Around 160 million years ago, in the Late Jurassic, Britain lay further south, and was submerged under a warm sub-tropical sea. These tropical conditions were just right for coral reefs to flourish. When the corals died, they became buried under successive layers of debris and sediment, and after millions of years became fossilised. They formed the Corallian Limestone that now underlies this area of Oxford.
A coral fossil found in the Coral Rag in Headington.
The Corallian limestone has been quarried extensively in Headington. A type of limestone called Coral Rag was the commonest stone for building in mediaeval Oxford and can still be seen in the City Wall and St Michael's Tower. These walls, built even as long ago as the 11th century, are still unweathered and almost as good as new. By the end of the fourteenth century, however, the founders of nearly all the colleges were using a different type of Corallian Limestone, called the Headington Hardstone and Freestone. This rock was very useful to builders because it can be cut in any direction, and can therefore be carved. The earliest stone to be quarried appears to have been the best quality and seems almost indestructible, being reused from old buildings. Headington stone was also exported further afield, and was used for building Eton College and Windsor Castle. Early in the eighteenth century, however, the good veins of Hardstone seem to have run out and Headington began to supply a very inferior product, which has had to be replaced at great expense. The Radcliffe Camera is an example of a building where the Headington stone is generally in poor condition.
The lower stage of the Radcliffe Camera is built of Headington Hardstone.
It's free to join Oxfordshire Geology Trust. By becoming a member, you join our mailing list and hear about all of our up-and- coming events directly from us.
Click here to download a membership form