Geology: freshwater shelly imestone
Rock Unit: Durlston Formation
Age: Lower Cretaceous
Provenance: Isle of Purbeck, Dorset
Purbeck Marble is a sedimentary rock; a limestone with a crystalline carbonate cement. The term marble is here not used in a geological sense, but instead as used by the building trade where any stone that will take a good polish is referred to as a marble. There are traditionally recognised three different beds of stone, all outcropping on the Isle of Purbeck in Dorset. They are known as the Blue Marble, the Red Marble and the Green Marble; it is the Blue Marble that predominates at Canterbury Cathedral. Each of the marble beds contains numerous small freshwater snail shells of the genus Viviparus (formerly known as Paludina).
Purbeck Marble has been used since Roman times and because of its ability to take a polish it has been highly prized as a decorative stone for columns, funerary monuments and fonts. The limestone was quarried close to the coast and taken by ship from Swanage. The Crown owned many of the quarries and the first stones have been described as being taken from a shallow mine running out to sea south of Swanage. The stone became very popular with medieval masons and their patrons in the thirteenth century, especially for tall, slender and clustered columns. It was also favoured for both its ability to take a good polish and to pleasingly colour contrast with the principal building stones. Marble columns were usually cut from relatively shallow beds and consequently positioned “end-bedded”, that is to say a horizontal bed was set vertically in the building fabric. Unfortunately this could lead to failure and splitting of the stone with a consequential tendency to delaminate along the line of the bedding plane.
Later Victorian restorations indulged in coating some of the marble columns within the cathedral with various preparations, including linseed oil, presumably to protect and polish the stone. Hugh Braun the architectural historian, denounced, “The Victorian blacking which makes the shafts stand out like drain-pipes” as “a shocking desecration”. In 1931 Sir Charles Peers, the cathedral seneschal, commented: "The cleaning of Purbeck and other marble shafts in the 19th century took the form, not of repolishing, but of making up the corroded surfaces in a dark mastic composition, leaving what was left of the marble unpolished”. Many of the shafts in the east end can be seen today to bear the scars of this treatment.
Many cathedrals have used Purbeck Marble in their internal decorations including Westminster Abbey and Salisbury. At Canterbury it has been used extensively in columns in the Choir and Trinity Chapel as well as in the nave and the crypt. In recent years Purbeck Marble has been worked and cut on site to replace stone columns and decorative features in the exterior facade that have weathered beyond repair. The stone usually contains a small iron content within the carbonate cement, which is liable to oxidise and rust in a wet or damp environment over long periods of time. Newly installed Purbeck Marble on the cathedral exterior may therefore have a more limited life span than most modern replacement stones, but nevertheless one extending well beyond the present century.
Where to see exmples:
Main cloister north walk
Choir south aisle - awaited
click to enlarge and read caption