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 on the western side of the cutting, but this parking is out of view from where the earthcache is. We opted to park our vehicle on the eastern side of the cutting eastward bound and parked as far off the road that was possible.
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) In your own words describe how to identify Dwyka tillite.
2) How did these rocks get here?
3) On the southern side of the cutting there is a fence pole on the edge. Describe the veins that can be seen running vertically below it?
As an optional request please take a picture of you and/or your navigational device with a clast in the background and post it with your log.
Please do not post any spoiler pictures.
During the Carboniferous Period (about 300 million years ago), Africa as part of Pangea was located at much higher latitudes and experienced colder climatic conditions. Southern Africa, as we know it today, was an ice sheet dominated by glaciers during these times. These ice sheets migrated towards the south and south-west from what is today Zimbabwe and Botswana. En route these ice sheets ripped up pieces of rock from the floor of the glacier and carried a multitude of rock types over thousands of kilometres.
Inevitably, the continents moved to warmer latitudes, resulting in climatic warming. The ice sheets began to melt and released all of the rock material that had been consumed by the glacial fronts. In modern glaciers, the dumping of material from retreating glaciers is called “terminal moraine”. This terminal moraine during the Carboniferous Period became cemented to form a rock type called “tillite”. Even while the southern part of the basin was filling, the mountainous hinterland feeding it with sediment was convulsed by phase after phase of folding, as the Cape Fold Belt was formed.
The tillite deposits in our end of the world are referred to as the Dwyka Group rocks of the Karoo Supergroup. What you see in the Grahamstown area is the infolding of a wedge of Dwyka tillite at the base of the Karoo, into the topmost sediments of the Cape, an intimate intermixing of these two sedimentary sequences.
Tillite is a grey-brown (bluish-black when fresh and brownish tan when weathered,) fine-grained rock mass with numerous small foreign clasts of various rock types and colours. Many of the clasts are polished (have a smooth surface) due to the friction when they were picked up by the moving ice-sheets. The scattered glacial erratics of all rock types embedded in the hard, dark matrix make this rock easily identifiable. Dwyka Group rocks are fairly easy to spot in the veld, as they weather to form an irregular landscape that looks like tombstones – so called “tombstone weathering”.
The featured picture was taken about 55 paces west of the 95,2E blue kilometre marker board. We mention this fact as satellite reception was extremely poor at the site in this cutting.
Note the clast in the top left of centre in the second picture and the missing clast in the lower left corner
In this cutting the tillite is more or less the most easterly there is to be seen and the tail end of the folded mountains. Tillite outcrops can be seen in many places, amongst others in quarries around Grahamstown and at the southern entry in Ecca Pass.
Tillite makes good aggregate and crusher rock; its fair adhesion to bitumen and resistant to abrasion make it a useful top coat on tarred roads. It is a poor aquifer, much of its ground water tending to be very salty or brackish.
Acknowledgments and recognition
Field Guide to the Eastern and Southern Cape coasts By Roy Lubke, Irene De Moor, I De Moor
dwaf Albany Coast
Addo Geology