Septarian Nodule: Properties, Facts and Photos

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 mud ball 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 formed during the Cretaceous Period, which began approximately 145 million years ago and ended 66 million years ago, around the time the dinosaurs disappeared.

Septarian nodules formed beneath the water as minerals and organic matter accumulated around a centre mass. Gradually, over millions of years and with the help of different geological processes, the mass hardened and cemented together to form a nodule.

The minerals are likely to have included sandstone, which is compacted grains of sand, shale, compacted mud, and siltstone and limestone, which are primarily calcium carbonate.

The mineral aragonite is the crystalline form of calcium carbonate. This chemical compound occurs naturally in rocks, most notably as limestone.

Siltstone is fine-grained silt, sand, clay or other materials carried in water before being deposited as sediment. 

Septarian nodules may have formed in shallow lakes as the tide 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. Both photos are by Stone Mania.

Pictures are clickable and redirect to the original image.

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.

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