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Rock stars

A cache by Sterreman Send Message to Owner Message this owner
Hidden : 10/30/2009
2 out of 5
1.5 out of 5

Size: Size: not chosen (not chosen)

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Geocache Description:

A Cache between the Rock and the Heavenly Place. The flat topped hill on which South Africa’s National Observatory is situated is a good example of a typical dolerite sill with a matching example dyke conveniently close by….and a nice surprise!

From the cache position, you have a lovely view of the domes of the South African Astronomical Observatory (SAAO). Quite a photogenic spot actually, especially with these special rocks in the foreground which are often snow-covered in winter. Be warned – it can get very windy here which can make it impossible to do the bonus question (see Question 5 below)! The site is open to the public during the day for visits, the ideal opportunity to do this cache (as well as the other one here.) [I work for SAAO, based in Cape Town, but often come to work in Sutherland. I would love to meet you, so feel free to contact me when you come and do this cache to see if I’ll be here.]

We pick up the geological story well after the sedimentary rock layers of the Karoo Basin formed, but before the break-up of the super continent, Pangea (meaning Entire Earth). [If you want to know how this happened, visit my Cutting edge (GC1XGM6) EarthCache, about 100 km south of here.] All the strata were still horizontal and in tact then, with the surface probably about 2000m above where you are standing now.

About 130 million years ago this grand continent began to split up into Gondwana (the southern part) and Laurasia, then into various smaller pieces, triggering volcanic activity. Because of this movement, and due to the enormous pressure of the molten rock (magma) from below, vertical cracks formed, creating pathways for the magma to push up into from deep within the Earth. Quite often the rising magma was unable to reach the surface and produce volcanoes, but the pressure was so great that the molten rock was able to lift the higher layers of rock (which was horizontally stratified and therefore was weakest along horizontal planes) and squirted horizontally into the gaps made, forming sheets between the sedimentary layers, sometimes hundreds of kilometers in extent (see sketch below). These “sills” cooled quickly through contact with the local rock, solidifying into very small crystals (unlike, e.g. the deep igneous intrusion of Paarl Rock, which cooled very slowly and formed large crystals). The result is a fine-grain granite known as “dolerite” (Greek: doleros, meaning "deceptive"). The vertical feeder channels also coagulated in what are known as “dykes”. The sills and dykes are thus younger than the sandstone and mudstone deposits.

Sill Dyke sketch

Over the millennia the sedimentary layers eroded away to the level we see today. However, the very hard dolerite resist weathering, producing flat top “mesas” (flat tableland with steep edges), a familiar sight throughout the Karoo. Because the harder rocks weather more slowly, these hills are in effect held up from their tops, and only wear down as the softer rocks below undermine the harder layers. The plateau that you, and the SAAO telescopes are on, is in fact such a dolerite sill. And if you look NNE, just past SALT (bearing 19°, 24 km away), you will see a very distinct flat-topped hill, called Tafelberg (Table Mountain), which is also a remaining piece of a sill.

There is also a good example of a dyke conveniently nearby – look NNW (bearing 330°, distance 4.2 km). This rocky outcrop is officially named Skuweklipkop (directly translated; “Rough Stone Hill”) although our local astronomers have an equally appropriate nickname for it (see Question 3 below). This feeder channel would have created a sill far above it, now entirely eroded away.

local features

You may wonder why I brought you here since these features are better visible from elsewhere on site, as can be seen in the above pictures (something to look out for when you drive out). It is because the surprise I mentioned, is right here! If you are at GZ (ground zero) – and weather permitting – you are probably sitting on it! You are amongst a heap of washing-machine-sized boulders. Now find a fist-sized stone and start tapping these boulders (try the other boulders too). What do you hear? If you could quickly learn to play the lithophone, we must arrange a rock concert!

But wait ... there’s more! Normally EarthCaches do not involve any hidden treasures, but for your tapping convenience, I left a hammer amongst the rocks at GZ. Please hide it again afterwards and shout if it breaks (you will see that it is home-made).

To appreciate how unique these rocks are, verify for yourself that Wikipedia only lists five such sites worldwide. It is interesting that most of these sites claim that scientists are still unable to explain the physical mechanism that cause the rocks to sound like this. However, my source, Prof Brian Warner, explained: “When the intrusion of molten rock occurred, the liquid rock cooled so quickly that the crystals didn’t have time to grow large, with the result that the dolerite is (a) almost microcrystalline and (b) almost uniform in constitution. This resembles a metallic structure. The rocks can vibrate coherently because there are no internal inhomogenieties to damp the sound waves.” It is interesting to note that in Afrikaans, dolerite is also referred to as “Ysterklip” (Iron Rock). It makes one wonder if it was perhaps the acoustic properties of dolerite that lead to this name.

(Source: Prof Brian Warner
Department of Astronomy, University of Cape Town , South Africa AND
School of Physics and Astronomy, University of Southampton, UK.)

To claim “Found it” you must email me satisfactory responses to the following:
Any logs not accompanied by an email will be deleted.

  1. Send me a picture of you/your party with your navigation device taken at this spot.
  2. Search for an example piece of dolerite that broke off, revealing its internal structure and colour. If you compare this structure and colour with your average granite kitchen tabletop, name one similarity and one difference between the two.
  3. Where do you think the local astronomers’ nickname of “Camel Rock” for Skurweklipkop originated.
  4. Say for argument sake that magma was never forced in between the stratifications at this site, meaning the local geology just composed of regular sedimentary mudstone and sandstone layering. Do you think the Observatory could still have been build here? Explain your answer?
  5. For a bonus, first download this video clip and tell me if you recognise the tune [hint: it fits in with the purpose of this site]. Then see if you can improve on my attempt by sending me a sound/video recording of your rock music composition. [How about a Metallica piece!]

Note: Do not post any spoiler pictures/hints to this page, even if encrypted.

Additional Hints (Decrypt)

Ernq gur grkg pnershyyl, nyy gur pyhrf ner va gurer. Rznvy lbhe nafjref gb

Decryption Key


(letter above equals below, and vice versa)



43 Logged Visits

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Coordinates are in the WGS84 datum

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