Septarians were formed during the cretaceous period around 50 to 70 million years ago.
Sea levels were much higher then & the Gulf Of Mexico reached inland to southern Utah, where
many septarian nodules are found. They are also found in Madagascar.
Periodic volcanic eruptions killed the smaller sea life which sank to the seabed & started
decomposing the minerals in the shells and carcasses and formed nodules or mud balls. When
the oceans eventually receded, the mud balls dried out & began to shrink and crack into the
beautiful patterns that you see inside the septarian.
Over the eons, calcite leeched down into the cracks and formed calcite crystals which grew to
fill the cracks. The interface between the calcite and the bentonite clay transformed into
argonite which is the dark brown crystal layer.The bentonite mud was eventually replaced
with limestone which completed the transformation of the entire nodule to stone.