I was once sitting in a cathedral built from local limestones, and commented to my friend David (who was sitting next to me) that it would take some worthwhile heat and pressure to turn it into marble. The look of shock on his face was truly priceless as he asked me whether one kind of rock can change into another. I gave him the elevator pitch about metamorphic rocks, which have been transformed from one type of rock to another by 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 later (between 27 and 12 million years ago) subjected
to heat and pressure as the Apuan Alps formed. The
quarries are enormous. The scale (in terms of size) of the quarrying
operations almost beggars belief - the faces and quarried blocks truly do dwarf
the trucks and other plant 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 Mint Cake*. 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 Mint Cake 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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