Reefs include hard substrates which arise from the sea floor and which are continuously submerged or emerge at low tide, such as rocks, rocky sandflats and mudflats, and biogenic concretions or encrustations (e.g. mussel beds or honeycomb (Sabellaria) reefs), but also stones and erratics on submarine moraine ridges. As a result of the specific glacial and postglacial developments in the North and Baltic Seas reefs often occur in intimate association with habitat type 1110 (sandbanks). Elevations containing much glacial debris and displaying mixed substrates (e.g. glacial debris, marl, sands, mud) are included in this habitat type if hard substrate dominates.