Thursday, November 5, 2015
Visit to a tropical coral reef
Bollihope, just south of Frosterley in Weardale, can be a bleak place at this time of year. Acres of brown heather and withered bracken, with a few small conifer plantations. But if you follow some of the burns that flow down into the bottom of the valley you soon find yourself standing on a tropical coral reef, albeit a 325 million year-old one.
This is the subject of Thursday's Guardian Country Diary
This is the rock known as Frosterley marble, which is not a true marble at all but a fine grained, dark limestone full of fossil Dibunophyllum bipartitum coral. In this slab you can see one almost complete piece of the horn-shaped coral in longitudinal section and two pieces in transverse section. The contrast between the fossil and the stone matrix becomes much greater when you make it wet or polish the surface.
The state of preservation of the coral structure is astonishing. This particular specimen is embedded in a slab that must have once been a small waterfall until the burn changed course. It has been worn smooth by flowing water.
Medieval craftsmen spent months cutting and polishing the rock by hand to achieve a smooth, marble like black surface that revealed the fossils in this detail. Their work can be seen in pillars of the Chapel of the Nine Altars in Durham cathedral and in the pillars in the Bishops' Palace at Auckland castle in Bishop Auckland, as well as in smaller objects like the font in Frosterley church.
It's also in the flagstones on the floor of the bar of the Black Bull pub in Frosterley.
Frosterley marble floor decoration in the Laing Art Gallery, Newcastle-upon-Tyne