It’s not every day you get to slice open a rare piece of rock as if it was a lump of butter, but this is what the Cotswold Water Park Trust did as part of a fundraising exercise earlier in the year. Now the stunning piece of local geological history is on public display in the Waterside Café for the first time.
The 1m diameter rock, known in geological circles as a Septarian Nodule, and easily weighing half a ton, was cut in half by specialist stone cutters at a quarry in nearby Tetbury to reveal a spectacular maze of calcite crystals. These are formed through the action of organic matter on the Jurassic seabed, approximately 160 million years ago.
Septarian concretions or septarian nodules, are concretions containing angular cavities or cracks, called “septaria”. The word comes from the Latin word septum; “partition”, and refers to the cracks/separations in this kind of rock. Cracks are highly variable in shape and volume, as well as the degree of shrinkage they indicate.
The process that created the septaria remains a mystery. The concretions are all composed predominantly of calcite. They demonstrate several phases of formation including a precompactional phase which started with rapid growth in soft or partly compacted water rich sediment and then a later episode of brecciation (fracturing). Chemical data demonstrates a dominantly organic source origin for the early formation of the concretions. Oxygen isotope analysis indicate formation at the same temperatures (13-16°C). Concretion growth in mud with low oxygen levels started with bacterial sulphate and formation of iron pyrite (Fools gold). Later cracking of the concretions also started at this stage and the expanding cracks were also partially or completely filled with brown and clear calcite.
The chemical changes that followed as the concretion was buried deeper was finally halted when marine pore water was flushed out from the compacting clays by fresh water. It was at this at this stage, concretion growth ceased. Basically mineralising pore waters were squeezed out of the clay.
In the Jurassic era, this whole area was a warm shallow tropical sea, with marine reptiles and an abundance of ammonites, belemnites and other related creatures. The ancient sea bed formed the deposits of Kellaways and Oxford clays we see today which become exposed underneath the Ice Age gravels, following the extraction of sand and gravel.
The Nodule from the Cotswold Water Park is a rare example of an unusually large Septarian Nodule from the Jurassic period. There is evidence of some fossil ammonites on the surface of the rock, but once cut in half, the calcite deposits of the nodule were revealed, creating this beautiful geological sculpture.
Similar concretions can be seen in other parts of the world and often grow to enormous size such as the Moriaki boulders in New Zealand, These formed very quickly in soft sands that are much younger than the concretion found in the Cotswold Water Park.