Septarian Geode (Septarian Nodule)

Septarian Geode (Septarian Nodule) Rock Professor Image

Utah Septarians are found near Zion National Park in Southern Utah. They are a beautiful rock that started their formation approximately 150 million years ago when the gulf of Mexico reached what is now southern Utah. Volcanic eruptions killed the sea life and they became trapped in the sediment and formed mud balls.

The ocean receded and the balls were left to dry and crack.

The ocean returned depositing more shell life above them. As this decomposed, calcite from the shells was carried down into the cracks of the mud balls, and calcite crystals formed. A thin wall of calcite was transformed into aragonite dividing the bentonite clay (shale) exteriors from the calcite centers. Because of this dividing wall (septum in Latin) the geodes are called Septarians.

Composition: Calcite (the yellow centers), Aragonite (the brown lines) and the bentonite clay (outer grey rock). Occasionally the fossil or some of the fossils which started the formation of the rock is noticeable in the rock ♦ Localities: Utah, New Zealand, worldwide.