This biotope is typically found encrusting the upper faces of wave-exposed and moderately wave-exposed circalittoral bedrock, boulders and cobbles subject to strong/moderately strong tidal streams in areas with high turbidity. The crusts formed by the sandy tubes of the polychaete worm Sabellaria spinulosa may even completely cover the rock, binding the substratum together to form a crust. A diverse fauna may be found attached to, and sometimes obscuring the crust, often reflecting the character of surrounding biotopes. Bryozoans such as Flustra foliacea, Pentapora foliacea and Alcyonidium diaphanum, anemones such as Urticina felina and Sagartia elegans, the polychaete Pomatoceros triqueter, Alcyonium digitatum, the hydroid Nemertesia antennina and echinoderms such as Asterias rubens and Crossaster papposus may all be recorded within this biotope. There are two variants. The first (unit A4.2211) contains significant cover of barnacles (Balanus crenatus) and bryozoans. The second (unit A4.2212) has a dense turf of didemnid ascidians as well as scour-tolerant bryozoans such as F. foliacea, sponges such as Tethya aurantium and Phorbas fictitius, colonies of the serpulid worm Salmacina dysteri and patchy occurrences of the ascidians Distomus variolosus, Polycarpa pomaria and Polycarpa scuba. This biotope has been recorded from the Lleyn Peninsula, Lundy Island (including the wreck of the MV Robert) and the north-east and south coast of England.