And decorative it is! When cut and polished, the Frosterley Marble is stunningly beautiful. So much so that you can find it forming decorative pillars, fireplaces and statues in many public building across the north of England, including within the Chapel of the Nine Alters in Durham Cathedral, but it can also be found as far away as Mumbai!
It might at first glance seem a little odd that you can only find these fossils in a single place. After all, the Carboniferous rocks of the region are well known to form continuous layers across the landscape that stretch for many miles, creating the stepped hillsides so characteristic of the Yorkshire and Durham Dales. Many of these layers are so distinctive in their characteristics and thicknesses, that they have been given names by geologists, and by miners and quarrymen long before them. The Frosterley Marble is part of an especially thick layer of rock called the Great Limestone (formed around 330 million years ago), which can be tracked across a large swathe of northern England. So why can’t these particular rocks, abundant in wonderful fossil corals, be found outcropping all across the Pennines? The answer lies in the nature of the environment in which they formed. Those familiar with the geology of limestone, or indeed the geography and biology of modern reefs, will know that these ancient corals were born, lived and died in a tropical sea. For a decent chunk of the Carboniferous Period, much of what is now the UK was much closer the equator than it is today, and was under water. In these warm, shallow seas vast coral reefs thrived, which would later be preserved as the Frosterley Marble, and indeed, all the limestones of the region. But reefs are complex, dynamic places. If you’ve ever been lucky enough to visit the Bahamas, or even just looked at that part of the world from space on Google Maps, you’ll see a patchwork of deeper water and small islands, with shallows between them. Many modern corals can only be found down to a certain depth (due to their reliance on the photosynthetic microbes they co-exist with), and so form thin bands around the margins of these islands where the water is just the right depth for them. Carboniferous England was no different. While limestone rock, formed mostly from microscopic sea creatures, was being laid down across a vast area, this particular coral reef likely existed only around the margin of an island. A thin band of the corals is all that was preserved, and we just happen to be lucky enough that the Bollihope Burn cuts through the rock in just the right spot to reveal them. Which brings us onto something else unusual about the Frosterley Marble: the corals themselves. The corals are of a species named Didunophyllum bipartitum, which is found in other layers of rock too. Often called solitary, horn or rugose corals, these are very different from many of the corals found in today’s seas. While almost all modern corals are colonial (meaning lots of individual organisms living together as part of a single structure) these corals were a singular creature. The last of these rugose corals disappeared at the end of the Permian, around 252 million years ago, but in these Carboniferous seas they were thriving.
Author: Jake Morton
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