Darwinian Contact EarthCache
-
Difficulty:
-
-
Terrain:
-
Size: 
(not chosen)
Please note Use of geocaching.com services is subject to the terms and conditions
in our disclaimer.
An important geological site in South Africa as far as early
geologists’ gaining an understanding of plutonic intrusions goes.
The Seapoint Contact is an exposed outcrop that shows the intrusion of granites into the older sedimentary layers.
The coarsely crystalline pinkish granites have intruded into the
darker banded Malmesbury sediments on the beach just across from the President Hotel. There is ample parking right at the cache site.
This contact is not a single spot, but rather a progression
from the darker Malmesbury sediments on the right when facing the ocean (north) through to the pinkish granites to the left (south) of the cache site. The actual contact is some 150m across. If you feel so inclined, you can trace the full progression of siltstones, sandstones and shales from around 3 Anchor Bay, all the way to this point. This contact between these two visibly different rock types illustrates the process whereby the hot molten granites (light) intruded slowly into the preheated sediments (dark) that were overlying them. All of this occurred deep within the earth’s crust. The resulting mix of rocks is known as a migmatite. Historically, scientists (geologists more specifically) questioned the origins of granites: did it come up from the depths or was it formed in situ as a precipitate from sea water? The debate had raged for many years, and at the time (in the early part of the 19th Century) Charles Darwin (he of the “Origin of species” and evolution fame) was on his travels, and decided that a visit to this very outcrop in the Cape of Good Hope may resolve the debate. The outcrop had been brought to his attention by comments in a paper read to the Royal Society in 1813, based on observations by a Royal Naval Captain – one Capt. Basil Hall. After calling in the Cape, Hall had written, “I came after a short ascent to a space where many yards of the rock were laid perfectly bare, and I found myself walking on vertical Schistus, or on what might be called Killas. This rock was in beds highly inclined, and stretching from east to west, which is nearly the direction of the mountain….. On looking forward a little higher up I saw another portion of rock that was also laid bare, and which appeared to be Granite. I now had no doubt of reaching, in a few minutes, the precise junction of the two rocks, and I ventured to predict….that we should immediately see veins from the main body of the granite penetrating into the rocki on which we were now standing. In this I was not deceived; the contact was the finest thing of the kind I ever saw …” More than 30 years later, Darwin walked over the same contact. After careful scrutiny, he found himself in agreement with Hall’s interpretation of the
granite being intruded into the fragmenting “roof” of schist, with
fragments and slivers of the schist being ripped off and deposited or “floating” in the granite itself. To Darwin, the interpretation was infinitely more plausible than the alternative (which was still very popular back then) that the granite formed by the conversion of the schist into granite.
(Acknowledgments: Geological Journeys.;
Norman & Whitfield; 2006).
LAYMANS VERSION: This area used to
be very deep under the surface of the earth when all of this
happened many millions of years ago. The darker sedimentary rocks (Malmesbury Group rocks) were heated by rising molten magma (lava) from deep within the earth. As the rocks were heated, they became pliable and soft (a bit like warming plastic or toffee slowly). The hotter granite magma from below, was slowly pushed through any
cracks and gaps into the overlying dark rocks. So what happened is that along some of the edges of the these cracks, the rocks melted together and formed a mixture, in other places the dark rocks were merely deformed (pulled and pushed out of shape) as the granite flowed in and I n still others, chucks of the dark rock were pulled off and are now “floating” in the granite. Look and see if you can see examples of any of these – pictures in the logs will be great.
Another thing you can look for in some of the rocks sticking into the ocean is how the 2 types of rocks weather (break down) at different speeds, leaving ridges of harder material. Contacts like this exist all over the world, but very seldom are they so exposed and easily accessible to people. This is what makes this contact particularly useful and interesting.
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
beacon visible and include in your log. Also include any other
interesting photos of things you saw. [Optional]
2) Look at the rocks. Look particularly for the large light coloured crystals (feldspars). From the crystal size – do you think that rocks cooled fast, or slow?
3) Look up towards Lions Head (the mountain closest to this
point – before the large Table Mountain). There is a large rocky outcrop on the base of the mountain (i.e. before the small “head” of sandstone). From it’s color, do you think it is Malmesbury Sediments, or Granite?
4) Explain briefly one way how sedimentary rocks may form.
5) Were there any other interesting things you saw at the cache that you’d like to note?
Additional Hints
(Decrypt)
Erzrzore gb fraq lbhe rznvy!