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Septarian Stone a Geological Curiosity

septarian nodule with distinctive yellow calcite crystals filling most of the centre section

Septarian the Geology Behind this Unique Rock

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

This spherical or oval shaped stone forms inside sedimentary rock.  It’s 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. They divide the stone into sections.

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

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

The unique pattern of cracks that radiate outwards from the centre of septarian are often filled with yellow calcite crystals. 

Septarian stones form in part through a geological process known as concretion.  During this process mineral-rich fluids fill spaces between sedimentary particles.
unpolished rough septarian concretion The process begins when minerals being carried in water seep into porous sedimentary rock.  Over time as the water evaporates just the 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 whilst one that forms around a fossil will have a shape that resembles the fossil.

As the concretion continues to grow it becomes surrounded by layers of sediment such as clay, silt and sand.  This is likely to happen 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 then seep through cracks and spaces. Over long periods of time these minerals crystallise.

Research carried out some time ago on septarian concretions from Dorset in the UK state 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, 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 for this happening continues to puzzle geologists.
the inner sections of two septarian nodulesThe 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 final photo are courtesy of The Fossil Shop.

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

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