Article from the August 2002 issue of
Pink, Pink Everywhere!
by Eileen A Lawley
The name says it all. The Côte de Granite Rose. The Pink Granite Coast. And it was indeed everywhere. Houses and churches are built of it. Walls, fence posts, as well as menhirs standing erect to act as gate posts ever watchful and vigilant. Pavements, pedestrian crossings and kerbstones under our feet. Plant containers, which are absolutely immovable, and even numbers carved on posts to describe the features of the scenery. And not forgetting statues, sculptures, and gravestones as well as spheres and cylinders to mark the edges of the road. No knocking over these bollards and riding away with no damage. Everywhere you could look the luscious pink cleavage planes of the feldspar glinted and sparkled like diamonds in the sunlight. This is a small area of coast between Perros Guirec and Tregastal in Northern Brittany, only about 6-8 miles long. If you are ever in the area the granite formations are an absolute feast to the eye. For a geologist, well "Worth a journey" to quote the Michelin Guide.
The coarse-grained granite, estimated to be 5-6 kilometres deep, was intruded 300 million years ago as part of the Variscan Orogeny into much older gneiss which is about 2000 million years old. So there is plenty more granite for houses. The area is centred on a very small and very shallow (!) harbour called Ploumanac'h (translation "The Monk's Bowl"), formed where two rivers which cut through the granite reach the sea.
Boats enter this harbour via a narrow entrance between the piles of granite boulders which form bastions guarding the entrance.
We are in the area sailing in our boat and in the past we have entered Ploumanac'h with our own boat, which was a very spectacular if rather nail-biting experience, but now the boat is too deep and we are likely to tip over in the sand as the tide ebbs. From the sea you can get a very good impression of the granite piles but once safely moored in a nearby harbour, we took a walk along the coast, on the Customs Footpath.
The imagination can run riot recognising the formations. The Devil has his own rock showing an evil profile and there are also castles, apparently standing steadfast against the sea. Animals are everywhere, rabbits, tortoises, a chameleon and of course trilobites. An elephant's foot has finer-grained aplites forming the toes, and there are faces, a skull, a clog, and a bottle poised on a boulder ready to pour, what else but a rosé wine. Evidence of the forces of wind and water are everywhere. Erosion has worn away the covering gneiss and rainwater has circulated in minute cracks to alter the granite, dividing it up into the boulders we see today. Erosion by wind, salt and waves has continued the process cutting channels and forming washbasins which sometimes eventually become holes. Vertical channels called draperies have been carved by sea spray running down the rocks. Fissures and cracks cut deep in the rocks form all sorts of crosses, St George, St Andrew and the Maltese Cross. Several apparently separate boulders are piled on top of each other but the aplites running continuously through the pile betray their origin from one solid block of granite. There are also fractures through which fresh water has percolated which have now been filled to form long quartz veins, a few inches wide, running straight out to sea.
As we continued our journey in our boat around the coast, the pink gradually changed to grey and we thought we had bid farewell to the pink granite. This was not in fact so, because we entered the marina at Trebeurden, where the harbour wall has been formed by the simple act of dropping granite blocks 50cm cubed and weighing about 3-400 kg haphazardly into the sea to form a dyke. No longer carefully craftsman built harbour walls with the high labour costs of today. We had to stay here several days while the gales blew by, but never grew tired of the beautiful freshly cut granite blocks, which in as short a time as 10 years are beginning to show signs of erosion from the water. Part of the coast has been made a nature park to "preserve the landscape", but while the sea continues to break against the rocks and the wind throws salt-spray against the granite piles then the only certain fact is that the features will continue to change.
by Eileen A Lawley
Why is it pink?
“Why is it pink?” a reader asks.
Reply of 6 March 2003 by the author, Eileen A Lawley:
I read in the tourist info that the granite contained "pink feldspar"
which I had already deduced from studying the minerals in the granite (quartz,
biotite and pink feldspar). My cribsheet on common minerals in igneous rocks
describes orthoclase as pink so I deduced that the pink feldspar is orthoclase
which is an alkali feldspar containing mostly potassium (instead of sodium or
calcium).
The feldspar crystals can be really quite large, 1-2 cm so that the whole rock
appears very pink.
I reread some books today and found that microcline can also be pinkish but
this mineral usually forms shorter crystals. >From the shape of the "pink
feldspar" , ie long crystals, I stick with my original conclusion.
I am not a professional geologist or petrologist or anybody like that so it
may be that I could be proved wrong by somebody with more expertise than me.
Remarks of 11 March 2003 by Ole Nielsen:
Personally I don’t know. I have never been there. But to speak in general
terms granites can be pink because they contain pink (hematite-pigmented) (alkali)
feldspars like orthoclase. As far as I know the colour of granites is mainly
determined by the feldspars and only to a small degree by “darker minerals”.
NB pink quartz does exist.
But I am no expert either, so if you, dear reader, happens to know what makes
this specific granite pink please share your knowledge with us.
The question however reminds me of a world-famous pink granite that I HAVE seen, namely Aswan granite.
Aswan, Syene, Syenite and Pink …
What we today call Aswan, the Ancient Egyptians called Swen. The Copts called it Souan, meaning ‘trade’ (or ‘market’). The Greek called it Syene – and some of you may remember a city called Syene or Seveneh in the Bible and described as the southern limit of Egypt. From etymology and theology to geology is sometimes a short way - as from Syene to syenite, the name that was given to the rock quarried at Syene. Curiously enough the word syenite is still used for a specific kind of rocks, but today the rock that was quarried in Aswan is not considered to be a syenite but a granite or granitoid. Unlike granite syenite contains little or no quartz. Syenites can be from gray to pink. Most of the granite used in the ancient Egyptian tombs, temples and obelisks came from the quarries in the Aswan area. The granites were transported via the Nile to many important sites such as Karnak, Luxor Temple, Giza and Edfu , In fact both monumental “red” (pinkish) and monumental “black” (hornblende) granites were quarried there. Tourists can still visit an old quarry from that time, and they can see an unfinished obelisk, which unfortunately developed a flaw during quarrying before they got it out in one piece (the obelisks at Karnak were all in one piece - as large as 36 m tall and some weighed more than 300 tons).
You do not have to go to Egypt to see one of the great Egyptian granite obelisks. London, Paris, Rome and New York each have one, but that is another story (From rather doubtful sources I have been told that Josephine's parting words to Napoleon Bonaparte before he left for Egypt in 1798 were: "Good-by darling! If you go to Thebes, do send me a little obelisk." In vain it seems, because Paris had to “wait” until 1831 before it got its Egyptian obelisk (made of pink Aswan granite), many years after Napoleon had met his Waterloo, a few kilometres from my place).
Ole Nielsen
|
||||||||
|
© OUGS Mainland Europe.
33723 |
Last modified on Monday, May 09, 2005 at 20:44
