HOW OLD ARE THE POOLE’S CAVERN SPELEOTHEMS?

Over the years visitors on guided tours have been given a variety of ages for the Poole’s Cavern speleothems but the only thing we know for sure is that the  hyperalkaline speleothems must post-date deposition of lime waste on the slopes of Grin Low and hence are less than 250 years old. Any suggested ages for the ‘normal’ speleothems are guesses but Professors Gina Moseley (University of Innsbruck) and John Gunn (University of Birmingham and BCRA Chairman) are aiming to establish more accurate ages for some of these speleothems.

Where to start?

Speleothems are composed almost entirely of calcium carbonate in the form of calcite, but they also contain trace elements such as uranium. After deposition uranium is subject to radioactive decay and one of the daughter products is thorium. By measuring the exact amounts of two isotopes, uranium-234 and thorium-230, in the calcite, and calculating the ratio between them, scientists can determine how long ago the calcite was deposited.

The measurements have to be made in a specialist laboratory using a mass spectrometer and after visiting Poole’s with her students in 2024, Professor Gina Moseley agreed to date some of the Poole’s speleothems. As Poole’s Cavern is a Site of Special Scientific Interest permission to sample the speleothems was needed from Natural England and in February 2026, when permission was granted, Gina visited Poole’s with Professor John Gunn to collect samples.

Professor Moseley drilling the flowstone at the entrance to the Roman Chamber. Sediments below the flowstone pre-date calcite deposition and those above post-date calcite deposition.

How are the samples collected?

This is done by drilling a core through the speleothem in much the same way as cores are obtained from trees to study their age. Three sites were sampled, a flowstone layer at the entrance to the Roman Chamber, a flowstone beneath the Dome and the Wedding Cake, a large stalagmite boss adjacent to the Bridge.

Normal practice is to carefully seal the hole left behind when the core is extracted to maintain the visual integrity. However, the hole near path level in the Wedding Cake has deliberately been left open. This was because the drill was not long enough to reach the centre of the stalagmite which is where the earliest calcite was deposited and it is hoped that Gina may be able to return with a longer drill bit so that we can find when this stalagmite started to grow.

Images of the wedding cake with the drill used for coring and the final white calcite core when it had been extracted

What happens next

The cores are now on their way to the University of Innsbruck where they will be carefully cut in half along the centre line. This is so that for each core one half can be dissolved as part of the analytical process whilst the other half will eventually be returned to Buxton and displayed in the Poole’s Cavern Museum. Once dissolved the liquid will go through a series of complicated chemical processes before it is placed in a mass spectrometer and the amounts of uranium-234 and thorium-230 will be measured. Finally, the ages will be calculated  to tell us when the calcite was deposited