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All the flowers spotted at Rock Edge on World Biodiversity Day, 22 May 2022

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The Rock Edge Site of Special Scientific Interest is a remnant of the limestone quarries formerly worked in Headington. The rocks exposed in the cliff face are of Upper Jurassic age, around 140–150 million years old. The Friends of Lye Valley have “adopted” this site for its abundance of nectar- and pollen-bearing limestone-loving wild flowers which are a vital source of nourishment for the invertebrates (moths, butterflies, soldier flies, hover flies etc) which breed in the Lye Valley fen, just to the south. We aim to increase the biodiversity of the site by sowing local wild-flower seed, including two sorts of scabious which support the nationally rare scabious bee and other bee species. We work with the Oxford City Council in management of the site and run joint conservation work parties and family fossil events in conjunction with the Oxfordshire Geology Trust.

Bringing back Headington’s Limestone Wildflowers: A Friends of Lye Valley project

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Above: Campanula glomerata, the clustered
bellflower, photographed on the walk in Rock
Edge led by Judy Webb on 2 August 2014

Left: Jurassic coral rock exposure in the former
quarry (which was known as the Crossroads Pit)

Here is a link to a list of plant species recorded so far for the Rock Edge Nature Reserve in Headington. There will be more to find, but this is the majority of the species:

• Flowers of Rock Edge species list (Excel table)

The most uncommon in a county context are highlighted yellow. These are not yet rare, but can be described as ‘very local’ which means restricted to certain special areas and in general declining nationally. Most of them are specific limestone/chalk specialists except the great burnet, which is usually a damper meadow specialist.

Rock Edge’s Geology and Fossils
by Oxfordshire Geology Trust

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Volunteering at Rock Edge

Friends of Lye Valley and Oxford City Council’s Countryside Volunteers joined forces on Thursday 25 June 2015 to help clear the paths across Rock Edge, trimming overhanging branches and encroaching nettles. Then volunteers cleared some of the growth which hides the Jurassic coral rock exposure.  A new plant was discovered for Rock Edge – hart’s tongue fern.  A good time was had by all – especially at tea break!

Clearing the coral rag

Tea break at Rock Edge

Clearing growth from around the hart's tongue fern

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Geoconservation event at Rock Edge on Saturday 21 March 2015

This was a big success, and the Friends of Lye Valley and the Oxford Geology Trust cleared a good long section of the 155-million-year-old Jurassic coral reef exposure, which is a nationally important site.

Also by lucky chance, the site was visited in the afternoon by a group of members of a local Geology Trust who were thrilled and thanked us for having cleaned it up so they could examine it in detail!  Friends of Lye Valley member Keith Frayn took these two photographs of the trust members enjoying the better access to the exposure. 

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Greater knapweed at Rock Edge


Nemophora metallica the brassy longhorn moth,
formerly Nemophora scabiosella) at Rock Edge


Andrena hattorfiana (scabious bee) at Rock Edge


Small copper butterfly on field scabious at Rock Edge


Andrena hattorfiana (scabious bee) and
Bombus pascorum (Common carder
bumblebee) on field scabious at Rock Edge

The Flora and Fauna at Rock Edge Geological SSSI/LNR

Rock Edge is a Geological SSSI. Sites of Special Scientific Interest are designated by Natural England (NE) and mean the site is of national importance for the geology visible. This means that the whole area has some legal protection from damage to the important exposed cliff section of 155-million-year-old Jurassic coral reef formed in a warm, tropical, shallow sea. The fact that the site is also designated a Local Nature Reserve (LNR) by Oxford City Council means that there is some recognition already of the wildlife value of the grassland and scrub, although LNR status means there is no legal protection for the wildlife at all. Much of the grassland near the roads at Rock Edge is species-poor and nutrient-enriched, possibly from dog excrement. This has also encouraged too much nettle and bramble growth.

Areas in the centre of the site on the ex-quarry floor have thin dry soil where the original limestone is visible, and these will be highly alkaline as a result of the calcium carbonate which composes the bodies of the fossil corals from the Jurassic reef. These areas are also nutrient poor (poor in nitrogen and phosphorus) as can be seen from the sparse, dwarfed vegetation that is yellow-green, not deep-green (plants can only be deep green in areas of high nutrient soil). A low-nutrient alkaline soil favours attractive wildflowers, whereas a high nutrient soil favours rank, species-poor tall grassland.

In these sparse, thinly-vegetated. nutrient-poor areas exists relict limestone grassland flora with important indicator species such as upright brome (Bromopsis erecta), hairy oat (Avenula pratensis) and a restricted range of attractive typical limestone species including a variety of flowers useful to insects, the most important of which are the common and greater knapweeds (Centaurea nigra and Centaurea scabiosa), field scabious (Knautia arvensis), and small scabious (Scabiosa columbaria). Very small amounts of hoary plantain (Plantago media) are found alongside the recently discovered clustered bellflower (Campanula glomerata). I feel it likely that this may be the only site in the city where small scabious and clustered bellflower survive in an original limestone grassland setting. There are several patches of common rest harrow (Ononis repens), which is also an original limestone grassland species, but the several patches of Lucerne (Medicago sativa) may have moved into the quarry from farmer’s fields nearby (before housing) where it would have been grown as forage or a soil improver. A couple of clumps of great burnet (Sanguisorba officinalis) and a big clump of comfrey (Symphytum officinale) occur on the Windmill Road side and may indicate an area of damper grassland just inside the wall. In the spring a small population of cowslips (Primula veris) occurs: these were probably introduced.

There are very small patches of bird’s foot trefoil (Lotus corniculatus). This is important to support the caterpillars of the common blue butterfly and the day-flying black-and-red six-spot burnet moth. Common butterflies using this site include: meadow brown, ringlet, gatekeeper, speckled wood, and marbled white – all of which have caterpillars feeding on grasses, but needing the abundant flowers such as knapweed, scabious, and ragwort for nectar sources. In the spring, orange-tip butterflies are seen: these will be using the garlic mustard on site as a caterpillar food plant.

Rarer insects (Nationally Scarce) include the brassy longhorn moth (Nemophora metallica), which breeds on scabious, and the ‘scabious bee’, Andrena hattorfiana ('Nationally Scarce'), which collects nectar and pollen almost exclusively from field scabious (Knautia arvensis) and small scabious (Scabiosa columbaria). Field scabious pollen is pink, and the it is collected on hairs down the whole length of the back legs, resulting in the bee looking like it is wearing pink ‘bloomers’ when the legs are fully loaded. It nests in light, free-draining soils in areas that support populations of field scabious. Further investigations would be necessary to locate the nest holes and actually prove it is breeding in soil on the Rock Edge site and not just visiting from a breeding site nearby. There are attractive picture-wing flies (Tephritids) such as Terellia colon, which breed exclusively in the flower heads of Greater Knapweed (Centaurea scabiosa), so is also dependent on the management which promotes this plant species to flower abundantly, and it needs the seed heads left for some time. There will be many more important insects yet to discover.

Scrub species found include elderberry, spindle, hawthorn, buddleia, and mock orange shrubs (the last two being garden escapes). Climbers include wild hop and greater bindweed. Some trees such as yew and holly have probably arrived on their own (from bird-dropped seed) and domestic apple (from a discarded apple core?). Others such as cherry plum, sycamore, Norway maple, whitebeam, and small-leaved lime will have been planted. Bramble or blackberry scrub threatens to ramp away in more enriched soil areas, accompanied by abundant nettles.

Both the scabious flowers required are present on site and increasing in flower numbers since late summer cutting with hay removal management introduced a couple of years ago. Obviously it would be good for the scabious moth and scabious bee populations if this management of the grassland on site continues in order to ensure continued seed germination and flowering of the species on which this bee depends. Cutting and hay removal will also prevent dense tall grass and a thatch of dead leaves which would not allow open warm areas of the soil necessary for the burrowing of the bees to make nest holes. With this proper cutting management and with a certain amount of soil disturbance, it may well be that more relic limestone grassland species (like the clustered bell flower) will be encouraged to recur from the seed bank dormant in the soil.

Judith Webb, August 2014