Portland screws and horses' heads

 

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 eastern end of Chesil Beach, is not what you’d call beautiful.  It’s been quarried extensively and it’s a post-industrial landscape rather than a scenic one.  At the southern tip of the island there’s a famous bird observatory, there’s Pulpit Rock, and because of the island’s southward slope, under certain conditions there is truly a ferocious tidal race where the land dives under the sea.  It's also where I saw Viper's bugloss and a carline thistle for the first time; I've enjoyed the times I've visited the island.

Even if you can’t get to the Isle to see it in situ, Portland Stone -  a Jurassic limestone laid down in a warm, shallow sea - has been used in many places.  Christopher Wren chose it for his St Paul’s Cathedral - mind you, he did have shares in the quarry!  Think also in terms of Buckingham Palace, most of Whitehall, Maritime Greenwich, Regent Street – and outside London in a huge number of places including Cambridge University’s Senate House, the Port of Liverpool Building, Baskerville House in Birmingham, Nottingham’s Council House...  You can no doubt add many locations to this list yourself. It was even exported to New York where it graces the UN Headquarters.  It is so popular with architects and stonemasons because it is soft enough to work (carve and shape) well but well-enough cemented to be erosion-resistant.  John Maine RA used Portland Stone to create ‘Sea Strata,’ the exterior and entrances to London’s Green Park tube station.  This looked amazing when it was newly built, but has sadly become begrimed and will take a lot of work to clean.

Laevitrigonia gibbosa - the famous "osses' head"

The photo at the head of this page is of Roach Stone  – the uppermost bed of the Portland Stone series,above the Grove Whitbed (I’ve posted a photo of oyster-rich Grove Whitbed on here previously). Roach stone, instantly recognisable, is full of voids (trace fossils) left where shells have dissolved away; the pointy ones that are so prominent in the photo above are the famous 'Portland Screw’ Aptyxiella portlandica, a gastropod; there are also oysters and Laevitrigonia gibbosa - distinctive trigoniid bivalves - present (right).  The latter are sometimes known as ‘Osses heads’ (horses’ heads) because that’s what the organism is are said to have resembled.  As to why there are so many fossils of so few species, this may have been down to a period of exceptionally high salinity where these species could thrive but others could not.

Roach stone has rightly taken it's place among Portland Stone architecture, used on the BBC 

The Keynes Building, Cambridge

headquarters in London, the Economist Building in the same city, the World Conservation and

Exhibitions Centre at the British Museum, which boasts an amazing 1,600 sq. metres of it, the Keynes Building (King's College, University of Cambridge, right bottom; there are windows, just not in the ground floor at this end of the building!  The blue plaque honours the late, great, Alan Turing.) and in Oxford, several University buildings including the Beehive Building (St John's College) and the ill-designed Blue Boar Quad. It wasn't the Roach that caused the problems, by the way.  On the south coast it was used extensively for the Cobb in Lyme Regis and - unsurprisingly, and unsparingly - in Portland's Breakwater.  It was also used, along with Whitbed facies stone, in the Verne High Angle Battery, a gun battery that was built in the 19th Century to protect Portland Harbour.  

Incidentally, this is one stone that is of great interest to visually impaired people, as the voids are so tangible!