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. Be sure to park well into the shoulder as this road can be very busy.
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) At the listed coordinates what are the small pieces of dark rock in this dyke?
2) Also in this dyke what rock is mottled and coarsely crystalline and from the grain size state whether this rock cooled quicker or slower than the darker rock.
Take a walk to the second site and examine this dyke carefully as you will need to answer the questions hereunder. You find this site at S32 53.124 E027 28.597.
3) At the second waypoint how does this dyke differ from the first one?
4) What rock has intruded and made this dyke?
5) At the second waypoint estimate the width and the height of this dyke.
6) Would you say that this is a regular phenomenon and give your reason?
As an optional request please take a picture of you and/OR your navigational device, at the listed coordinates, and post it with your log.
Please do not post any spoiler picture of the second waypoint.
GEOLOGY
The break-up of Gondwanaland (an extensive southern super continent composed of Africa, South America, India, Australia and Antarctica), which occurred towards the end of the Karoo period in Jurassic time (+/- 170 million years ago), caused faulting, uplift and volcanic out-pourings and resulted in intrusion of dykes and sills of dolerite.
This region is predominantly of the Beaufort Group, which is made up of mudstones and sandstones, interrupted only by the Karoo dolerite that cuts through it. The first outcrop of dolerite is visible about 15km west of King William’s Town, which is seen around S32 56.793 E027 18.134. There is much dolerite in this region onwards through to the other side of Kokstad, occurring mostly as big sills (horizontal) and inclined sheets and only very occasionally as dykes (vertical).
Dolerite
Dolerite is an igneous rock (see hereunder) formed below the Earth's surface, a form of basalt (extrusive volcanic rock), containing relatively little silica (found in nature as sand or quartz). Dolerite is a medium-grained basalt and forms in shallow intrusions, such as dykes, which cut across the rock strata, and sills, which push between beds of sedimentary rock. When exposed at the surface, dolerite weathers into spherical lumps.
Igneous Rocks
Igneous rocks are derived from deep within the Earth’s crust or upper mantle, where they arise as molten magma. This either reaches the surface as volcanic eruptions, producing extrusive lava, which cools very quickly or, alternatively, settles at a level just below the surface. If the molten magma reaches a level close to the surface, it cools relatively quickly, particularly if it intrudes as a thin body, resulting in an intrusive rock nearly as fine-grained as surface lava. If, on the other hand, a large volume of magma intrudes at depth where it is insulated from the atmospheric chill, it has time to grow easily discernible crystals of the rock-forming minerals, and becomes coarser grained. The most conspicuous examples of fine-grained intrusive rocks in South Africa are vertical, or nearly vertical – dykes and horizontal sills of Karoo dolerite.
The distribution of Karoo dolerite
The southern strip of South Africa, with its history of folding and thrusting and mountain building, did not favour the intrusion of dolerite, which was happening on a large scale over the whole of the rest of the Karoo Basin: the rock formations were too tightly packed, with less of a regime of tensional cracks opening up than was prevalent further north. Near to KWT you are almost out of the fold belt.
Identifying Karoo dolerite
· massiveness, meaning it shows no internal structure like bedding;
· colour – dark grey to almost black, on a fresh surface, sometimes with the crystalline texture visible;
· rounded shape (may also weather spheroidally) – boulders on surface;
· very conspicuously red soil that forms over it; and
· positive topographic expression, forming ridges and cappings.
Other rocks, especially some of the sediments in this part of the world, also weather spheroidally and have to be scrutinised quite carefully before being certain they’re not dolerite.
The first site
At the listed coordinates there is a pale dyke-like feature intersecting the medium-grey intrusive rock. Take a closer look as it shows an unusual reversal of geological roles, where a sedimentary rock has intruded into an igneous rock. If the sediment is not fully lithified when the dolerite comes rumbling hotly through it, the heat is transferred from the dolerite to the sediment. This renders it ‘plastic’ and therefore capable of flow. The dolerite contracts slightly as it cools and shrinkage joints may form, into which the sediment can flow.
The dyke is nearly a metre thick and has a blurred contact with the dolerite. It shows, in addition, inclusions of two kinds of dolerite in the sandstone: one dark and very fine-grained, the other mottled and coarsely crystalline. The result is an extremely ‘mixed-up’ rock; a sediment that has not only intruded an intrusive rock, but has collected fragments of the ‘host’ dolerite in doing so.
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
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