Septarians were formed during the Cretaceous period, 50 to 70 million years ago when the Gulf of Mexico reached what is now Southern Utah. Decomposing sea life killed by volcanic eruptions, had a chemical attraction for the sediment around them, forming mud balls. as the ocean receded, the balls were left to dry and crack. Because of their bentonite content they also shrank at the same time trapping the cracks inside. As decomposed calcite from the shells was carried down into the cracks in the mud balls, calcite crystals formed. A thin wall of calcite was transformed into aragonite separating the bentonite heavy clay exteriors from the calcite centers. Because of this, the nodules are called Septarians. Septarians are composed of Calcite (The Yellow Centers), Aragonite (The Brown Lines) and the Outer Grey Rock is Limestone. Occasionally the fossil or some of the fossils which started the formation of the rock is noticeable in the rock.