Concretions
Concretion, like concrete, means a hard cemented material. A concretion consists of the same material as the rock around it, plus an additional mineral cementing it together. Nodules, such as flint nodules, may look like concretions but they are composed of a different material to the rock surrounding them, such as flint nodules found in Chalk.
Concretions grow within sedimentary rocks such as shale or sandstone. When the rock is being laid down the cementing mineral is present dissolved in small amounts within the fluid between the grains. However, under certain chemical conditions, perhaps related to microbial activity, reactions within the sediment cause the mineral to become concentrated around a centre to such an extent that it deposits the cement a little at a time as thin layers. At the centre of concretions is often an object acting as a nucleus, such as a plant or animal remains or another rock or fossil. The minerals that cement concretions include calcite, iron carbonates and oxides or silicate minerals.
Concretions can be shaped like cylinders, sheets, nearly perfect spheres and everything in between. In size they can range from as small as gravel to as large as a truck. Moeraki boulders in New Zealand are concretions that have grown to over 2m in size in around 4 million years. The size of concretions is controlled by the chemical conditions of the sediment, the availability of the cementing mineral and rate at which fluids can migrate within the sediment.
Some concretions may have a hollow centre, a centre filled with loose powdery clay or sand, or a detached hard lump resembling a nut. The loose powder shows that iron oxide formerly cementing the grains has been drawn away from the middle and towards the outside, contributing to the hard iron oxide shell. If a loose ‘nut’ is present, there has been some internal shrinkage when the concretion dried out. Sometimes the ‘nut’ can be heard rattling inside the concretion.
Concretions are usually harder than the rock that contains them and thus tend to survive erosion and weathering to end up exposed in soil layers and on pebble beaches. This means they are often found and excite interest, sometimes being mistaken for fossil eggs or meteorites.
Septarian concretions
In septarian concretions the outside of the developing concretion becomes a hard shell, with dehydration caused by chemical reactions of the inside material resulting in shrinkage cracks. These cracks may subsequently be filled in with secondary calcite. As calcite is harder than the surrounding rock, weathering may cause these calcite filled cracks to actually stand proud of the rest of the nodule.
Limestone septarian concretion
Ironstone septarian concretion
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