We have put together a series of earthcaches along the N2 in the Eastern Cape and this is just one of them. We got really excited at the prospect of putting together these earthcaches after doing some research. Little did we know just how varied our possibilities were and how diverse the geology is in this province! We hope you enjoy them and learn a great deal of our beautiful part of the planet.
A word of caution however – this cache is not for children and extra care needs to be exercised when walking along this road as it is extremely busy. There is reasonable parking at the listed coordinates, but make sure that your vehicle is parked well off the road.
An Earth cache is a special type of Virtual Cache that is meant to be educational. Therefore to log a find you must demonstrate that you have learnt something from the site and experience.
Send your answers to us in an email via our profile page.
Any logs not accompanied by an email will be deleted.
Logging Tasks:
1) Describe the landform (scenery) you see on the land side of the N2.
2) What is the speed limit for this section of the road?
3) Given no other interference how far inland would these dunefields have crept in a century?
4) Looking at these dunes in the distance, which side is the slip-face and give the reason for this?
As an optional request please take a picture of you and/OR your navigational device, at the listed coordinates with the dunes in the background and post it with your log.
Please do not post any spoiler pictures.
Algoa Bay is the last of the ‘half-heart’ bays that characterise the southern coast, and also the largest, shaped by both the Swartkops River and the biggest river you’ll cross on the Cape south coast, the Sundays River. This bay, too, reached far deeper into the interior in the geological past and, like those further west, it has largely been filled with sediment washed off the mountains of the interior, on and off over the past 150 million years. The thickest of these sedimentary packages is the Uitenhage Group, deposited during the late Jurassic and early Cretaceous, around 140 to 120 millions years ago. On a lesser scale, sedimentation continues to this day – the beaches of Algoa Bay are sandy in contrast to the rocky coastline along the Tsitsikamma and north of Kenton-on-Sea. Long, sandy beaches in a windy environment mean sand dunes. Only a few kilometres from the coast, you’ll see the dunefield. To an extent, the dunes are vegetated, but by no means entirely, and flanking the mouth of the Sundays River you will find slopes of shifting, bare sand. From just east of Port Elizabeth there is a stretch of 100 km of modern Quaternary (2 million years ago to present) coastal dunes. The highest dunes in South Africa that relate to the present coastline are to be found in this dunefield and near Woody Cape the dunes rise to about 90m in height.
Coastal dunes and dunefields form where there is an adequate sand supply and sufficient wind energy to move the sand. The coastline of the Eastern Cape possesses both these attributes and hosts very impressive coastal dunes and dunefields.
Coastal wind energy in the Eastern Cape is high and dominated by the prevailing southwest wind, which is the main dune-forming wind in the region. Where the coastal configuration is such that the southwest wind blows offshore, the easterly wind is the main dune-forming wind. Coastal dunes are inextricably linked to sandy beaches, which are supplied with sand by waves and longshore currents. Thus, many large-scale coastal dunefields are located down drift of significant river systems.
Mobile dunes with coastal dunefields are orientated transverse to the prevailing wind with their steep slope (slipface) oriented downwind. Along the straight eastern coast, north of Cape Padrone, trains of transverse winds move along-shore in a northeasterly direction. These dunes generally abut on vegetated and/or fossil dune ridges that are aligned parallel to the shore and are referred to as buttress dunes.
Among the coastal dunefields of the southern coastal zone the Alexandria coastal dune field stands supreme. It is not only the largest of its kind along the South African coastline, but it is nearly untouched by human interference. The position of the Alexandria dune field along the northern shoreline of the log-spiral Algoa Bay is a direct consequence of the interaction between the local wind climate, the ready sand supply on the many kilometres of beach, and the configuration of the bay shoreline. The dunefield as developed at present is about 6,000 years old, and represents a steady accumulation of sand at an average rate of about 170 000 cubic metres per year. The landward edge of the dunefield is 30 to 40m high, and is encroaching the hinterland at a rate of about 25cm per year. The dominant wind that drives the dunefields is the southwesterly gales, but during summer easterly gales serves to temporarily reverse the westward march of the dune crests.
Acknowledgments and recognition
Geological Journeys by Nick Norman and Gavin Whitfield
A field guide to the Eastern Cape Coast by RA Lubke FW Gess & MN Bruton