The fabric role of 1396-97, preserved in the library of Canterbury Cathedral, records that the Prior at Canterbury purchased three stone types to re-build the south-west tower; including 82 tons of a “northern stone”.
It has been suggested that the northern stone is possibly a reference to a Magnesian Limestone of North Yorkshire. This dolomitic limestone is a hard, creamy-white, oolitic stone of Permian age. A limestone is called a dolomite when much of its calcium content has been replaced by magnesium. It is usually fine-grained with a distinct crystalline texture. The Huddlestone Quarry, which was very active in the Middle Ages, and supplied York Minster, for instance may have supplied the stone for Canterbury. Fifteenth century accounts record the stone’s use in the Minster fabric. Some dolomitic limestones are notably prone to decay by chemical weathering because they produce magnesium sulphate and calcium sulphate as a weathering bi-product. The former is very soluble and cavernous decay of the stone can often result. There are no known extant blocks of this stone in the cathedral at Canterbury.
Another suggestion put forward is that northern stone may be from Barnack in Cambridgeshire. This seems less likely as Barnack Stone, a Jurassic limestone, is a very hard wearing stone and it would be reasonable to expect blocks to still be present in the modern church building and identified as such during the various phases of restoration. Unless further evidence arises (either documentary or archaeological) we can only speculate on the source and nature of this stone.