The Marine Life Information Network

Information on the biology of species and the ecology of habitats found around the coasts and seas of the British Isles

Substratum or habitat types

Rock and sedimentary substrata
Hard (immobile)
Immobile hard substratum e.g. solid rock, concrete but inc. soft rocks , such as chalk .
Artificial (man-made) Man-made structures e.g. of metal, wood or concrete. 
Bedrock Any stable hard substratum, not separated into boulders or smaller sediment units. Includes soft rock-types such as chalk, peat and clay (Hiscock et al., 1999). 
Large to very large boulders  Boulders >512 mm; likely to be stable (Hiscock, 1996).
Small boulders 256 - 512 mm. May be unstable.
Hard (mobile)
Mobile hard substratum, e.g. cobles, pebbles that are regularly moved by wave action
Cobbles 64-256 mm. May be rounded to flat. Substrata that are predominantly cobbles.
Pebbles 16-64 mm. May be rounded to flat. Substrata that are predominantly pebbles.
Sediment / soft
Definitions from Hiscock (1996) and Long (2006)
Coarse sediment Gravel / shingle 4 -16 mm. Clean stone or shell gravel including dead maerl. >80% gravel.
Sandy gravel 50 -80% gravel, 20 -50% sand.
Gravelly sand 50 -80% sand, 20 -80% gravel.
Sand and muddy sand  Sand 0.063 -  4 mm. >80 % sand
Coarse clean sand 0.5 - 4 mm. >80 % sand.
Medium clean sand 0.25-1mm > 80 % sand.
Fine clean sand 0.063 - 0.5 mm. >80 % sand.
Sandy mud 50 - 80 % mud, 20 - 50 % sand
Mud and muddy sand
As defined by Long (2006). 
Muddy sand 50 - 80 % sand, 20 - 50 % mud.
Mud <0.063 mm (silt / clay fraction). >80% mud.
Mixtures of a variety of sediment types, composed of pebble / gravel / sand / mud. This category includes muddy gravels, muddy sandy gravels, gravelly muds, and muddy gravelly sands.
Muddy gravel 50 - 80 % gravel, 20 - 50 % mud.
Gravelly mud 50 -80% mud, 20 -50% gravel.
Muddy sandy gravel 50 -80% gravel, 20 -50% mud and sand
Muddy gravelly sand 50 -80% sand, 20 -50% mud and sand
Sandy gravelly mud 50 -80% sand, 20 -50% mud and sand
Other substrata or features
Substratum (other) Definition
Biogenic reef An elevated structure on the seabed built by calcareous or other concretion-forming organisms, or by chemical precipitation (Hiscock, 1996) e.g. reefs of Modiolus modiolus or Sabellaria alveolata.
Burrow dwellers Occupies or shares space in burrow constructed by other organisms.
Caves A large hollow in the side of a vertical rock face or cliff.
Crevices / fissures Narrow openings (Thompson, 1995)
Macroalgae Macroalgae surfaces, such as the fronds of kelps or fucoids.
Maerl Calcareous nodules (rhodoliths) of maerl that may build up to form maerl beds
Seagrass Habitat associated with seagrass bed communities.
Strandline A line on the shore composing debris deposited by a receding tide; commonly used to denote the line of debris at the level of extreme high water (Lincoln et al., 1998).
Salt marsh A flat, poorly drained coastal swamp inundated by most high tides (Lincoln et al., 1998).
Overhangs An overhanging part of a rock formation (Thompson, 1995).
Other species The surface of other species, e.g. shells or carapace.
Peat (fossilized) A pure organic soil, the result of plant material accumulating in waterlogged conditions (from Fitter & Manuel, 1986). Fossilized peat may also be exposed and provide a unique substratum in the intertidal.  
Rockpools A pool of water among rocks left behind by the ebbing tide
Under boulders Habitat associated with the underside of boulders.
Wrecks Remains of sunken vessels (boats and ships).


Connor, D.W., Allen, J.H., Golding, N., Howell, K.L., Lieberknecht, L.M., Northen, K.O. & Reker, J.B., 2004. The Marine Habitat Classification for Britain and Ireland. Version 04.05. Joint Nature Conservation Committee, Peterborough. Available from: <>

Fitter, R. & Manuel, R., 1986. Collins field guide to freshwater life of Britain and north west Europe. London: Collins.

Hiscock, K. (ed.), 1996. Marine Nature Conservation Review: rationale and methods. Peterborough: Joint Nature Conservation Committee. [Coasts and seas of the United Kingdom. MNCR series.]

Lincoln, R., Boxshall, G. & Clark, P., 1998. A dictionary of ecology, evolution and systematics (2nd ed.). Cambridge: Cambridge University of Press.

Long D. (2006). BGS detailed explanation of seabed sediment modified Folk classification.

Thompson, D., (ed.) 1995. The Concise Oxford Dictionary of Current English. 9th ed. London: Oxford University Press.

Wentworth, C.K., 1922. A scale of grade and class terms for clastic sediments. Journal of Geology, 30, 377-392.