Calcareous and ultrabasic scree, with boulders, rock debris and riverine gravel derived from sedimentary and metamorphic rocks, ultramafics and basic volcanics occurring through the western Mediterranean, from lowlands to the high mountains. Epilithic lichens and bryophytes may be very diverse, particularly in the mountains, where they are mostly found in crevices and other shady and humid microsites of immobile boulders. The vascular plant vegetation comprises hemicryptophytes and chamaephytes adapted to the mechanical disturbance caused by mobile screes, shortages in water supply and lack of fine-grained soil. The habitat becomes scarcer and more scattered to the foothills and lowlands and more prone to be affected by human disturbances, such as quarrying and infrastructure development but high mountain screes are usually well preserved and therefore in a very natural state.