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. Make sure that your vehicle is parked well into the yellow shoulder and beware of possible falling stones. Alternatively, find parking after the cutting.
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) While standing at the listed coordinates, what shape banding do you see near the top of the cutting on the opposite side of the road?
2) There is a blue kilometre marker board nearby. What is written thereon?
3) Name some uses for this clay.
4) How is silcrete formed?
5) In a few words describe how Liesegang banding is formed
As an optional request please take a picture of you and/OR your navigational device, round about the listed coordinates with Liesegang Banding of your choice in the background and post it with your log.
What you will see
In this cutting you see silcrete of the Grahamstown Formation and you see the highly kaolinised bedrock and Liesegang Banding.
Silcrete
The silcrete is a relatively recent Tertiary surface phenomenon, formed as the weakly acid ground water rises by capillary action, bringing with it minute amounts of dissolved silica, which leave a deposit as the water evaporates. Silcrete is a hard layer of secondary cherty rock formed by the upward movement of silica in solution and its deposition near-surface under dry conditions.
The rock below the silcrete has been intensely kaolinised and quite a conspicuous capping of silcrete has developed. This tells geologists that this is the African Surface of erosion. The silcrete layer, as found in this area generally, typically caps the plateau surface above Grahamstown.
Kaolin
Kaolin is a member of the clay mineral family, with a distinctive white colour, compact consistency and a slippery feel when wet; the result of extensive and long-lived chemical weathering of certain rock types; a common component of the peneplain soil surface horizons developed in the Dwyka tillite in the Eastern Cape. It was found that the thickness of the kaolin horizon varies considerably, but reaches 35m in places. Peneplain means extensive land surface of flat to very gentle relief formed by the long-lived erosion of land to its base level.
The development of extensive white kaolin, as a result of palaeo-weathering (old surface that has subsequently become buried) of Witteberg, Dwyka and Ecca sediments during the prolonged period of Tertiary peneplanation, has given rise to numerous good-quality kaolinite deposits, and resulted in a local pottery industry. It is also used as component of porcelain ceramics and as filler for paper and other products and is an important traditional white pigment and cosmetic.
Liesegang banding
Of particular interest is the flow of solutions through the rock that have produced spectacular Liesegang banding. This is the colour-banding resulting from rhythmic precipitation of material from solutions passing though sedimentary rocks, in this instance, the kaolin. This is due to movement of ‘waves’ of groundwater through the permeable rock, and along bedding and joint lanes. Over long periods of time, the water leaches soluble material – mainly iron – from the rock and carries it along in solution. Sooner or later the water evaporates or the chemical environment changes, the material is no longer stable in solution, and it precipitates. Water moves along a ‘front’, as though in a wave. If everything were equal, these fronts would be straight. However, there are subtle but important differences in permeability, so the front becomes distorted and sinuous. The result of all this may be either colour banding, sometimes in very beautiful, brightly hued patterns, or hardening alongside joint-planes, giving rise to the pitted, box-work appearance. Sometimes this banding can easily be mistaken for bedding.
The phenomenon was first noted over one hundred years ago (in 1896) by the German chemist Raphael E. Liesegang, and has aroused the curiosity of chemists for many years. It was first noted when he dropped a solution of silver nitrate on to a thin layer of gel containing potassium dichromate. After a few hours, sharp concentric rings of insoluble silver dichromate formed. When formed in a test tube by diffusing one component from the top, layers or bands of precipitate form, rather than rings.
Acknowledgments, sources 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
Wikipedia