Peculiarly shaped, relatively large (up to 30 cm in diameter) concretions of quartzitic sandstone occur in a single horizon of Upper Albian loose sands in the Cracow Upland, southern Poland. They are characterized by hollow interiors adorned with mass-aggregated moulds of the borings of diverse sponges, polychaetes and bivalves. These moulds represent the siliceous filling of borings in limestone clasts that had been subject to dissolution, leaving a hollow within the concretion that had formed around them. Synsedimentary block-faulting and jointing affected the Jurassic limestone-basement, causing the uplift of a local horst (the Glanów Horst), to within the littoral zone so that it became exposed to abrasion. It is inferred that a hurricane or catastrophic storm surge swept limestone debris fallen from the cliff out to the sandy offshore, where nucleation of soluble silica was presumably favoured by the decay of the soft tissue of live or dead rock-borers. After filling the empty borings and solution of the limestone clasts, the nucleation progressed intensely, finally completed by precipitation of siliceous sinter in the hollow interiors of some of the concretions during subsequent diagenesis and/or epigenesis.
