A redefinition of the travel restrictions hereabouts means that “driving to the countryside and walking (where far more time is spent walking than driving)” it is now “likely to be reasonable” within the terms of Regulation 6 of the The Health Protection (Coronavirus, Restrictions) (England) Regulations 2020. That means that, rather than plan another post about the fascinating ecology of Lough Down, I can look a little further afield. As both Heather and I are writing chapters for a forthcoming book on the Natural History of Weardale, we turned our eyes to the hills, largely for exercise and a change of scenery, but also as part of our background research for these chapters.
We parked the car at Westgate and followed a path alongside Middlehope Burn, a tributary of the Wear with a long history of lead mining and, as such, a case study in how man has shaped the ecology of Weardale, both terrestrial (Heather’s domain) and aquatic. The first part of the walk is through Slitt Wood, where the stream cascades over a series of low step-like waterfalls, alternately sandstone and limestone, illustrating the bedrock geology of the area. The air is full of birdsong and there are patches of primroses feasting greedily on the light that is still plentiful on the forest floor at this time of year. However, this idyll is short-lived as, passing through a gate we emerge into a grassed area surrounded by derelict mine buildings. Early on a Saturday morning in the midst of the pandemic, we have the place to ourselves and it is a struggle to imagine this place as a busy industrial site. Similar sites are scattered throughout Weardale and the surrounding dales; all are now closed but once they would have employed large numbers of people. There would have been the miners, working underground, of course, but also gangs of people (including women and children) breaking down and sorting the ore as it was brought to the surface, plus ancillary workers involved in construction, both above and below ground.
A couple of hundred metres beyond the site of the main shaft at Slitt Mine, I spot an adit (a shaft driven horizontally into a hillside) and make my way towards it. These are intriguing habitats for ecologists interested in the interactions between man and nature and I was intrigued to see what was growing in this one, White’s Level. The mine’s levels and shafts act as natural drainage channels, collecting water that has percolated through the rocks but, because the miners have driven the levels along the mineral veins, the water comes into contact with lead, zinc and cadmium during the course of its underground journeys, emerging with concentrations far in excess of those deemed safe by toxicologists. However, the channel immediately downstream of the entrance of White’s Level was lush with vegetation. I could see thick wefts of filamentous algae giving way to beds of water-cress (Rorippa nasturtium-aquaticum) and bog pondweed (Potamogeton polygonifolius). The latter two were surprising as, in my experience, most of the streams draining north Pennine adits are dominated by algae rather than by higher plants.
The stream flowing from White’s Level to Middlehope Burn, April 2020. The left-hand image shows the beds of water-cress very clearly whilst the right hand image shows the filamentous algae growths immediately below the entrance. The picture at the top of the post shows Middlehope Burn at High Mill Falls, just upstream from Westgate.
The water cress had a distinctive purplish tinge which is probably a response to stress. We’ve encountered this type of colour-change in response to stress elsewhere (see “Escape to Southwold”). In this post, and in “Good vibrations under the Suffolk sun …” I talked about how plants have to regulate the amount of energy from sunlight in order that their internal photosynthetic machinery is not overwhelmed. Those posts were both written after a hot weekend in July, but this was a chilly and overcast April morning in the Pennines where the prospect of plant cells being overcome by heat seems faintly ludicrous. Here, instead, is my alternative hypothesis.
Although White’s Level and the other mines in the northern Pennines were driven by the demand for lead, lead is a relatively insoluble element and zinc, which is found alongside the lead in the metal-rich veins of the northern Pennines, is more soluble and, therefore, has a greater toxic influence on the plants and animals in these streams. Zinc affects the metabolism of plants in several ways, one of the most important of which is to reduce the effectiveness of the chlorophyll molecules which are responsible for photosynthesis. It does this by nudging the magnesium atom, which lies at the heart of every chlorophyll molecule, out of place.
Macrophytes in the stream flowing from White’s Level to Middlehope Burn, April 2020. Left: Potamogeton polygonifolius; right: Rorippa nasturtium-aquaticum.
What this does, then, is alter the balance of the equation that tries to balance energy inputs and photosynthesis. If your chlorophyll molecules are hobbling along, then the point at which they are overwhelmed by even the meagre Pennine sunlight shifts so that the need for the plants to manufacture their on-board sunscreen kicks in sooner. Just a hypothesis, as I said: if you have a better explanation, please let me know.
A few hundred metres further on, there is another lush growth of water cress in the stream flowing out of another adit, Governor and Company Level, this time even extending beyond the metal grille designed to keep the curious from harm. I most associate watercress farms with the headwaters of chalk stream, which are characteristically spring-fed and, therefore, have very stable conditions. The adits of the northern Pennines are, this respect, very similar to springs insofar as their flow, temperature and chemical conditions vary little over the course of a year. In that respect, it is perhaps less of a surprise that we find water cress growing so prolifically here. The zinc, admittedly, is a complication we don’t find in most springs but, that apart, the adits could be thought of as man-made springs, creating a series of almost unique, but largely overlooked habitats.
In the next post, I’ll talk about the algae that I found in the White’s Level channel.
A prolific growth of water cress in the drainage channel below Governor and Company Level, April 2020.
Some other highlights from this week:
Wrote this whilst listening to: Still working through Dylan’s back catalogue: John Wesley Harding, , Nashville Skyline and Self-Portrait, the latter a blip in an otherwise superb run of albums. Next up is New Morning but I want to re-read the chapter in Dylan’s Chronicles Volume One where he describes the genesis of this album before listening.
Cultural highlights: My book group looked at Pride and Prejudice but, being deep into The Mirror and The Light, I did not had time to read this. We watched the 2005 film version starring Kiera Knightly instead. Turned out that three of the six participants in the book group had also watched the film the night before our Zoom meeting, rather than (re-)reading the book itself.
Currently reading: Finally finished The Mirror and The Light which was, definitely, worth the effort. Started Kate Atkinson’s Big Sky.
Culinary highlight: Home-made tortellini filled with mushroom paté, served with a consommé made from turkey stock from the freezer. Culinary ambition hereabouts always goes sky high in the week of the MasterChef finals.