The
City of York, with the most complete medieval city walls still standing in
England, is deservedly one of the UK’s great tourist attractions; I have visited it many times, for pleasure, business and as an English Language teacher/field trip leader with students from Spain.
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| A few views of York Minster |
Set
in the low lying Vale of York, with the underlying bedrock buried beneath Quaternary sediments, there are no building stones
found in the immediate vicinity. The Minster, the churches, various other
historic buildings and the walls themselves are constructed of Magnesian Limestone - imported along the rivers from the town of Tadcaster, where the
Romans had fully exploited the hard rock for building stone.
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| A few details of the Magnesian Limestone |
Although
the Magnesian Limestone has been used to build some spectacular and long lasting
buildings and ancient monuments, it possesses physical characteristics that
make it particularly unsuitable for intricately carved stonework.
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| Variations in colour and texture within the Magnesian Limestone |
During
the transformation from limestone into dolomite, when the Permian Zechstein Sea
lapped up against the ancient sea shore of eastern Britain, it has been
estimated that the volume of the rock shrunk more than 12%. This left the rock
formation full of cavities, vughs and cracks – at angles that are oblique to
both the planes of bedding and jointing.
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| A few views of the city walls in York |
Some
of these have been filled wholly or partially with the mineral calcite, during
subsequent diagenetic processes – known respectively as shakes and vents in the
stone trade. A formal history of the restoration of the English Houses of
Parliament, largely constructed in a very similar limestone from South Yorkshire, would no doubt confirm the observations of Charles Dickens, when he described this stone as being “the worst ever used in the metropolis” - having seen the need for constant, expensive repair.
