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Portland screws and horses' heads

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  Roach stone with many 'Portland screws' England’s ‘Jurassic Coast’ is rightly famous.  Stretching along the English Channel coastlines of Devon and Dorset, it’s rocks bear witness to the entire Mesozoic – the Triassic, Jurassic and Cretaceous Periods (together known as the ‘Age of the Dinosaurs’) from East to West.  That’s a span of 185 million years, in the space between Exmouth in the west to Old Harry Rocks at Studland Bay, Dorset. Even if geology isn’t your thing, you’ve seen quite a few bits of it on film or television – West Bay, Bridport, is the setting for Broadchurch, Lyme Regis was home to Mary Anning (and of course to Trey), Kimmeridge Bay is famous for its fossils, Weymouth hosted the maritime events in the 2012 Olympics, and Lulworth Cove and Durdle Door are rightly famous. Chesil Beach and the Fleet are very important for their geomorphology and the lagoonal fauna of the brackish water in the Fleet. The Isle of Portland, tied to the mainland at the eas...
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Onion at the Royal Academy

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Following on  from the 'mintcake' Carrara marble I blogged about previously:  This column, in London's Royal Academy of Arts, is cipollino (little onion) marble - the green colouration comes from bands of chlorite and epidote.  Looking at it, it is easy to see how it got its name!  Both the ancient Greeks and the Romans used to quarry it, mainly for use as columns.  That still seems to be the case now; Westminster Cathedral (The Catholic one at Victoria) used it for the same purpose and, if memory serves, for a number of panels as well.   We happened to be at the RA for an exhibition and I was pleased to see this column there; I greeted it like an old friend.
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Slabs of inedible mintcake

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  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. ...
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Larvikite

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This, as the header of this post suggests, is Larvikite.  You've probably seen it cladding buildings and sparkling in the sun.  It is an intrusive igneous rock - one that is formed from magma that solidified underground. In this case, this happened near what is now Larvik (hence the name) in Norway. Larvikite is similar to granite in many ways but it has far less quartz than granite has. The feldspar minerals are iridescent because of some rather cool optical interference, rather like a kingfisher's feathers, and the iridescence varies as the angle you're looking at it from changes.  A polished slab is a so-and-so to photograph because the camera just looks through it to your reflection and you wind up with a selfie.* You also see it used in large, unpolished, blocks as coastal defences, and it's still very much worth a close look then. Incidentally, a chap by the name of Sir Montague Maurice Burton (of menswear fame) used Larvikite on the outside of his premises  ...
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The Whin Sill

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  While I have been 'curating' - she says grandly - photos to put on this blog, I've been reminiscing about when I took some of them.  Take this one, for example:  I was on an Open University geology summer school (SXR260, anyone?), and it was the first time I had been to Teesdale (County Durham, in northern England). One of our field trips was to see the Whin Sill (a lens-shaped igneous rock intrusion that took place in the  late Carboniferous ). There's an amazing exposure of the sill here at High Force Waterfall!  I can just imagine the heat of the magma as it forced its way through the country rock, and the cracking, creaking and groaning as it happened.  The basalt columns, by the way, are the same structures formed by the dolerite cooling that you see at Fingal's Cave and the Giants' Causeway.  Further along the Tees, at Low Force, you can walk across the exposed tops of such columns to see the hexagonal shape of the columns from above. I was...
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Rock bang full of oysters

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You've just got to love Portland Stone, with its distinct facies.  This is grove whitbed, a Jurassic limestone famous for being absolutely full of oysters.  I took this one, I think, at Green Park tube station in London. The station entrance, on Piccadilly, is literally a work of art; called Sea Strata, it was created by John Maine RA with the building clad in different facies of Portland Stone. This is the highly fossiliferous Grove Whitbed, which is full of oyster shells.
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A twist(er) in the tale

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We (the local geology and palaeontology group) were in a huge quarry In Rutland one Saturday. We'd been collecting marine invertebrate fossils in the morning - I'd collected enough fossil shellfish to make a decent gumbo.  There were mussels, horse mussels, clams, sea urchins, all from a warm, tropical sea in the Jurassic, all living in their own trophic niches.  Some of my friends found fish teeth – bony fishes and hybodont (peg-toothed) sharks. During the morning, the sky had become more threatening, and we had to run to try (and fail) to avoid the downpour that came with a thunderstorm.  Luckily, this quarry has excellent facilities including a warm, dry room with a coffee machine, chairs and tables. It’s not a pretty room but it’s a very welcome one.  Anyway, this was a good time to have lunch, so we did, then we started to move out to another area with rocks and fossils deposited in a freshwater environment. I was a little behind some of the others, as I’d s...
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