Slitt Wood SSSI

Lots of things are not as we expect or would want them to be at the moment so it was with some relief we realised that clarification of the guidelines on exercise meant that our regular sorties from home on bikes or on foot can now be extended to include walks further afield, as long the walk is long enough to justify the drive to get there.  Martyn and I are both working on contributions to a book on the natural history of Upper Weardale, so what better excuse for a walk further up the valley! 

We opted for Slitt Wood SSSI, which stretches north along the banks of Middlehope Burn, from Westgate where it joins the Wear.  The burn has cut its way down through the alternating layers of Carboniferous limestone, shale and sandstone which make up this part of the north Pennines, creating picturesque waterfalls where it cascades over harder bands of rock. 

Middlehope Burn above High Mill

The banks are covered in flowering primroses, violets and barren strawberries at the moment, with plenty of wild garlic, wood anemones, dog’s mercury and opposite-leaved golden saxifrage in the mix too. What makes this site botanically special, however, is the metallophyte flora – specialist plants which can tolerate the high levels of heavy metals in the soils around the old mine workings a little further up Middlehope Burn.

The North Pennines AONB lies atop the Northern Pennine Orefield, where mineralising fluids driven by magma intruding into the sediments from below filled fractures and faults in the Carboniferous sediments. As they cooled and crystalised, these fluids gave rise to veins of metals such as lead, zinc, cadmium and iron as well as fluorite and barium minerals.  Today’s idyllic-looking north Pennine landscape is far from natural, having been quarried for building materials and mined for metals since at least Roman times. Walking up Middlehope Burn you come across plenty of evidence of lead mining, particularly from its 18th and 19th Century heyday.  Large, flat washing floors by the burn where the ore was sorted from waste material are still so contaminated with heavy metals that a very small number of plant species can grow on the thin soil – a vegetation assemblage known as calaminarian grassland.

It was too early in the year to find perhaps the most iconic of these calaminarian species, Spring sandwort, in flower but the tiny, fragrant leaves of wild thyme were easy to spot and I found my first mountain pansies of the year.

Left; Minuartia verna, Nenthead, June 2016: Right; Viola lutea

Spring sandwort, also known as Leadwort because of its ability to grow on contaminated sites, also grows more generally amidst short grass on thin, calcareous soils – I’ve often seen it near Cow Green reservoir in Upper Teesdale, for example.  It is a delicate little cushion-forming plant, less than five cm tall, so being able to tolerate high concentrations of metals gives it a useful competitive advantage over some more robust species in these environments.  Plants have two choices if they want to grow in the presence of toxic metals; they can exclude the metals at the roots or they can transport them rapidly through the plant and store them safely in cell walls and vacuoles or excrete them via pores on the margins of the leaves known as hydathodes. Spring sandwort adopts the latter strategy (Neumann et al, 1997) and is a hyperaccumulator of metals such as Zinc, concentrating the metals many-fold in its leaf tissues.

Mountain pansy, Viola lutea, is another tiny plant which likes calcareous soils and tolerates heavy metals – a facultative metallophyte. A little later in the year it grows abundantly in the upper Pennines but I was delighted to find a few flowering here in mid April.  Despite its specific name, lutea, the flowers are as often purple or bicoloured as yellow, in this part of the world.  There is some suggestion that some of these forms are actually hybrids between V. lutea and the larger V. tricolor (wild pansy).  Work by Sychta et al. (2018) on a range of Viola species showed that even cells  suspended in liquid culture retain their ability to tolerate very high concentrations of lead and zinc. They do this by accumulating the metals and then sequestering them in cell walls and vacuoles, out of harm’s way.

As well as the presence of toxic metals, the thin soils around old mine workings tend to have low concentrations of essential mineral nutrients and a limited ability to retain water.  Metallophytes can cope with these harsh conditions, making them very important early colonisers and key to limiting the spread of contaminants and starting remediation work.  Their roots help stabilise the soil and the plants themselves provide shelter for other germinating seedlings, as well as contributing much-needed organic matter to the soil on their demise. Succession will eventually permit the growth of other, less specialist, flora, though on the washing floors at Slitt Mine and Middlehope Shield there has been limited scope for this.

Above Middlehope Shield the valley opens out into an area of boggy ground, once the site of reservoirs supplying the waterwheels which drove rollers to crush the lead ore.  Silted up, these are now a rich habitat for sedges, rushes, marsh marigolds and orchids (later in the year).  A dam was built inside White’s level when the mine closed in the 1860s to provide a large, underground reservoir and the water flowing from it today supports a fine expanse of pondweed, watercress and mint.  I wouldn’t fancy the latter as salad ingredients, though, knowing how much lead and zinc remains in the mine water!  Much more on the aquatic life in Martyn’s blog.

Site of the old reservoirs, Marsh marigold (Caltha palustris) and the entrance to White’s level

In stark contrast to the terrestrial and aquatic contamination, lichens such as Usnea and Evernia. draping the blackthorns trees testify to the cleanliness of the air nowadays – it would have been a very different story in the 19th century, with the smoke of the mineral lines steam trains filling the valley.

Having ambled up the valley, we walked from the top towards Rookhope along the Weardale Way to justify our drive, spirits lifted by the sight and sound of lapwings, larks and curlews.  We met no-one along the eight mile loop apart from one or two people in their own gardens as we dropped back down into Westgate.  Despite that, and the fact that we see many more people on our regular walks from home, we both still felt a niggle of uncertainty which means we won’t be making this a regular thing.  Roll on the time when can go back to our favourite places without that guilt and can have friends living just a stone’s throw away join us for a walk!

Neumann D., Nieden U., Schwieger W., Leopold I. & Lichtenberger O. (1997) Heavy metal tolerance of Minuartia vernaJournal of Plant Physiology, 151, pp 101-108.

Sychta K., Słomka A., Suski S., Fiedor E., Gregoraszczuk E. & Kuta E. (2018)  Suspended cells of metallicolous and nonmetallicolous Viola species tolerate, accumulate and detoxify zinc and lead. Plant Physiology and Biochemistry, 132, pp 666-674. https://doi.org/10.1016/j.plaphy.2018.10.013.

One comment

  1. Always an interesting read! Thankyou.
    And I envy you being able, albeit with a niggle of uncertainty, being able to drive to a walk (we are still limited to a 1 km radius of home, for an hour, though there are signs that will improve in a couple of weeks).

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