Linksfield Ridge EarthCache
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Linksfield Ridge is a prominent rocky outcrop across the eastern part of Johannesburg city. A number of undeveloped zones still exist along the ridge and a number of other geocaches can be found among the rocks on this ridge.
This cache actually has 4 points for you to examine during your exploration of the area.
Point 1: Setting the scene: S26 10.122 E28 05.553:
The brow of the ridge; look southwards and along the length of the ridge to get an idea of the traverse of the ridge and the cross section. Note the exposed rocks here on this spot for your comparison with the other areas. This is also one of the places you can access the remaining waypoints. The other is on Ridge Road. This area offers good panoramic views of the geology of the Johannesburg area and the Witwatersrand Supergroup in particular. The quartzites that form the ridge belong to the Orange Grove Quartzite formation. South-westwards to the communications tower in Observatory is another ridge formed by the Brixton Quartzite formation (see earthcaches Topsy Turvy Highland Quartzite; GC1DT0J and Brixton Quartzite - Tower View; GC1BTE3), and the lower ground between the two ridges is underlain by the more easily weathered shale. Although the Contorted Beds (see Contorted Beds in Braamfontein; GC1BTDQ) within the shale does form high ground through Bellvue, Hillbrow and Braamfontein. The valley in the middle distance (south westwards), Bezuidenhout Valley (or Bez Valley), is underlain with rocks from the Ventersdorp Supergroup, consisting of easily decomposed lavas and sedimentary rocks. The Ventersdorp Supergroup is younger, and normally overlays the Witwatersrand Supergroup, and the presence of these rocks in this valley is due to major faulting; the boundary fault on the north side of the valley progressively truncates the Hospital Hill subgroup between Observatory and Bedfordview. More on some of the faulting activity can be experienced at the earthcache next to the Scottish Memorial (Topsy Turvy Highland Quartzite; GC1DT0J). Langermans Kop (great cache on there too; Johannes who?) to the south, consists of quartzites of the Mondeor Formation, the uppermost unit of the Witwatersrand Supergroup. To the south of Langermans Kop is the faulted quartzite ridge, and even further south, a line of mine dumps marking the Main Reef (the gold bearing ore body); generally in a line near the M2 highway. The hills in the very far distance are those of the Mondeor Formation and Ventersdorp Supergroup.
Point 2: Fossilised Ripple Marks S26 10.039 E28 05.393
These are a little difficult to see; especially in summer when vegetation can overgrow the rocks; and also with ongoing development and fences being erected. Keep your eyes open on the southern side of the road; as there are a number of examples exposed; some better than others; as you drive up the road. Be careful when stopping as the road is very narrow. The ripple marks vary in shape and size, and can be seen imprinted on the overlaying quartzite (a number are on the underside of the rocks and are negative images or casts remaining in the harder overlaying rock strata. The actual ripple marks have long since fallen away and have been weathered. These ripples were formed in the silty sediments of the Orange Grove Quartzites, and are similar to those seen at Point 4 below. The ripple marks themselves were formed by the movement of a water current over sediments in shallow water. You can observe similar ripple marks in mud bars, sand banks and other shallow soft sediments in any modern day river or estuary.
Point 3: Base of the Witwatersrand Supergroup S26 10.067 E28 06.003
There is a guarded boom gate at the entrance to Kallenbach Drive here. But if you explain that you are looking at the park at the end of the drive there will be no problem accessing the gated area. Also mention that you will only be 10 minutes. Point to the geological information plate (Point 4 below) right near the intersection at the boom and explain the link between the 2. The guards were very affable to letting us in even at dusk.
At this locality, the base of the Witwatersrand Supergroup can be seen resting on basement granite. This is best seen to the right of the small locked access gate but can be a bit obscured by vegetation at times. This granite differs from some other exposed granites in the area (e.g. earthcache A really Gneiss Earthcache ! (Tonalite Gneiss) GC1BZ8C), in that the feldspars have changed to a soft material called mica, although quartz grains can still be recognised in the matrix. This is believed to be the result of weathering and the soil forming processes that operated here when the surface prior to the Witwatersrand Supergroup was deposited (so you are seeing an early fossilised soil nursery!). The lowermost layer of the Witwatersrand Supergroup is a pure quartzite, but immediately above the granite, at the base of the quartzite is a thin conglomerate layer. The quartzite shows fine banding, called planar bedding. Some cross bedding can also be seen. The nature and disposition of the cross bedding indicates that the water in which this sand was originally deposited, flowed both northwards and southwards. This could only have been caused by the ebb and flow of tides, and this gives strong evidence that this was formed originally on the edge of a sea (so I hope you brought your sun block and surfboard with :) ). The planar layering of the quartzite must have been horizontal when the sands were laid down, but has since been tilted to its present attitude of about 30° to the south. The contact surface between granite and the sediment often contains veins of hard white quartz, which fill any openings that were formed on the contact as the rocks were tilted. Consider this earthcache too for more on a contact - Witwatersrand Contact; GC1DT48.
Point 4: Siltstone S26 10.062 E28 05.643
A blue information board from the mid 1980s SA Geological Society trail still is in place here. This marks an exposure of a siltstone zone in the Orange Grove Formation. In comparison with the quartzite at Point 3 above, the siltstone is much finer grained, and an altogether softer rock with a shaly texture. The environment of deposition was different to that of the quartzite formation (remember they were formed in a shallow sea shore area). The siltstone was formed in deeper water and further from the shoreline. Think about the grain size sand in quartzite to silt and mud in the siltstone and the need for calmer water with less movement to keep the small particles from being swept away with constant tidal movement. The reddish colour of the rock comes from iron oxide (think of rust) and the differing colours come from the differing proportions of the various minerals in the various layers. During the deposition of the silt, particles of different sizes tended to become separated separately, resulting in nearly flat layers or bedding planes. Subsequent tilting of the whole rock sequence has lead to the current attitude of about 30° to the south. The structural processes that lead to the tilting imparted a cleavage or foliation to the siltstone. This can be observed when you try and break a piece of the rock off. It tends to split along these cleavage lines which are at a steeper angle than the bedding.
(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 information sign in the background. [optional]
2) Describe the geology of the Johannesburg area from what you can see from Point 1. Use the description – but reply more on your observations and experience in your travels to the cache site. I don’t expect a thesis – just in your own words :) .
3) Did you find any fossilized ripple marks – post a photo and leave a description in the log to aid other cachers in finding them.
4) Describe how you think soil forms from rocks.
5) This is quite a large area and there are many interesting facets to the area. Describe some of the interesting things you saw (not just from geology – but also plants, birds, animals, buildings etc). You may post these in your logs too.
Additional Hints
(Decrypt)
Erzrzore gb fraq lbhe rznvy! Fhafrgf sebz urer pna or nznmvat!