The Rock Garden (Robinson Dyke) EarthCache
The Rock Garden (Robinson Dyke)
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Difficulty:
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Terrain:
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Size: 
(not chosen)
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This cache is located in suburban Joburg and actually forms part of
the front lawn of a private home. I’m glad I don’t need to mow the
grass!
These outcrops and boulders are part of the Robinson Dyke, one of the many sub-alkaline and femic (i.e. iron and magnesium rich) dykes belonging to the 1000 – 1400 million year old Pilanesberg Dyke swarm. This swarm extends in a northwesterly direction to the Pilanesberg Complex in the North West Province (around the Sun City / Lost City / Pilanesberg Reserve area) – {see Pilanesberg National Park Alkaline Ring Complex by cincol (GC18GG9)}. And then onwards towards Botswana. These dykes, which are commonly composite in nature, have dark femic margins of tholeiitic basalt or andesite grading into a more salic (acidic) centre, which contains finer grained, reddish-grey syenite or trachyte. The dykes intrude the approximately 3200 million years old granitic rocks of the Johannesburg Dome and caused the growth of the large feldspar crystals (porphyroblasts) and the reddish-pink iron discolouration
seen in the rocks flanking the dykes. This dyke can be traced intermittently from the old Robinson Deep Gold Mine in the Booysens area to well beyond the Northriding area – a distance of over 20km.
(Acknowledgments: guidebook to Sites of Geological & Mining
Interest on the Central Witwatersrand.; Geological Society of South Africa; 1986).
A dyke is a vertical igneous intrusion into older existing “bed rock” or country rock (i.e. rock that was already there. A horizontal intrusion is called a sill and a single “shaft” intrusion is called a pipe.
In order to qualify to log this cache, you need to answer the following questions and s=email the cache owner. Any logs not accompanied by an email will be deleted.
1) Take a photo of you and your GPSr at this spot with the outcrop visible and include in your log [Optional].
2) Look at the exposed rock (try find a “fresh” exposure and not a weathered one). Describe from the “crystal” or grain size whether this rock cooled quickly (like a dolomite or obsidian with small grains) or more slowly to produce larger grains.
3) Describe how you see weathering occurring. Explain how it occurs and especially what you see at the edges of the rocks compared with the middle.
Additional Hints
(Decrypt)
Erzrzore gb fraq lbhe rznvy!