The ferruginous sandstone of the gothic “Maagdentoren” is suffering from a specific biological deterioration process triggered by perforating activities of mason bees. The damage due to these perforations causes extensive loss of material, so that a durable conservation of such degraded stone blocs becomes questionable. In order to evaluate the conservation possibilities of stone blocs damaged by perforating mason bees, an investigation of the internal structure by means of X-ray tomography was carried out. This investigation revealed that the cumulative effect of the digging work by multiple generations of mason bees may result in networks of perforations. Bioturbated sandstones were found to be most suitable for attack by mason bees because of morphological and geometrical compatibility between the original layered burrowings by marine organisms and those by the mason bees. As a conclusion the conservation is not recommended of sandstone blocs for which the load bearing capacity is endangered by the branched and layered perforations.