Helderkruin Earthcache EarthCache

This is an easy earthcache in a beautiful little park..

This diagrammatic north-south cross section through the Witwatersrand ridge/plateau, below the Johannesburg city center, shows only the major subgroups of rocks, each of which is made up of many separate layers of varying composition. The harder layers form the hills, and the softer layers the valleys. The oldest rocks of the Witwatersrand Supergroup form the Orange Grove Quartzite layer. This forms the escarpment in the north, from which the Witwatersrand derives its name. The granite to the north of the Orange Grove Quartzite escarpment is around 3200 million years old, and is an exposed portion of the underlying Kaapvaal Craton.

"Witwatersrand NS cross section" by Oggmus

At this spot what you are able to see here is some of the Orange Grove Quartzite layer.

The Kaapvaal Craton, one of the first microcontinents to form on Earth through plate tectonics, was assembled about 3900 million years ago. About 3000 million years ago local cooling of the underlying asthenospere caused subsidence of the south-eastern portion of this microcontinent below sea level. The floor of this newly formed “Witwatersrand sea” consisted of smoothly eroded granites. Sandy sediments brought in by rivers from the north started being deposited on the granite about 2970 million years ago. This sandy layer eventually became compressed to form the Orange Grove Quartzite, the lowermost layer of the Witwatersrand Supergroup. This quartzite formation forms the escarpment from which the Witwatersrand derives its name

The Orange Grove Quartzite also forms the backdrop to the Witpoortjie Waterfall where the Crocodile River cascades over a 70m high drop, just north and slightly east from here.

The Witwatersrand plateau consists of a 5000- to 7000-metre-thick layer of mainly sedimentary rocks laid down over a period of about 260 million years, starting approximately 2970 million years ago.^{. The oldest rocks (laid down 2970 million years ago) form the northern scarp of the Witwatersrand plateau; the youngest (laid down 2710 million years ago) are those that form the southern edge of the plateau.}

^{As for the history- Gold is found in the conglomerate strata of the younger members of the Supergroup. The abundance of this gold is without equal anywhere else in the world. Over 50 000 tons have been mined from these rocks since this precious metal was first discovered here in 1886. This accounts for approximately 50% of all the gold ever mined on earth.}

^{If one looks east down the stream, there is a little spot where evidence that this was all an inland sea, is visible. Look carefully!}

^{To qualify to log this earthcache, please answer the following questions and email us your answers.}

1. ^{Look east and see if you can spot the evidence that this was once an inland sea. Describe what you see and if you have a camera handy, take a photo and include it in your email to us, although this is optional?}
2. ^{The little stream that runs along this section of the valley eventually forms a part of which (a) river, and (b) waterfall?}
3. ^{In your opinion, on examination of the rocks, would you think Orange Grove quartzite is hard or softer rock?}