This alliance includes calcicolous vegetation on limestone cliffs or outcrops of other calcareous rocks. It can also occur on walls which contain calcium in mortar. Typical species are the calcicole ferns Asplenium ruta-muraria and Gymnocarpium robertianum and those with a broader tolerance to calcium content, such as Asplenium trichomanes and Cystopteris fragilis. Unlike on siliceous rocks, these habitats host more nutrient-demanding, in some cases ruderal species.