Geological mystery solved …..

The rock below was presented to the East London Museum for identification by John Goldsmid.

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It was found on the beach near Hamburg (west of East London). A closer investigation reveals a linear inclusion (noted by the lighter coloured material) which seems to be inserted into the darker surrounds. Mr Goldsmid assured museum scientist Kevin Cole that the rock was not fabricated!

Two main rock types are found in the Hamburg area – sedimentary rocks (consisting mostly of sandstone) and igneous rocks (volcanic in origin). The above specimen did not fit neatly into either category ….

A start was made to unravel the mystery by doing a ‘streak test’- the specimen left no marks when struck along the surface of another rock. The second test was to determine whether any carbonate was present – here a weak acid proved to have no effect on the rock specimen surface.

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At this point Dr Billy de Klerk (Albany Museum palaeontolgist/ geologist) was contacted. Details of the discovery and the methods used were communicated along with pictures of the rock.

Considering the rock was very fined grained and had been abraded by wave action in the coastal zone, the following was concluded by Dr de Klerk:

It is a baked mudstone (very fine grained) that was cooked in contact with a dolerite dyke. Before undergoing contact metamorphism it had a small quartz vein develop along a joint in the rock during the tectonic folding event around 250 Ma.

To interpret this further (K. Cole): The Karoo Supergroup of rocks in the area contain sedimentary sandstone and mudstone (dating back 250 million years). The small quartz vein (noted as the lighter inclusion described above) inserted into mudstone at the time listed above. Approxiamately 183 million years ago large larval outpourings across southern Africa produced sills and dykes in this region. At the contact zone of the larva and mudstone the latter ‘baked’ into a new rock type called Hornfels – a fine grained granulose rock produced by thermal metamorphism. So here we have a metamorphic rock along with sedimentary and igneous rocks mentioned above.

Billy de Klerk summarises the find as mudstone hornfels (with a quartz vein) rounded by abrasion during wave action.

 Thank you John for this great little geological treasure! 

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About East London Museum Science

Conservation Biologist East London Museum South Africa
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