Septarian Nodule: Properties, Facts and Photos

Septarian nodules are distinctive geological structures characterised by cracked interiors commonly filled with calcite. Often described as prehistoric mudballs, they formed within sedimentary environments millions of years ago and are valued for their scientific interest and striking appearance.

Septarian: Meaning and Geology

The word 'septarian' comes from the Latin word 'septum,' meaning a dividing partition between two tissues or cavities. In anatomy, the nasal septum is the wall of cartilage and bone that separates one nostril from the other.

Septarian, correctly known as a septarian nodule or concretion, is a rock, not a mineral.

The cracks in septarian nodules are commonly filled with yellow and brown calcite. The exterior of the nodule is often limestone or marl, a lime-rich mudstone. Some septarian nodules also contain aragonite or siderite.

Septarian is sometimes likened to a prehistoric mudball because of how it formed.

Septarian Nodule or Concretion?

Although often known simply as septarian, the correct name for this geological structure is septarian nodule or concretion.

In geology, nodules and concretions are quite similar, hence the words tend to be used interchangeably.

A nodule is a small, irregularly shaped mass or lump of crystals or particles with a composition that contrasts with the surrounding rock. It may even be separated from the formation in which it occurred.

A concretion is a self-contained, cemented body of sediment. The word concretion comes from the Latin, 'to grow together' or 'to harden'.

Despite being quite common, concretions are often considered a geological curiosity because of the many unusual shapes, sizes and compositions in which they occur.

How Septarian Nodules Form

Septarian nodules occur in sedimentary rocks of many geological ages. Some of the most familiar examples formed in marine sediments during the Jurassic and Cretaceous periods. The Cretaceous Period, which followed the Jurassic, began approximately 145 million years ago and ended around 66 million years ago, coinciding with the extinction of the non-avian dinosaurs.

Sedimentary rocks associated with septarian nodules include sandstone, composed of compacted sand grains, shale, formed from compacted mud, as well as siltstone and limestone. Limestone is composed primarily of calcium carbonate.

Aragonite is a crystalline form of calcium carbonate, a compound that occurs naturally in rocks, most notably in limestone, including fossiliferous limestone.

Siltstone is a fine-grained sedimentary rock composed mainly of silt-sized particles, which are smaller than sand but larger than clay. These particles were carried in water before being deposited and compacted into rock.

Septarian nodules formed beneath the water as mineral-rich sediment accumulated and cemented around a central nucleus. Gradually, through various geological processes, the mass hardened to form a nodule.

Many septarian nodules formed in marine mud, on the seabed or in shallow coastal lagoons. In the Tatao area of Madagascar, geologists believe the soft mass may have been rolled by the movement of the water or carried by mudflows before it settled.

The newly formed structures then became buried under sediment and over time, cracks formed inside. Mineral-rich water then passed through the rock, leaving minerals behind that filled the cracks and crystallised. They were mostly calcite, sometimes with silica.

The crystals which formed are the bright yellow centres of many septarian nodules. In the Moeraki Boulders of New Zealand, brown calcite formed first along the walls of the cracks, with yellow calcite growing later towards the centre.

In some nodules, the clay-rich body is cemented with siderite, an iron carbonate, forming a rock known as clay ironstone.

The exterior of the septarian nodule was hard, but the interior contained distinctive angular cavities or cracks. These became known as 'septaria' from the Latin 'septum.'

The cracks are believed to have been caused by the dehydration and shrinkage of clay. Some geologists suggest they may have resulted from the expansion of gases generated by decaying organic matter in the centre of the nodule. They may also have been caused by fracturing or shrinkage associated with earthquakes or compaction.

Describing precisely how septarian nodules form is not easy. There are many different views and interpretations. Numerous questions remain unanswered, and it remains an ongoing topic of debate among geologists. However, the general formation process is relatively common in sedimentary rocks.

Large Septarian Concretions | Moeraki Boulders

The Moeraki Boulders are large, grey-coloured septarian concretions found on a stretch of coastline in New Zealand.

The boulders, buried for millions of years beneath mudstone, began to appear because of erosion from coastal cliffs. In years to come, more will likely emerge from the mudstone as geological changes continue to reshape the landscape.

The larger Moeraki boulders are estimated to have taken about four million years to reach their current size.

Almost identical spherical boulders can be found close to Hokianga Harbour on the North Island. Similar concretions can be found in many countries around the world.

The Moeraki boulders are famous primarily because of their shape and size.

Article Pictures

The picture of the septarian nodule at the top of our article is courtesy of Stan Celestian.

The piece in the second picture is in London's Natural History Museum. The septarian spheres are from our collection. Photos: Stone Mania.

The fourth image redirects to an impressive photo of the Moeraki Boulders on Koekohe Beach on the east coast of New Zealand's South Island. The photo is on Flickr, but you may only be able to view it if you join. Joining Flickr is free, and there are no additional charges.

Pop-up images
Aragonite, fossiliferous limestone, siltstone, shale, siderite and inside the concretion: Courtesy of Stan Celestian.

The photos in our article and the links for the pop-up photos link to the original image on Flickr. You may only be able to view them if you join. Joining Flickr is free, with no additional charges.