Bridge Over Troubled Waters EarthCache
Bridge Over Troubled Waters
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The cache is located in the Braamfonteinspruit, so please be extra careful during flood and high flow times. Parking is available on a number of sides (Victory Park or Parkhurst). A nice play area for kids, but supervision required.
The medium to coarse grained granodiorites exposed in the Braamfonteinspruit here, form part of the Archaean granite-greenstone basement of the Johannesburg-Pretoria dome. They are approximately 3200 million years old (compared with the 3500 million year old greenstones found in the Ancient Linden Earthcache nearby). The granatoid rocks in the stream form part of a large family of igneous rocks (i.e. rocks directly resulting from magma and lava from the inner parts of the earth) and are often collectively known as granites. Granites however include the likes of; granodiorites, adamellites and granites, which together constitutes the major part of the continental crusts (i.e. continents and land) that underlies most of the former Transvaal Province (i.e. Gauteng, Mpumalanga, North West and Limpopo Provinces). This suite of rocks is younger than the tonalitic gneisses approximately 1km upstream (A Really Gneiss Earthcache!) and although the contact between the two can not be seen, there is good reason to believe that the granodiorites were actually intruded into the tontalites a few million years later. Where similar granitic and tontalitic rocks are exposed close together, particularly in the region of Barberton in Mpumalanga and Swaziland, it can be clearly demonstrated and seen that the younger suite of rocks (these that are visible here) are derived by the partial melting of the older gneisses.
This younger granodiorite-adamellite-granite suite is considered to have caused the development and stabilisation of the continental crust in southern Africa. In later geological times, this granitic continental crust acted as a basement on which consequent sedimentary rocks (i.e. those caused by material being weathered and transported then deposited in a new location to forma new rock like sandstone) were deposited. This started with the Witwatersrand Supergroup (many of the other Earthcaches in Joburg are based on these rocks – e.g. Brixton Quartzites).
The granodiorite consists of quartz (glassy looking crystals); microline (pinkish crystals); plagioclase (white crystals) and biotite (black flaky crystals). Unlike the tonalite gneiss (A Really Gneiss Earthcache!) the granodiorite is not foliated but massive (i.e. not breaking into layers – but a solid chunk) and therefore must have formed after the tonalitic gneisses were deformed. Also evident are very coarse grained pegmatite veinlets (a hard whiteish streak that crosses the main rocks); altered amphibolite inclusion (also often called xenoliths) (amphibolite is similar to the granodiorites – but you’ll notice an exclusion of the glassy quartz and they are generally slightly larger grained with a black and white {GO SHARKS} color) and crush zones or mylonites (these are small – but really interesting and sometime very pretty – rocks found at the zone between two rock zones where large shear or pressure forces occurred during formation – keep an eye open for these difficult to spot bits) which were caused by intense shearing and micro brecciation (i.e. breaking up of the massive rock to form these metamorphosed inclusions).
(Acknowledgments: guidebook to Sites of Geological & Mining Interest on the Central Witwatersrand.; Geological Society of South Africa; 1986).
NOTE: This is a high muggle area – so please be aware of muggles while you are doing this cache! Also – although there is a wonderful little playpark here for the kids, it is right next to a river – so smaller kids need to be supervised – and older kids during the wetter months when the flow is high. Do Not enter the river area if there high flow of water.
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 an interesting aspect of these rocks visible and include in your log. [Optional]
2) Look for some of the interesting aspects of site that are mentioned in the text above and describe (or photograph) – or both – and include in the email.
3) Look at the sand on the riverbank – see the start of a sedminetary rock. Descibe what you see and how it many have formed.
4) If you have done the “A Really Gniess Earthcache!” just upstream. Please describe how the 2 rocks types are different (tonalite vs. granodiorite).
Additional Hints
(Decrypt)
Erzrzore gb fraq lbhe rznvy!