7. Peat and the Common

‘peat is an archive of the past…’

 

The raised mires each side of the Solway Firth were formed 8-10,000 years ago, as the glaciers retreated from the North of England and Scotland.

Then, according to this beautifully visual description by Scottish Natural Heritage, “ the landscape was littered with many pits, often within a layer of watertight glacial debris or till. These basins formed lochs and were colonised by a fringe of fen plants, which slowly spread across the lochs until no open water was left. …

“Plants in the outer fringes of the swamp soaked up all of the nutrients flowing in from the surrounding ground, leaving the centre of the swamp waterlogged and nutrient-poor – just the right conditions to encourage a bog to form. Sphagnum bog mosses thrive in these conditions and started to dominate the vegetation. The steady upward growth of the living Sphagnum layer and the slow accumulation of the dead vegetation below combined to produce peat.

“As peat accumulated, it began to rise above the level of the former loch surface. Once a layer half a metre or more in thickness had formed above the old surface, the peat became isolated from groundwater. The bog therefore became dependent on rainwater alone, deriving all its nourishment from the atmosphere, and as a result is poor in nutrients [my italics].

Sphagnum has the unusual ability to take up more than 10 times its weight in water, and its metabolism ensures that its surroundings become weakly acidic. It grows upwards, its lower parts dying as it grows; as surface plants die and are replaced, they are grown over by new individuals.

The layer of dead material, because of the acidic and anaerobic environment, does not decompose and becomes compressed, eventually forming peat.

Fifteen species of Sphagnum have been recorded on Bowness Common. Different species have different preferences – some will grow in pools, some prefer the drier conditions on hummocks; some will tolerate nutrients, others flourish at very low nutrient levels. Species compete with each other within the vertical range too, not necessarily growing at their physiological optimum but rather where they are most competitive. For example, S compactum can live in a fairly wide range of conditions, but when mixed with S papillosum it is out-competed and pushed to the margins of its range, to very wet and very dry bog surfaces, where it – but not S papillosum – can survive.

In drier conditions – where the bog has been disturbed by drainage for example – there will be a shift towards Sphagnum species that can live and grow on hummocks, because they are able to take up water more effectively than ‘wetter’ species; in this way, waterlogging of the mire can be maintained.

dry tussock dry mosses
Dry hummock with pleurocarp moss

(In even drier conditions, such as patches close to the SJR track, Sphagnum mosses are sometimes replaced by Pleurocarps, creeping mosses – bryophytes that do not store water and do not form peat.)

Interestingly, Richard Lindsay points out in his analysis for the RSPB, Peatbogs and carbon, a critical synthesis (2010)that “hummock-forming species tend to be more effective peat-formers than species found lower in the microtopography. Thus drier conditions may actually stimulate increased peat formation…”

Of the 10 species of Sphagnum found in a couple of hours along the boardwalk at RSPB Campfield on the edge of Bowness Common, the hummock-formers such as S. magellanicum, S. papillosum  and S. capillifolium are soon found. These species are also the main peat-formers.

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Peat forms at the rate of about 10cms every 100 years and, because the raised mires are essentially ‘trapped’ within post-glacial depressions, they grow upwards to form a shallow dome, like an inverted saucer.  But the growth of the Solway Mosses has not been a simple accumulation of peat. Cores show layers of peat separated by sand (watch the video of Alasdair Brock explaining this while taking a core sample on Wedholme Flow), indicating that in places there were several incursions by the sea, and there were regions that initially remained as ‘islands’. Nor did all the wetlands form in an empty bowl, because here and there melting glaciers had dumped their burden of rocks and till, leaving small, tear-drop-shaped hills or drumlins.

Frank Mawby throws more light on the Solway Mosses: “The classic image of a raised mire as a bog in a slight saucer-shaped depression doesn’t fit for Wedholme, Bowness and Glasson  – they all have a glacial topography and all have grown over ridges and mounds. Bowness, of course, is a classic of glacial topography in that it never overwhelmed the Rogersceugh drumlin, although it did almost cover another lower, parallel ridge to the north. But Drumburgh Moss is in a saucer and probably fits the classic mire model.”

Borings carried out by Walker, recorded in his classic paper Late Quarternary History of the Cumberland Lowland (1966, Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences, 251, pp. 1-210), along transects of Bowness Common, showed that “The highest point of Bowness Common crossed by the transects was at 15.6 m (51 ft.) O.D. over a boulder-clay hillock. The highest level of the bog surface above deep basin deposits was about 13.6 m (45 ft.) O.D.” This fits well with the ‘fifty feet of peat’ reported during building of the SJR.

DCIM100MEDIADJI_0032.JPG
Looking NE across Bowness Common: Rogersceugh Farm on its drumlin. March 2017                   (C) James Smith

The Rogersceugh drumlin rises about 80 feet above the Moss– and consequently has an astonishing 360o view, to the other Mosses, the Cumbrian fells and Dumfriesshire, and the Firth.

Archiving stories of the pastacrotelm diag rspb doc

From Richard Lindsay, Peat-bogs and carbon, a critical synthesis, 2010

Peat-cutting has its own lexicon – but the anatomy of the intact raised mires has wonderful words, too. The mires are fed only by rain and so are poor in nutrients: they are ‘ombrotrophic’. The living vegetation on the surface – the bryophytic mosses, and the vascular plants such as cotton grasses, heather, sundews, crowberry and bog cranberry – forms a thin layer, sometimes only 10 cms deep, called the acrotelm.

It is the acrotelm that is the most important layer for initiating peat-formation, because it is here that the Sphagnum mosses grow.

Below this is the progressively acidic and anaerobic catotelm, in which the dead plant material is preserved and compressed to form dark peat. A core driven down through the raised mire would eventually reach the bottom, a layer of boulder clay.

Vegetation growing on the thin, ‘living’ acrotelm responds in different ways to the environmental conditions – the amount of rain, warmth, nutrients, pollution. Different species – of plants and of microscopic animals – grow more slowly or out-compete others depending on their favourite conditions. Pollen, even radioactive particles (remember Chernobyl), blown in from surrounding areas is deposited or incorporated amongst the stems.

“Peatbogs are responsive systems with homoeostatic mechanisms that are not far removed from those found in living organisms … features having many similarities to tree-rings can be found in the equally-thin layers of peat which are successively deposited in a bog over millenia. These narrow bands of peat tell the same tale as tree-rings but over a much longer time-scale.” (Richard Lindsay, Peatbogs and carbon, a critical synthesis)

And it is not only physical remains that are archived. Carbon is ‘sequestered’: carbon dioxide that has been taken out of the air by growing plants and incorporated into molecules such as carbohydrates and proteins remains tied up in the undecomposed material, and is ‘locked up’ or stored.

As the level of carbon dioxide rises in the Earth’s amosphere, this is why peat-bogs, and their restoration, is especially important.

Cutting and draining

Peat has long been valued as a fuel, indeed in some places where the climate is harsh and there are few trees – the Isle of Lewis and the Flow Country of Sutherland are good examples – it is the only means of providing heat for warmth and cooking. Its importance gives rise to many Gaelic words descriptive of the peatlands themselves, and of the tools and the methods of cutting, drying and managing the peat-cuts. Lewisian artist Anne Campbell has collected together a list and glossary of the Lewisian words and phrases in a delightful little book, Rathad an Isean (Gaelic for The Bird’s Road – the narrow gap on the bank between the stacked cut peats and the face of the cutting).

model barrow and tools1
Malcolm Wilson’s scale model of peat-cutting tools used on Wedholme Flow (see Ask the Fellows who cut the peat)

 

Much of the peat-cutting in lowland Scotland and England was initiated or managed by the Dutch, especially in the early- and mid-twentieth century, and several Dutch words, such as stooling and sticker (stikker), were used here on the South Solway Mosses. Henry Engelen’s Cumberland Moss Litter Company worked on Kirkbride Moss, part of Wedholme Flow, and also briefly at the edge of Bowness Moss; men, wearing footboards to stop them sinking in – just like the navvies on Chat Moss (see Section 8, The railway navvies) nearly two centuries previously –  cut and stacked peat by hand in the 1950s and ‘60s . The peat was used for horticultural and other purposes, and was harvested by ‘milling’ on a larger commercial scale by Fisons, then Levingtons, then Scotts.

Unlike Wedholme Flow and Glasson Moss, which were commercially harvested for peat between the 1960s and 1990s, Bowness Common has remained relatively unscathed; peat was cut for fuel around the edges, and Fisons drained a southern section by the Rogersceugh track but seem not to have done much in the way of harvesting. Alasdair Brock (Natural England’s current Senior Reserves Manager for the North Cumbria National Nature Reserves) told me that most of the central part of Bowness Common is its original surface.

 

Nevertheless, during the construction of the SJR, longitudinal and cross-drains were cut right across the Moss from near Whitrigg to Bowness and, as noted previously, by July 1869 ‘the water had been running in river-like streams on each side of the proposed line for many weeks.’ The level of the bog was caused to ‘fall by 4-5 feet each side of the track.’

In 1988 Richard Lindsay, an expert on the effects of drainage on the hydrology and carbon-sequestration by peat-bogs, examined the peat around a 100-year-old drain on Wedholme Flow (Peatbogs & carbon section 9.1, p122). His measurements showed that the structure of the mire was affected for nearly 100m each side of the ditch, the original surface being lowered by a couple of metres (excluding the depth of the ditch).

This lowering results from three processes, consolidation, compression, and oxidation, all of which are very relevant to the effect of the SJR’s construction on Bowness Moss.

Water is held in spaces between particles, the interstitial spaces; within the acrotelm these may be large, and when the acrotelm is damaged by cutting it is predominantly this layer which drains rapidly.

The interstitial spaces in the catotelm are smaller and decrease in size with depth, but other, bigger, water spaces such as ‘macropores’ and ‘pipes’ are often widespread. Water escapes rapidly from these spaces too when a peat-face is cut and, because as much as 95% of the intact mire may be made up of water, the drained peat rapidly subsides and consolidates.

This initial consolidation can indeed be rapid. Frank Mawby passed on some startling information about Wedholme Flow: “I recall Pete Wanning [a foreman] telling me that when they began the drainage of the main dome the bog sunk so rapidly that almost every morning they had to re-do the railway line because it buckled and distorted the tracks.”

After consolidation, as Lindsay explains rather graphically and beautifully, ‘material suspended in air weighs much more than the same material suspended in water. Consequently as water is lost from the peat through primary consolidation a significant quantity of material formerly floating largely weightless … steadily emerges from the falling water table to become a substantial load on the peat column beneath.’ This compression forces out more water from the underlying peat, the water table falls even further and the level of the peat surface falls even more, driven by compression.

Finally, when oxygen from the air begins to penetrate the exposed catotelm, the previously anoxic peat begins to oxidise and break down, losing sequestered carbon as CO2 and ‘other more complex breakdown products’ as DOCs, dissolved organic carbon. This oxidation, too, reduces the level of the peat.

These processes explain why, as the army of navvies paddled in peaty water to dig the thousands of yards of ditches across Bowness Moss, the level of the bog subsided each side of the SJR’s future permanent way.