Tide-swept boulders and cobbles, often with a mobile component to the substrata (pebbles, gravel and sand), characterised by dense stands of the brown seaweed Halidrys siliquosa. It is can be mixed with the foliose brown seaweed Dictyota dichotoma and kelp such as Laminaria saccharina and Laminaria hyperborea. Below the canopy is an undergrowth of red seaweeds that are tolerant of sand-scour such as Phyllophora crispa, Phyllophora pseudoceranoides, Rhodomela confervoides, Corallina officinalis and Chondrus crispus. Other red seaweeds such as Plocamium cartilagineum, Calliblepharis ciliata, Cryptopleura ramosa, Delesseria sanguinea, Heterosiphonia plumosa, Dilsea carnosa, Hypoglossum hypoglossoides and Brongniartella byssoides may be locally abundant, particularly in the summer months. There may be a rich epibiota on H. siliquosa, including the hydroid Aglaophenia pluma, ascidians such as Botryllus schlosseri. There is generally a sparse faunal component colonising the boulders and cobbles, comprising the tube-building polychaete Pomatoceros triqueter, the crab Cancer pagurus, the starfish Asterias rubens, the gastropod Gibbula cineraria and the sea anthozoan Urticina felina. The bryozoan Electra pilosa can form colonies on the kelp. Situation: This unit can occur below the tide-swept Laminaria digitata zone of the sublittoral fringe bedrock or boulders (unit A3.221). Less stable substrata of boulders, cobbles or pebbles may support kelp and Chorda filum in the shallows (A3.123) or dense ephemeral seaweeds (unit A1.45). Sand-influenced rocky outcrops in deeper water may support a Flustra foliacea community (A4.134). This biotope is widespread and is found on the open coast in Wales, the south-west and the English Channel as well as more sheltered tidal rapids in the Scottish sealochs. It can form extensive forests or parks in certain areas (Dorset, Sarns). In Wales, the south-west and west of England the red seaweeds Spyridia filamentosa and Halarachnion ligulatum and brown seaweeds Dictyopteris membranacea and Taonia atomaria are frequent. In Scotland, kelp occur at a greater proportion of sites, solitary ascidians such as Ascidiella spp. are more common and the featherstar Antedon bifida and brittlestars Ophiothrix fragilis are found. Temporal variation: Higher diversity of red seaweeds during the summer.