One of several fairy rings in Lambrok Meadow, caused by the mycelium of a subterranean fungus.
The fungus secretes enzymes from the growing tips of the underground threads, called hyphae, that make up the mycelium. The enzymes break down the nutrients in the soil into molecules small enough to be absorbed through the walls of the hyphae. The mycelium grows outward, looking for more nutrient-rich soil, while in its now nutrient-deficient centre, the fungus dies off and eventually an expanding ring is formed.
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Oyster mushroom mycelium [1] showing radiating growth pattern of the hyphae and [2] the fruiting bodies of a fairy ring mushroom (Marasmius oreades).
Sometimes, the grass inside a fairy ring will be weakened or even die but the Lambrok Meadow fungus must be of the type that produces hormone-like chemicals called gibberellins, which alter the grass’s growth patterns, causing this luxurious dark green ring.
There are two types of fairy ring fungus. Those found in the woods are called tethered, because they are formed by mycorrhizal fungi living in symbiosis with trees. Meadow fairy rings are called free, because they are not connected with other organisms. Free fairy ring fungi are saprotrophic: they grow on rotting material.
There are about 60 species of fungi which can grow in this fairy ring pattern. The most frequently found is Marasmius oreades, which goes by several common names: fairy ring champignons, fairy ring mushrooms or Scotch bonnets. We are unlikely to be able to identify this one, though, until it develops fruiting bodies next autumn.
Southwick Country Park Nature Reserve 
Excellent, thank you for finding out for me.
It’s a pleasure, Julie. There are three or four rings at that end of Lambrok Meadow; if they produce fruiting bodies, send me pictures and then we will be able to identify the species.
Southwick Country Park
definitely. I will keep watch, thanks again
There are lots of cremation pits in the field near lambrok close in the same field is a bronze age burial mound
While a burial pit will leave a mark in the soil, it doesn’t usually leave such a distinct mark in the vegetation, not after two or three thousand years, anyway. Two of the rings DID have that odd sticking-out piece (right hand side of my pic) though and I can’t account for that. A fungal ring seemed most likely but a bronze age site would be a great thing to find.
I recently contacted the Historic Environment Records Data Manager at the Wiltshire and Swindon History Centre to ask if there were any historic finds in Southwick that we should mention in the Southwick Neighbourhood Development Plan. Records relating to Southwick Park included – medieval ridge and furrow earth workings and post medieval water meadows in an area close to the Lambrok waterway and further back on the site evidence of a probable medieval building and a slightly curving ditch or gully containing a single sherd of LATE PREHISTORIC POTTERY and a flint flake. It is considered that the ditch is probably PREHISTORIC.
How exciting! We knew about the ridge and furrow workings and the old water meadows but not that there had been a medieval building or a prehistoric ditch. If there has been a bronze age settlement in the area, perhaps it is associated with that. Do you know if these findings are mapped anywhere?
I’ll try to send you the details and reference numbers of these sites and the map of sites in Southwick I received from the Data Manager.
Thank you, Barbara.
Iv’e sent details and map to Suzanne.