Last year the Land of Iron commissioned an eco-hydrological assessment of Fen Bog(s) by consultants (Sheffield Wetland Ecologists). An eco-hydrological assessment examines the workings of a water system and its wider ecosystems. Sunday was International Bog Day so to celebrate the complexity and variety of bogs – here is a very very simplified overview of that assessment. Any misunderstanding or misinterpretation is all mine.
Fen Bog(s) is at the top end of the Newtondale glacial channel in the east of the North York Moors. It’s part of the Newtondale Site of Special Scientific Interest (SSSI) and the majority of it is also designated as a Special Area of Conservation (SAC). Most of the site is a Yorkshire Wildlife Trust nature reserve, other parts are owned by the National Park Authority, the North Yorkshire Moors Railway and the Duchy of Lancaster.
Fen Bog(s) is a large peatland/wetland site, and according to the report “is of exceptional biological, palaeo-ecological and telmatological (to do with bogs) interest, especially as there are no comparable examples in the region or, indeed, in most of England”.
The bog happens to be within the boundary of the Land of Iron Landscape Partnership Scheme. The scheme focuses on the landscape area impacted on by the short but intense period of ironstone mining and railway development in the North York Moors. Intriguingly part of the Fen Bog(s) site has been subject to long-term modification since the Whitby–Pickering Railway line (now belonging to the North Yorkshire Moors Railway) was built alongside/across the site. The Partnership commissioned the report in order to get an holistic assessment of the existing data (of which there is a lot), and to identify the gaps and address these through additional field investigations, with the aim of increasing the understanding of the eco-hydrological functioning of Fen Bog(s) in order to help inform future management decisions. This management needs to conserve and restore its environmental value as well as allowing the continued functioning of the railway.
The Whitby & Pickering Railway was first opened in 1837, as a single-track, horse-drawn enterprise carrying freight between the two towns. Newtondale connects through the central moorland which largely separates the north and south of the North York Moros. Soon after the line was doubled and substantially rebuilt for steam propelled haulage with services starting in 1847.
Benham (An Illustrated History of the North Yorkshire Moors Railway, 2008) explains that Fen Bog(s) proved a “major headache” for the railway builders and that “Stephenson resorted to the same technique employed at Chat Moss when building the Liverpool & Manchester Railway. This involved stabilising the land by ‘pile-driving’ fir trees into the bog and overlaying them with sheaves of heather bound in sheep skin, together with more timber and moss.” In addition deep drains were dug alongside the railway through the mire to try and keep water off the track. The extensive drainage has tilted parts of the bog. It has also been suggested that it meant the bog turned from a topogenous system (source water mainly from the land) to an ombrogenous one (source water mainly from precipitation) – but the report considers this is unlikely. The railway’s embankments and sidings were built and maintained using railway ash, basic slag, limestone and basalt – all base rich materials imported onto the site which still have an impact.
The summit of the railway is a short distance north of Fen Bog(s), near the former location of the ‘Goathland Summit’ signal box. South of this the railway track skirts the western edge of the wetland, it is built mostly along the steeply-sloping edge so that its upslope side is on mineral ground or shallow peat whilst the mire side is over deeper peat. The railway line has therefore partly obliterated, truncated and drained much of the western edge of Fen Bog(s). Towards the southern end of Fen Bog(s), the glacial channel curves west and the railway here crosses the bog to the other side of the channel, thereby cutting across and separating parts of the Bog(s).
Peatlands are strongly influenced by hydrology, chemistry, and vegetation.
The Fen Bog(s) report considers the hydrogeology including stratigraphy, surface profiles, and solid, wetland, and superficial (recent) deposits.
It also investigates the water supply in and the drainage out. All the different water features on the site are mapped – as pool, spring or seepage, stream/ditch with visible flow, water flow track, water filled ditch with no visible flow, damp channel, or seasonally wet channel. The main artificial drainage is associated with the railway including the drains on either side of the line, but there is also other historic drainage at the south end of Fen Bog(s) which was done to improve the land for agriculture.
Hydrochemical measurements were taken as part of the assessment to establish the current pH and also the electrical conductivity of the water at different points. There is a lot of variation across the site. It has been suggested that high pH readings i.e. alkaline are caused by leeching slag used in the construction of the railway track. Measurements from the recent assessment suggest that in terms of chemistry any effects of the trackway on the Bog(s) is either historic or localised. Because of the mix of chemistry Fen Bog(s) is classed as a Transition mire and this is reflected in its mix of vegetation (see below). The transition can be geographical or successional, or both.
There are a series of historic water table measurements at two specific points, from the 1970s to 1990s – one in ‘wet’ bog, rich in sphagnum, in the north, and one in relatively ‘dry’ bog, with a lot of heather, in the south. The report suggests the main reason for the more consistently higher water table at the northern monitoring point can be associated with the greater number and penetration of flow tracks across the mire, the number of groundwater outflows and a more consistent supply of telluric water (surface water and groundwater). Groundwater geology is always important in sustaining a high water table.
Development and status
Much of the depth of peat at Fen Bog(s) is believed to sit in a hollow which decreases at its southern end. It has been suggested this hollow may have been a glacial or post-glacial lake. However it appears as if the mire developed on a dry surface, that is through paludification, and not by infilling a water body (terrestrialisation).
The lower layers of peat cores and sections collected contain the remains of tree species (Birch, Willow and Alder) and other plants (Reeds and Sphagnum) that suggest wet woodland. Then the higher up layers on top contain more plants and silt indicating the formation of swamp and a rise in the water level. This may be a consequence of wetter climatic conditions but also may partly be to do with human activity. There is an increase in non-tree pollen suggesting the removal of trees at the time, and the report postulates that the build-up of water on the site may have been due to it being artificially damned at the southern end. Sphagnum increases in the top level of peat, from c. 1100 AD atleat until the 19th century. The development of a Sphagnum-dominated surface on a reed-monocot swamp requires some isolation of the surface from more base-rich water sources which means the margins with inflow must have remained largely free of Sphagnum and a dome of peat therefore developed in the middle of the bog.
Fen Bog(s) can therefore be considered an embryonic raised bog, which has developed upon a protracted phase of reed–monocot peat that, because of the topography of the trough and the occurrence of marginal inflows, has been susceptible to flooding with telluric water until relatively recently. Because the system has developed across a shallow watershed, it can be regarded as an embryonic ‘sattelmoor’ (saddle bog). The report notes that this assessment is based on the centre and eastern margin of Fen Bog(s) – the western margin has been modified too much by the railway development and associated drainage to be useful as evidence. The modification led to a tilt of the mire’s surface towards the west.
Vegetation over time is the raw ingredients of a bog. The report reviews and updates current NVC vegetation classifications across the Fen Bog(s) site. It’s quite a mosaic. As well as non-mire vegetation such as dry grassland, bracken, dry heath and wet heath, there is also:
- Weakly base-rich springs and soakways – base rich means a richness of chemical ions i.e. alkaline, a soakway is a narrow track of water flow where little or no water is normally visible. Supports plants such as Bog bean, Broad-leaved cotton grass*, Common butterwort*, and Black bog-rush*, as well as Sphagnum sp. and other bryophytes. Beyond the immediate Fen Bog(s) site there are base-rich springs and weakly base-rich soakways – where soils are acid rather than alkaline so it means the water ends up only weakly or not base-rich at all.
- Acidic springs and soakways – supports plants such as Common sedge, Yorkshire fog and Marsh violet, as well Sphagnum sp.
- Ombrotrophic bog – where the main source of water is precipitation. Supports plants such as Common cotton-grass, Cross-leaved heath and Bog myrtle.
- Minerotrophic Bog – where the main source of water is watercourses and springs. Supports plants such as Purple moor-grass, Common yellow sedge and Carnation sedge.
- Molinia mire – purple moor-grass dominated vegetation, also supports plants such as Sundews, Star sedge and Bog asphodel
- Nutrient-rich fen – these areas may be influenced hydrochemically either by base-rich springs, or by the base-rich material that make up the railway embankments/sidings. Supports plants such as Angelica, Tufted vetch and Water horse-tail
- Carex rostrata fen – base-rich mire supporting plants such as Bottle sedge (this is the Carex rostrata), Marsh marigold and Ragged robin.
- Pools and soakways with Carex limosa – supports plants such as Bog sedge* (this is the Carex limosa), Slender sedge*, and Bog pimpernel.
- Wet woodland – these remaining woodlands are similar to that which began the formation of peat millions of years ago. Supports plants such as Grey willow, Downy Birch and Creeping buttercup.
- Reeds and willow scrub – can also be classed as wet woodland. Supports plants such as Narrow buckler fern, Soft rush and Sphagnums.
- Tall swamp and reedbeds – each at different stages of development with their own characteristics. One site which supports bulrush is presumably mineral enriched from the track ballast but this shows no sign of spreading out into adjacent vegetation without the enrichment. Another site, not yet colonised by willow scrub, supports plants such as Marsh pennywort, Water mint and Branched bur-reed.
* notable uncommon vascular plant species
From the assessment the report goes on to outline the main management issues and to suggest restoration opportunities for the Fen Bog(s) site. These include vegetation control through gazing and fencing, monitoring the spread of reeds (Phragmites), clearing parts of the species poor scrub areas, retaining the wet woodland/scrub habitat, blocking and redirecting specific railway ditches, minimising the introduction of new embankment ballast material, and using engineered solutions to tackle subsidence problems. Interested parties will consider the recommendations and decide what is desirable as well as practically possible, in order to maintain this very important bog site that embodies a clash of natural and cultural heritage.
Postscript: There is a story that a steam locomotive sank into Fen Bog(s) at some point in the past, and remains there today. But this is just a story.