|Septarian from Muddy Creek, Orderville, Kane Co., Utah. Credit Rob Lavinsky|
A concretion is a hard, compact mass of matter formed by the precipitation of mineral cement within the spaces between particles, and is found in sedimentary rock or soil. Septarian concretions are concretions containing angular cavities or cracks, called "septaria". The septaria are the calcite filled cracks at the centre of the rock, indicating where the centres of the concretions have shrunk, possibly during dehydration during its long transformative journey.
Cracks are highly variable in shape and volume, as well as the degree of shrinkage they indicate. Although it has commonly been assumed that concretions grew incrementally from the inside outwards, the fact that radially oriented cracks taper towards the margins of septarian concretions is taken as evidence that in these cases the periphery was stiffer while the inside was softer, presumably due to a gradient in the amount of cement precipitated.
|Calcite inside Septarian concretion. photo: DweadPiwateWoberts|