Brixton Quartzite - Tower View EarthCache
Brixton Quartzite - Tower View
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Located in Henley Street below the Brixton Tower. These exposed Quartzite beds are easily accessible 24/7. Ensure you park in a safe space (I recommend towards the bottom of the rock outcrop looking over the SABC) as cars can speed around this corner.
The basal quartzite of the Brixton Formation of the Witwatersrand Supergroup is clearly exposed in this road cutting. The quartzite dips at 35° to the south and weathers to a pretty light brown colour. In the fresh rock the colour is a light green owning to the presence of a little fuchsite (a chrome bearing mica). Exhaust gases from passing motor vehicles results in a carbon deposit on the surface of the rocks and the primary colour is not seen. Perhaps take a small hammer with and you can break a small piece of rock to get to a fresh exposure.
The quartzite consists primarily of quartz and represents sand that was deposited here in a shallow sea about 2650 million years ago. In the course of time, and with increasing pressure and heat, the sand was eventually hardened to a quartzite. Today this quartzite is again being weathered and at the top of this road cutting one can see that the weathering is taking place to such an extent, that individual grains of sand are again being separated from each other.
If one walks up through the cutting, several interesting sedminetary structures can be observed in the quartzite. At the bottom, the quartzite is horizontally thinly bedded with poorly recognizable ripple cross-lamination. Thereafter follows a zone in which a side view of trough cross bedding (5 – 10 cm high) is exposed. The orientation of the cross bedding is mainly to the south, but a few have an opposite orientation, i.e. to the north. Still higher, a grit can be recognized with a front view of trough cross bedding orientated in an east west direction. It is overlain by thin units of flat laminated quartzite in which a small fold is present.
Since the quartzite and its sedimentary structures observed here were formed so long ago, how can we speculate that it was formed in a shallow sea? The answer lies in the resemblance between quartzite and sand structures deposited in modern seas. It is known that sea sand consists mainly of quartz, and it is also known that ripple cross-lamination formed as a result of the migration of sand ripples and trough cross-bedding, which in turn resulted from the migration of sand dunes under the influence of flowing water. The orientation of the cross bedding indicates the direction of flow. From this deposit, it is clear that a complex stream system existed with flow directions predominantly to the south, but also to the north or west. Such systems are typical of modern shallow sea areas where wave and tidal actions are responsible for flow in different directions.
The Brixton quartzite differs from modern sea sand in that shell fragments are absent. Crustaceans only developed around 500 million years ago, so that when deposition of the much older Brixton quartzite occurred, these animals were not present in the oceans. Flat lamination present near the top of the road cutting is known to form in very shallow water under high flow velocities, or on a beach, or in areas where the surf breaks. The presence of these laminations could also indicate depositions in a shallow sea. The fold in this layer was probably caused by subsequent earth movements.
(Acknowledgments: guidebook to Sites of Geological & Mining Interest on the Central Witwatersrand.; Geological Society of South Africa; 1986).
In order to qualify to log this cache, you need to answer the following questions and 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 Quartzite outcrop visible and include in your log [Optional].
2) Look at the exposed quartzite from the bottom to the top of the exposure. See if you can identify any of the structures or features described above. Include comments in your email – and photos too – if you found them interesting.
3) Look for the weathering at the top of the cutting, and explain what material is visible in the “new” sand.
4) What chemical is the main component of quartz?
Additional Hints
(Decrypt)
Erzrzore gb fraq lbhe rznvy!