Exploring Nature

The little coastal town of Sedgefield in South Africa's Garden Route is known to most travelers as a sleepy hollow lining the N2 with one traffic light to hinder your progress through it. If you don't blink you may even notice the wall of mosaic horses bordering the vets property. However, should you stop into the town and head down to the beach accesses at the lagoon mouth (Kingfisher Creek) or at Swartvlei Beach you will be treated to South Africa's southern most fossil dunes, and notably the highest vegetated dunes in South Africa, the highest towering at 215m.

Surrounded by the Indian Ocean and coastal lake system, Sedgefield is bordered by a series of parallel fossil sand dunes extending between Kaaimans River in the west and Brenton on Sea in the east. These dunes were formed over 2 million years ago and over time have been eroded to steep cliffs showing distinct layers and patterns caused by water, wind and sand erosion. Creating a dramatic buttress to the temperamental Indian Ocean, these cliffs offer nesting platforms for two colonies of Cape and White Breasted Cormorants and to those who are patient enough to witness the transformation, at sunset they change from a spectacular golden yellow to a deep, rich ochre.

During their formation in the Pleistocene, pieces of mollusk shell became mixed into the quartz sand grains as they were deposited into dune shapes. The hard nature of the dunes formed when rainwater seeped through the dunes, dissolving the calcium carbonate of the shell fragments and percolating into the sand lower down. During phases of minimum disturbance to the dunes, the dissolved material cemented the sand into hard and easily distinguishable layers. Looking at the cliffs flanking Myoli Beach and either side of Gerickes Point, the clear horizontal and diagonal layers seen on the exposed cliffs are testament to the different epochs of the dunes formation. The orientation of the dunes is also an indication that South westerly winds have been the prevailing winds in the region for more than 2 million years

The dune system comprises a series of three fossil dune ridges, or cordons, running parallel to the coastline. The southern most dune is submerged two miles out to sea and is a popular reef for skiboat fisherman. The central dune runs along the coastline forming the cliffs along the beach and reaches a height of nearly 204m, making it the highest fossil dune in South Africa. The third dune is easily seen as the cordon north of  Groenvlei with a remnant section in the west forming the island in Island Lake.

The dune system has been central to the development of the coastal lakes found around the outlying reaches of Sedgefield. During marine regressions, when coastlines retreated towards the continental shelf, erosion from flowing rivers cut through the softer and younger geological formations of the dunes. With subsequent marine advances and regressions, there has been a combination of erosion in the soft geology and deposition of sediment which has built up platforms and left depressions which have been filled as lakes. Thus the formation of these localized lakes is the direct result that combined climatic changes have on soft geology. Interestingly enough, the only other area where this effect can be found in South Africa is in the northern parts of Kwazulu Natal (St Lucia to Ponta de Oura).

Walking either east below the Myoli Dunes or west towards Gerickes Point, you will see evidence of recent dune slips where sections of the dune have broken free as a result of ongoing erosion to the dunes. The steep slopes of these fossil dunes is caused by the continuous sea erosion that undercuts the base of the dunes, transforming them from gradual sloping sides to dramatic cliffs. Careful inspection will reveal a red/brown layer (about 1.5m thick) punctuating the sand of the dunes. This layer varies in height above sea level and indicates an era when sediment was deposited over the then-existing foundation of the dune.