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The Lye Valley

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Catchment of Valley-head spring fen, drawn by Judy Webb

In the Lye Valley in Oxford for thousands of years there was once a very wet and wild tract of boggy land alongside the Lye Brook and Boundary Brooks. This was all within a very large open area of common land known in previous centuries as Bullingdon Green. The whole area was common grazing land until enclosures in 1856. Bullingdon Green stretched from Shotover around the area of the Lye and Boundary brooks and over what is land now occupied by housing in part of Headington and Wood Farm area and land now occupied by the Churchill, Park, Nuffield and Warneford Hospitals. The last portions of the ancient Bullingdon Green that are still green, grassy areas are the whole of Southfield (Oxford) Golf course, Warneford meadow and Warneford playing fields. Archaeological studies have shown that on the current Churchill Hospital site there was once a large Roman pottery industry, producing distinctive white wear that was exported all over the country. Broken fragments of Roman pottery may be found in the bed of the Boundary Brook, where they have washed out from the pottery waste pits. The wild boggy tract of land along the Lye Brook was known in the 1600s and 1700s as 'Hogley Bog', 'Hockley', or 'Hockley-in-ye-hole'. Later texts refer to it as 'Bogs under Bullingdon Green' or 'Headington Bog' and most recently as 'Bullingdon Bog'. Certainly the whole area was deforested long before recorded history and the abundant trees we see in the valley today are relatively recent re-invaders. Photographs by Henry Taunt from 1915 show a completely open landscape with just a few trees along the brook line. Along with the drier grassland the fens would have been rough-grazed and the reeds and rushes would have been cut for thatching, basket-making or as animal bedding, all these activities keeping the vegetation short and biodiverse. Grazing ceased early in the 1900s and the site became hemmed in by housing in the 1930s and '40s, resulting in the drying up of some of the springs. The lack of grazing and cutting led to scrub and reed invasion. Reversing this process by cutting and raking reed, removing scrub and trimming willows is bringing back the short vegetation. These are the main important jobs the OCV have to do on this site.

Old Hogley Bog (what we now call the N and S Fen)

A bog or fen is a type of wetland. In the old days the term 'bog' was used for any wet and peaty area with marsh plants. These days we use the term bog only for sites with acid pH (less than 4.5). In the Lye Valley the wetland has a very high pH (7.5+) because of the very lime-rich water feeding it, produced by the springs along the valley sides. Thus in the Lye Valley the wetland is known as a 'calcareous fen'. A fen is non-acid wetland on peat which is fed by groundwater rather than by rainwater. There are lots of types of fen but in the Lye Valley what exists is a remnant of the very rarest type of calcareous fen called a Valley-head spring-fen. This type of fen is also known as rich-fen because it has a high diversity of plant species adapted to the wet, high pH conditions. It is a habitat of high conservation value, which is why the North and South sections remaining are designated as an SSSI (Site of Special Scientific Interest) meaning they are considered nationally important and an incredibly rare survival in an urban context.

So why is there a fen here?

Ammonite fossil from Jurassic clay

Rust spring, North Fen

Around the Lye Valley and under most of the urban area of Headington the rocks are very porous limestone of the Jurassic coral reef (exposed at Rock Edge). Underlying this limestone is the heavy Oxford Clay, which is a non-porous rock. The clay can be seen exposed low down in the banks of the brooks and characteristic fossils such as the large 'devil's toenail' oyster shells and pyritised ammonites can sometimes be found on the bed of the steams. Rain falling in the area around the valley (the 'catchment') percolates down into the limestone, which holds the water in the pores in the rock (in an aquifer). The water cannot penetrate the Oxford Clay. The Lye brook has carved a channel down though the limestone and the underlying clay. In the valley sides, where the limestone lies on the clay, water emerges in a spring line at the junction. This creates the fen wetland. The springs have very limy water - full of dissolved calcium carbonate which it has picked up going through the rocks. This deposits on vegetation as soon as the water hits the air. This forms a crunchy whitish crust on leaves and stems which is the same thing as fur that forms in your kettle. Here it is known as tufa and can literally turn mosses growing in the water to stone (petrifaction). The springs have been running for thousands of years, since the last ice age. Over time, dead plant remains that have not decayed due to the waterlogging have built up as black peat interspersed with whitish tufa deposits. On the west side of the North Fen south of the Churchill hospital, the rocks are not limestone, but are the Beckley Sands, which have a high iron content. Thus the springs that emerge from this side of the site sometimes produce a bright orange cloudy substance and a reflective film on the surface of the water. Both of these phenomena are iron oxide (rust) which forms on exposure of the iron-rich water to the air - see the photo of the 'rust spring'. Many people think this is pollution, but it is an entirely natural deposit. In past centuries people used to cut the peat for fuel, so most is gone, but just over a metre depth is still left under the fen vegetation. Without the springs continuing to flow, the fen species will die and the peat will dry out, oxidising and liberating vast amounts of CO2 from the carbon stored in it.


In the Lye Valley there are still 20 types of plant that are on the Rare Plants Register for the county meaning they have been reduced to occurring in less than 10 county sites and may therefore be considered as under threat of being lost. This table shows a list of the important wetland plant species in the Lye Valley SSSI fens.

Bog Pimpernel

Marsh Lousewort

There are many important rushes and sedges (plants that look similar to grasses) but the most obvious and spectacular rare flowers are the marsh helleborine orchids, grass-of-Parnassus (which has a starry white flower and is not a grass!) bog pimpernel, marsh lousewort, marsh valerian, and marsh pennywort. Some of these are reduced to as few as two other sites in the county. Colourful ragged-robin, yellow flag and purple loosestrife are common plants which thrive here and walking over the fen releases the fragrant minty perfume of the crushed leaves of the abundant water mint. In May large numbers of common spotted orchids are a good sight. All these plants like short vegetation and bare peat areas for seed germination. Allowing the common reed to take over will quickly eliminate them by competition, but areas of dense reed are left for some invertebrates specific to reed and for reed warblers and reed buntings.

Because of a long history of human occupation near the site, a number of non-British plants, possibly from nearby gardens or allotments, have become established. These vary from harmless, to very damaging. In the harmless category a white semi-double rambler rose puts on a good show in the North Fen. Other, more unwelcome plants colonising the sites are Japanese Knotweed (N. Fen) Michaelmas daisies (N. Fen) Buddleia (N. Fen) and Himalayan Balsam (S. Fen). This last necessitates strenuous efforts at removal in order to preserve the rare native flora of the fens from being swamped.

Important Invertebrates of the fen

Marsh whorl snail

Scarlet tiger moth

This wet, short, mossy fen habitat is also home to a variety of uncommon or rare insects such as the tiny round Water Penny Beetle, the diminutive Marsh Whorl Snail and the spectacular Banded-general Soldierfly, which is striped yellow and black like a wasp, but is completely harmless. A whole range of scarce flies enjoy breeding in the wet conditions and the rarest is a small brown daddy-long-legs fly known as the 'Dimple-Cheeked Damsel'. The most spectacular moth on site is the Scarlet Tiger, which has a caterpillar feeding on comfrey. The caterpillar of the Mullein Moth feeds on water figwort and is more spectacular than the adult moth. Glow worms are a still a feature of the site and these enjoy feeding on the abundant marsh snails. A range of common butterflies such as orange tip, speckled wood, comma, meadow browns, skippers and common blues are regularly seen, and the eggs of the rare Brown Hairstreak have recently been detected on blackthorn in the drier bank areas. If you are not much impressed by insects just consider that these wetland areas provide perfect breeding conditions for very large numbers of insects such as flies and midges which are abundant food for amphibians reptiles, birds and bats.

Larger animals

Smooth newt

Slow worm from Town Furze allotments

Small ponds in the North Fen area are important breeding sites for common frogs and smooth newts and these provide food for grass snakes which have been recorded laying eggs in the compost heaps of Town Furze allotments, up near Girdlestone Road. These allotments have lizards and slow worms as well. All these reptiles are found foraging and basking in the sunny areas of the N fen. Lizards particularly like the boardwalk and the trimmed willow pollards just next to it. Bats regularly hawk up and down the valley and roost in the abundant ivy-covered trees. Larger animals such as badgers, foxes and muntjac deer may regularly be seen. Badgers have a very large sett very near the North fen area. Muntjacs are not popular as they love to eat fen orchid flowers! Bird watchers can have a very pleasant walk as there are a wide range of common birds to be seen, with successful breeding in the quieter scrubby areas.

Threats to the site

Marsh Helleborine

Grass of Parnassus

Lack of (or insufficient) management has been a major threat to the site in the past, and as a result plant species have been lost (see list of important wetland plants linked to above). However management is much better now and many species are increasing their populations as a result (for example nearly 2000 Marsh Helleborine orchid flowers were counted last year (2011) and the highest number of Grass of Parnassus flowers ever seen (124) was noted. OCV is playing a major part in this encouraging success.

However other major threats remain. Invading Himalayan Balsam is a problem in the South Fen and Japanese knotweed in the North Fen. These plants are being tackled.

Far bigger problems concern the hydrology of the sites. Continued house building and paving over of gardens etc. in the rainfall catchment areas for the springs is resulting in lack of water penetrating the ground to supply the underground aquifer and there is a reduced spring flow to the fen in some areas. This means the peat in the fen is less wet and squidgy than it should be in some parts, meaning plants and invertebrates that like it very wet are struggling. Predicted climate change means that this area of the south-east can expect much less rainfall over a year, so the springs will be producing less eventually, anyway. Building developments coming to within the catchment area need to make strenuous efforts to infiltrate all roof and paving water. In some cases no development at all should be allowed if the fens are to live.

The second hydrological problem concerns another effect of the surrounding development. Road run-off water is directed into the watercourses of the Lye Brook and Boundary Brook via large surface drains and this adds to the natural gentle brook flow. After a rainstorm, a huge amount of water that should have gone into the ground is instead collected from roads and paved areas and is directed via drains into the Lye and Boundary Brooks. This intermittent gushing of a lot of water creates tremendous damage - the volume and force erodes the stream banks and causes flash-flooding further downstream in the Florence Park area of the city. For the North Fen the erosion of the stream banks means that portions of the bank vegetation are ripped away suddenly and the bottom is scoured out deeply. From a gentle stream that was in a very shallow channel the stream channel has been deeply carved to a depth of 1.5m. This exposes the fen peat next to the channel to drying out so that the peat near the brook is far too dry and wetland plants and animals have all gone from this zone. For the South Fen the situation is even worse because it is subject to the combined water flow of the Lye and Boundary Brook, meaning a much greater volume and force of water to cause erosion. Here the channel next to the fen is scoured to a depth of 1.78m and the adjacent peat is therefore too dry for many metres into the fen area. These major hydrological issues are the current subject of a great deal of research. It is to be hoped a solution can be found before these remarkable sites are lost to the city.

Judy Webb - Independent Environmental Services Professional, Oxon

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