On the shore at Sea Point, south of Cape Town, sits a dramatic geological contact (figure 1) that has been famous for some two centuries. This complex and spectacular feature was first described by Clark Abel in 1818,1and then visited by the young Charles Darwin in 1836, on his journey around the world on HMS Beagle.2
It is at this point that once-molten magma intruded the sediments of the Malmsbury Group, believed to have occurred at a depth of some 10 km.2We now observe the results of this event frozen before our eyes, a complex intermixing of the dark sedimentary rocks with the pink-brown igneous granite.
Heat from the intruding granite has altered the Malmsbury sediments, turning them into hornfels. The zone of alteration is called a contact aureole and extends for at least 100 metres from the contact.3
The contact between the two rocks consists of a complex of interlocking, intermixing shapes, like intertwining tongues or flames (figure 2). The molten granite looks like it has been swirled and sheared with the sediments, like the mixing of two different coloured cake mixtures in a bowl.
Figure 2. Closer view of the interfingering between the light brown granite and the slate-grey altered sediments. The complex intermixing suggests both, to a certain extent, were plastic and flexible when the intrusion occurred.
Such long swirls of granite are what we would expect from a pool of magma intruding the sediment while the magma was molten, plastic and easily deformed. However, the intertwining shapes of the dark sediments at the contact are almost as intermixed, suggesting that they also were plastic, to a certain extent, at the time of the intrusion. Does this mean that there was not much time between the deposition of the sediments and the intrusion of the granite?….
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