Slabs of inedible mintcake

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Brick-shaped slabs of unpolished marble with their long sides aligned vertically.

I was once sitting in a cathedral built from local limestones (from more than one source), and commented to my friend David that it would take some worthwhile heat and pressure to turn it into marble. He asked me, in total shock, whether one kind of rock can change into another.  The look on his face as he was asking was priceless!  I gave him the elevator pitch about metamorphic rocks, which have been transformed from one type of rock to another by the actions of heat and pressure.  Marble is one such metamorphic rock.  The purer the limestone was to start with, the purer (and more valuable!) the resulting marble.  

Carrara Marble, one of the most famous stones in the world, was originally laid down as a pure marine limestone some 190 million years ago during the Jurassic, and then subjected to the aforementioned heat and pressure between 27 and 12 million years ago as the Apuan Alps formed.  The quarries, in Northern Italy, are enormous.  The scale (in terms of size) of the quarrying operations almost beggars belief - the faces and quarried blocks dwarf the trucks and so forth in use.

You normally see marble polished to within an inch of its life - it's the fact that it can be so finely worked in all three dimensions, and the finish it can take, that make it so sought-after.   Think in terms of Michelangelo's David and Canova's Three Graces. More prosaically, it is used architecturally as columns and in slabs and for steps/floors like the area leading up to the high altar in the cathedral we were sitting in. 

What I particularly like about these slabs cladding the Rotunda in Birmingham (UK) is that they have escaped the polishing treatment and look rather like Kendal Mintcake*.  It's wonderful to see the stone sparkling in sunlight and I couldn't resist taking a couple of photos on the way past.

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*Kendal Mintcake is pretty much a bar of mint-flavoured sugar, much favoured by mountaineers and walkers/hikers for its calorific value. It doesn't hurt that it tastes very nice, too!!

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