Septarian nodules were formed 50 million years ago when waters from the Gulf of Mexico reached into southern Utah. The have a siltstone exterior with aragonite veining and calcite centers. These septarian nodules, when opened, reveal a pattern of divided walls. The name Septarian came from the Latin word “septum” meaning”dividing walls”. Septarian Nodule Bookends Pair Measures 5.0″ X 10.0″ X 3.0″, 8.0 lbs.