Why Does Septarian Have Cracks?

The cracks that give septarian its name have been argued over for more than a century. This article looks at what research into concretions from Dorset revealed, and at one geologist's proposal that the answer lies in earthquakes.

Septarian Stone and the Mystery of Its Cracks

Septarian, also known as septarian concretion or septarian nodule, is a spherical or oval-shaped stone that forms within sedimentary rock. Its most distinctive feature is the network of cracks that radiates outwards from the centre, dividing the interior into angular sections. These cracks, known as septaria, are usually filled with yellow calcite crystals, producing the striking patterns for which the stone is known.

Our main article covers what septarian is made of and how it forms. This article is concerned with the cracks themselves, and why there's so much disagreement about how they formed.

What the Dorset Concretions Revealed

In 2001, geologists D.J. Green and Duncan Pirrie published a study of septarian concretions from Osmington Mills in Dorset. They examined their composition and stages of development to establish how the concretions formed and when the distinctive cracks appeared.

The concretions came from the Nothe Grit Formation and dated from the Oxfordian stage of the Late Jurassic period. They had developed around large oyster shells, which acted as starting points for their growth.

The surrounding sediment contained quartz, feldspar, mica and clay, along with fragments of marine organisms. The concretions themselves were mainly calcite, with smaller amounts of pyrite, glauconite and silica.

The minerals helped the researchers reconstruct the order of events. Calcite first cemented the sediment around the shells before it had been significantly compacted. After burial, a local build-up of pressure within the sediment caused radial fractures to develop. These cracks were subsequently filled by further deposits of calcite, including an iron-bearing form.

The study revealed when the different stages occurred, but it did not fully explain why the cracks developed their distinctive shapes. How the cracks within septarian concretions form continues to puzzle geologists.

Brian Pratt and the Earthquake Theory

Also published in 2001 was an academic paper by Brian R. Pratt of the Department of Geological Sciences at the University of Saskatchewan in Canada. His research examined how and why cracks formed in septarian concretions.

Pratt examined septarian concretions from marine and marginal-marine shales and mudstones. When he compared their internal cracks, he found evidence of more than simple shrinkage. Some resembled shrinkage cracks, while others showed signs of tension and shearing. Broken and displaced fossils, fragmented sections and sediment forced into cracks indicated that some of the deformation had occurred suddenly.

The concretions would not have been equally hard throughout when this happened. Some areas were still relatively soft, while others had become rigid. Sudden shaking could therefore have caused softer material to lose water and contract while harder areas fractured or broke apart.

Earlier explanations involving dehydration, gas and pressure during burial could not account for the full range of features. Pratt proposed that earthquake shaking could have created the brief but powerful and constantly changing stresses needed to produce them.

His theory explains why the cracks vary so widely in shape and direction, and why some concretions show evidence of several separate cracking events. However, what caused the initial shrinkage that left parts of the concretions vulnerable to cracking remains unclear.

Article Photos

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

The rough septarian concretion is courtesy of James St. John.

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

Pop-up photos
The mica is from our collection. Calcite: Courtesy of Stan Celestian.

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