Many mechanisms have also been proposed to explain the formation of the very characteristic internal cracks (or cavities) pattern called "septa". It includes the desiccation of clay-rich, gel-rich, or organic-rich, cores leading to the shrinkage of the concretion's softer center. Some theories suggest the expansion of gases (CO2, CH4) produced by the decay of organic matter. Less satisfying theories even consider the shrinkage of the concretion interior by sediment compaction or its brittle fracturing by earthquakes. Several mechanisms might also be combined. In the Oligocene Boom Clay Formation (Rupelian age: ~ 34 – 29 Ma) in Belgium, flattened septaria are found in horizontal clay layers enriched in calcium carbonate, so their spatial distribution suggests they formed at the bottom of the ancient sea under special conditions.