Pair of Septarian nodules, Belgium,Oligocene (25 million years old), on bases, h. 42 cm

Note: Septarian nodules form around anucleus of decomposing organic matter. During the fossilisation process, thismass will dehydrate and dissecation occurs. Shrinkage cracks will thus open inthe nodule. These cracks are sometimes filled with dolomite, celestine, or inthis case, calcite.