DIRK WIERSMA / SCIENCE PHOTO LIBRARY DIRK WIERSMA / SCIENCE PHOTO LIBRARY
Septarian nodules are round to ovoid concretions, compact bodies formed by accretion in deposits of claystone, limestone or rhyolite (a volcanic deposit). In septarian nodules the interior is dissected by a network of cracks that result from drying up and shrinkage of the initial concretion. The cracks are usually later filled up by precipitation of some other mineral like agate or calcite, often visibly crystalline. The cracks are often restricted to the interiors of the nodule and only seen after splitting or cutting the concretion. In the example here seen, a claystone nodule from Taouz, Southern Morocco, the cracks run out to the periphery, by exception Actual diameter is 36 cm.
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