Septarian Concretion or Septarians nodule were formed during the Cretaceous Period, around 50 to 70 million years ago. Sea levels were much higher then and the Gulf of Mexico reached inland to Southern Utah where many of the septarian nodules are found. They are also found in Madagascar where conditions were similar.
Periodic volcanic eruptions killed the smaller sea life which sank to the sea bed and started decomposing. The minerals in the shells and carcasses attracted sea floor sediments which accumulated around the carcasses and formed nodules or mud balls. When the ocean eventually receded, the mud balls dried out and began to shrink and crack into the beautiful patterns that you see inside the septarian nodules.
Septarian concretions or septarian nodules, are concretions containing angular cavities or cracks, which are called “septaria”. The word comes from the Latin word septum; “partition”, and refers to the cracks/separations in this kind of rock. 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. Frequently, minerals such as calcite (septa formation) are deposited in these cracks, but other filling minerals (for example, siderite, pyrite, barytes or quartz) may also be present. In the center of septaria there is often a nucleus of originally organic material.