What is Septarian?

Septarian Stone - Geological Curiosity

Septarian, also known as septarian concretion or septarian nodule, is a fascinating geological curiosity.

This spherical or oval-shaped stone, which forms inside sedimentary rock, is composed primarily of the minerals calcite, aragonite and siderite.

Septarian has a distinctive pattern of cracks and lines that radiate outwards from the centre of the nodule. These cracks and lines divide the stone into sections.

Septarian typically forms in shallow marine environments or coastal mudflats. The finer details of how it forms are not fully understood, and many views and interpretations exist.

The formation of a septarian nodule takes place over millions of years. The septarian we marvel at today began forming around 110 to 125 million years ago.

The unique pattern of cracks radiating outwards from the centre of the stone is often filled with yellow calcite crystals. 

Septarian forms in part through a geological process known as concretion. During this process, mineral-rich fluids fill spaces between sedimentary particles.
The process begins when minerals carried in water seep into porous sedimentary rock. Over time, as the water evaporates, only minerals are left behind.

The accumulation gradually builds up and eventually forms a hard solid mass.

The shape of a concretion is determined by the shape of the original particle or fossil around which the minerals were deposited. For example, a concretion that forms around a shell is likely to have a circular shape, while one that forms around a fossil will resemble the fossil.

As the concretion grows, it becomes surrounded by layers of sediment such as clay, silt and sand, probably as the concretion rolls back and forth with the natural motion of the water.

When the water dries up, the outer layer cracks due to dehydration, which creates the characteristic 'cracks' or 'septaria.'

The newly created structure is then buried under sediment. Minerals such as silica or calcite seep through cracks and spaces, and over time, the minerals crystallise.

Research carried out some time ago on septarian concretions from Dorset (in the UK) states they formed around large oyster shells during the Late Jurassic period. The stones are said to have been made up of quartz, feldspar, mica and clay.

Small amounts of pyrite were also detected, as well as a significant amount of shell from sea creatures.  

Cracks in the septarian that radiate outwards from the centre were filled with calcite. The stone was believed to have formed in two stages, the calcite formed during the first stage, and the cracks formed after the structure was buried.

Pressure from compaction is likely to have caused the cracking.

Exactly how the cracks within septarian concretions form has been debated for more than one hundred years. The precise reason continues to puzzle geologists.
The following information, which I've simplified, comes from an academic paper written in 2001 by Brian R. Pratt, Department of Geological Sciences, University of Saskatchewan, Canada.  His discussion focuses on the cracks in septarian stone and how and why they formed. 

Septarian concretions have been documented mainly from marine environments. Almost all have been dated to the Phanerozoic age. The Phanerozoic is the current period in Earth's geologic time scale.  It began about 541 million years ago and continues to this day.

Some believe the cracks were caused by dehydration as organic materials or clay dried out. The generation of gases or excessive pressurisation during burial has also been considered. 

The wide range of shapes in which the cracks in septarian concretions occur is of particular interest. Geologists are keen to understand what kind of stress was involved in order for these to form.

A recent theory suggests cracking was triggered by strong shaking during earthquakes. What caused shrinkage within the concretion that led to them becoming prone to cracking remains unclear.

Septarian stones are fascinating geological structures. Once polished, they can be quite spectacular and are highly sought after around the world.

Article Photos

The septarian nodule at the top of our article was photographed by Stan Celestian.

The septarian concretion in the second photo was photographed by James St. John.

The septarian slices in our last photo are courtesy of The Fossil Shop.

The first two photos are clickable and redirect to the original image.

Pop-up photos: Aragonite - Courtesy of Stan Celestian. Siderite - Courtesy of Ron Wolf. The mica is from our collection.