrace of +30 m can also be seen. A younger lower +6 to +8m narrow wave-cut terrace can be followed along the coast in Betty’s Bay.
The above sequence of events ties in with the geomorphic development of the Southern African subcontinent following the fragmentation of Gondwanaland.
Further Reading.
Botha, G.A., 2006. The coastal Cenozoic deposits. In: The Geology of South Africa. Geological Society of South Africa and Council for Geoscience. Editors: M.R. Johnson, C.R. Anhaeusser, R.J. Thomas
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Cawthra, H.C., 2014. The marine geology of Mossel Bay, South Africa. University of Cape Town PhD thesis.
Malan, J.A., 1990. The stratigraphy and sedimentology of the Bredasdorp Group, Southern Cape Province, South Africa. University of Cape Town MSc thesis.
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Malan, J.A., 2023. South coast geology – Stanford, Agulhas, Arniston, De Hoop, napier, Caledon excursion. Filed trip guide. Overberg Geoscientists Group.
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Roberts, D.L., Karkanas, P., Jacobs, Z., Marean, C.W., Roberts, R.G., 2012. Melting ice sheets 400,000 yr ago raised sea level by 13 m: Past analogue for future trends. Earth and Planetary Science Letters 357–358, 226–237.