Froggy Pond Boulder Hop EarthCache
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Really interesting rocks on one of the most beautiful beaches in the world. You may even be lucky enough to spot penguins or whales while doing this cache!
Once down on the beach, look back inland and up the mountain. Here you can see the Peninsula Formation of sandstones from the Table Mountain Group – which make the bulk of the famous Table Mountain. Here at Froggy Pond (not just this particular spot) – but all along the nearby coast (try walking eastwards below the golf course – or closer to the naval base at the penguin viewing area too) – the basal granites arte exposed and readily accessible.
Look out for loose blocks perched on rounded slabs. Southwards of here, the granite can often be found to contain xenoliths (i.e. inclusions of non-granite materials). These xenoliths are often from the overlaying sedimentary Malmesbury Group (often slightly darker siltstones or shales). These non-granite inclusions originated when the hot molten granite magma was pushed upwards at high pressure from the depths of the earths’ crust where the granite was generated. As it was forced into the overlying Malmesbury sediments, it literally tore pieces of the rock off and these became embedded in the granite (very similarly to peanuts that end in a peanut brittle once the hot toffee sets solid).
Note the twinned crystals of feldspar (the large light coloured crystals common in the granite) that can be up to a few centimeters across. These were the first crystals to form in the slow cooling magma, and because the granite intruded at depth (i.e. well below the earth’s surface), they cooled very slowly, and the crystals had plenty of time to grow. If you walk about, you’ll find some dolerite dykes that have intruded into the granites much later. These are seen as darker ridges cutting across and through the various granite boulders. You can often see them following the same line across a number of boulders on the beach. The dolerite is also an intrusive magma originated rock, but very different from the granite. The difference is two fold: (1) the dolerite is darker – mainly because it is mafic (i.e. basic- or non-acidic) whereas the granite is acidic and (2) the dolerite is fine grained because it is so thin (i.e. it cooled so much faster than the surrounding granites). (Acknowledgments: Geological Journeys.; Norman & Whitfield; 2006).
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 some of the granite boulders visible. [optional]
2) Look at the feldspar crystals in the granite and describe one (i.e. size; colour; shape; surrounding rock; any other interesting facts on the crystal).
3) Look for a dolerite dyke – take a photo and post it too if you can see one. A dyke is a vertical intrusion – what is the horizontal equivalent of a dyke called?
4) Look up the mountain and describe how the rocks there differ to the rocks on the beach.
5) Did you see anything else interesting at the cache site (kelp; penguins; whales; baboons)? Make a note of these.
Additional Hints
(Decrypt)
Erzrzore gb fraq lbhe rznvy! QB ABG srrq gur onobbaf!