Friday, 4 September 2015

The City Walls in York


The City Walls in York

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.

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.

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.

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.

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.

The Bars