Septarian Nodule: Properties, Facts and Photos

Septarian nodules are distinctive geological structures characterised by their cracked interiors filled with yellow 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 septum is the cartilage in the nose that separates one nostril from the other.

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

Septarian nodules are composed primarily of three different minerals. The yellow crystals are calcite, the brown lines are aragonite or siderite and the outer shell is limestone.

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 contrasting composition. That means they're not all the same and may even be separated from the formation in which they 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 formed in marine sediments during the Jurassic to 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.

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

The materials involved typically include sandstone, composed of compacted sand grains, shale, formed from compacted mud, as well as siltstone and limestone, which are primarily composed 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 were carried in water before being deposited and compacted into rock.

Septarian nodules may have formed in shallow lakes as the movement of the water caused an accumulated mass to roll gently back and forth. Over time, with the addition of new layers of sticky mud, the size of the mass increased. During the hot summer months, as the water receded, the mud dried out, causing it to crack. 

The newly formed structures then became buried under sediment. The cracks slowly filled through seepage with a coarse crystalline substance such as silica (quartz) or calcite from the shells of dead marine creatures.

The crystals which later formed are the bright yellow centres of the septarian nodule. A thin wall of calcite was also transformed into aragonite or siderite, separating the heavy clay exterior from the crystallised centre.

The exterior of the septarian nodule was hard with a network of ridges, 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 be 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 caused by earthquakes or compaction.

Regardless of how the cracks formed, they subsequently filled with hardened minerals as groundwater changed and receded. The minerals were mostly silica or calcite. 

Describing precisely how septarian nodules form is not easy. There are many different views and interpretations. Numerous questions remain unanswered, and it's 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 largest septarian concretions 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 third photo 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 the photographic website Flickr, but you may only be able to view it if you join. Joining Flickr is free, and there are no additional charges. 

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