Thorne Moors

Summer scene on the Moors Photographer Keith Heywood

Summer scene on Thorne Moors (photograph by Keith Heywood).

 

1. GEOGRAPHY AND NOMENCLATURE

2. EXPLOITATION AND CONSERVATION

3. HABITATS

4. REFERENCES
 

1. GEOGRAPHY AND NOMENCLATURE

Thorne Moors is a peatland on the Yorkshire–Lincolnshire border. It is 15km north-east of Doncaster, and lies to the north-east of Thorne, SSW of Goole and WNW of Scunthorpe. Located in the lee of the Pennines, Thorne Moors has a mild lowland climate, with modest precipitation (quoted figures range from 538–660 mm per annum) and a high evapotranspiration potential. It is situated at the heart of the Humberhead Levels, in the southern part of the physiographic region known as the Vale of York.

The Humberhead Levels are defined as the footprint of the pro-glacial Lake Humber, and have become formalized as one of the Natural Areas created by the former English Nature (now Natural England) (e.g. Anon.1999). However, the complex limits of Lake Humber, both in space and time, do not allow such a neat definition of the Humberhead Levels to be more than a simplified convenience. The Humberhead Levels Natural Area covers c.2275 sq. km (c.880 sq. miles), and extends for almost 70 km from north of Selby in Yorkshire southwards to Retford in Nottinghamshire. To the west lies the dipslope of the Lower/Upper Magnesian Limestone, which essentially forms the easternmost Pennine foothills. The barrier in the east is two scarp slopes. These mark first, the rise of the Chalk forming the Yorkshire Wolds, and second the Lower Lias around Scunthorpe in Lincolnshire. This eastern frontier is broken by the Humber Gap, whereby a number of significant rivers gain their outlet to the Humber Estuary and, ultimately, the North Sea. North of the Humberhead Levels stretches the mid-Vale of York; southwards lie the Trent and Belvoir Vales and the wooded land of Sherwood.

The bedrock of the Humberhead Levels is Triassic in age, comprising the Sherwood Sandstones in the west and the Mercia Mudstones to the east. Thorne Moors is positioned on the north–south boundary of these two formations, which can be traced through Retford and Goole northwards. Both formations are masked by a thick veneer of Lake Humber clay–silts, and Thorne Moors developed on this lacustrine drift. The bedrock is only emergent as isolated outcrops through the obscuring mantle. Thus the Sherwood Sandstones reach 51 m OD at Brayton Barff near Selby, and the Mercia Mudstones attain 41 m OD in the Isle of Axholme and 82 m OD at nearby Gringley Beacon. In addition to these rockhead ‘islands’, there are low-lying drift ‘islands’, for example the oldest part of Thorne, and Lindholme in the midst of Hatfield Moors.

Thorne Moors, like many other parts of the lower Ouse–Trent network, is largely bounded by anthropogenic alluvium (warp), the exception being to the south-west. This artificially-induced deposit was achieved by a technique known as flood-warping. By this, silt was progressively accreted in embanked compartments from stretches of tidal river. With skilled operation, sediment-laden water was channelled via specially dug warping drains at each suitable tide, with a potentially fertile soil being gradually created inter alia on the lower parts of the peat moorland. Although the process was mostly economic during the 1820s–70s, it continued to encroach Thorne Moors until the 1930s. In parts where flood-warping was not undertaken, the earlier pattern of reclamation is discernible. This was executed along ribbon-like strips that extended as the peat was dug out. Those thrusting into the peat from the direction of Thorne were known as ‘cables’, and collectively still present a distinctive arrangement of boundaries.

The Humberhead Levels are positioned towards the northern extremity of the Lowland Zone of Britain. As a result of this, Thorne Moors has biogeographical significance as a meeting place of northern and southern flora and fauna. The moorland lies within 8 km of the confluence of the Rivers Ouse and Trent that forms the head of the Humber Estuary. The major physical elements of the Humberhead Levels – by their sheer extent, not striking form – are the broad floodplains of rivers in their lowest reaches that converge here. Of the five great river systems of England and Wales, the Humber–Ouse–Trent is one of the most extensive, stretching eastwards from the Pennines and the Peak District to the Yorkshire and Lincolnshire Wolds. The other main rivers of the Humberhead Levels are the Idle, Torne, Don, Went, Aire, Foulness and Derwent. In total, all these catchments drain 26,000 sq. km, around 20% of the English land surface. Most of the rivers were originally tidal, although this influence is now foreshortened and somewhat regulated.

As “a veritable land of rivers” (Gaunt 1994), the Humberhead Levels have a subdued geomorphology and generally minimal or no altitude. Much of the region lies below high tide level in the embanked Rivers Trent and Ouse. The Levels have always comprised a landscape naturally wet, even waterlogged. The meandering, sluggish rivers could not cope with heavy rains in their western catchments, and flooding was exacerbated by the tidal nature of the lower river courses. The landscape is maintained in its current state by channelized rivers, man-made flood-banks and powerful pumps. The Humberhead Levels have been transformed into a region of intensive arable farming, with wetlands and other natural habitats progressively reduced, fragmented or eliminated. The landscape familiar today is graphically illustrated by the vertical aerial photography of the so-called ‘Millennium Map’ of England (Getmapping plc 2001). The ‘Millennium’ coverage of the Humberhead Levels depicts, for mile after mile, a ‘culture steppe’ essentially comprising a micro-mosaic of fields. The significant exceptions to this are isolated, like the few urban centres, river mouths and the peat moors. They are overwhelmed when viewed from the altitude of the camera.

Thorne Moors comprises a floodplain raised mire complex. It has been reliant on precipitation rather than groundwater for irrigation throughout much of its 5000 year existence. It probably had a polyfocal origin, formed by the coalescence of separate mires into a single structure. Now, although severely reduced and damaged as a hydrological unit, Thorne Moors represents the largest extant area of raised mire in Britain. Hatfield Moors, 3 km to the SSW, is the second largest, and differs in having a sandy substratum and some characters of wet heath. Separated by a former arm of the River Don, their proximity belies very different substrates and starting points. However, both became components of a massive area of mires and other wetlands, much of it formerly designated as Hatfield Chase, extending across the lower Ouse, Don and Trent floodplains. This was essentially the Humberhead Levels south of the River Aire. At their greatest extent, these wetlands were surpassed in size only by those of the Fens and The Wash, with which they were to some extent contiguous through the Lincoln (Witham) Gap.   Although the wetland landscape of Hatfield Chase is now almost erased by human exploitation, the main surviving elements, the peatlands at Thorne and Hatfield, retain their individual characters. Further, both differ from other extant British mires by being ‘Continental’ in nature: western outliers having affinities with those of the Baltic lowlands. They are not, as long assumed, of the more usual ‘Atlantic’ types of oceanic Britain, merely situated at the drier eastern end of the wetness gradient. They are the sole surviving British examples of a largely non-British biotope. With the almost total destruction of mires in the Fens, Buckland & Smith (2003) observed that “the Humberhead Levels…provides the only remaining refuge for the unique floral and faunal associations of the eastern raised mires”.

At its maximum, the peat of Thorne Moors probably stretched over c.4000 ha, but at least eight centuries of reclamation and peat removal have reduced both that extent and the depth of what remains. Today, Thorne Moors amounts to 1918 ha, with a depth rarely exceeding 2.5 m and frequently much less. The peat still encompasses five continuous parish territories. Besides Thorne Waste, these are Snaith & Cowick Moor, Rawcliffe Moor, Goole Moor, all in South or East Yorkshire, and Crowle Moor in North Lincolnshire. Other former parish shares of the peat are now removed or flood-warped, especially in the so-called Marshland parishes to the north-east. Thorne Waste, lying close to Thorne, the largest peripheral settlement, has long been regarded in size and position as the leading parochial component. It still extends to 1309 ha, which is over 68% of the surviving area. The term Thorne Moors is applied to the aggregated whole, including the Lincolnshire sector, and since 1993 the county boundary has followed the line of Swinefleet Warping Drain.


 

2. EXPLOITATION AND CONSERVATION

For centuries, the peat of Thorne Moors was exploited manually, mainly as an unrefined fuel. Other uses emerged in the 19th century, especially moss litter as horse bedding. However, a decline in the 20th century was reversed when trade became increasingly sustained by horticultural demands. From 1963, this led to an expansion of exploitation by machines. In 1987, the existing methodologies (and coexisting vegetation) began to succumb to new technologies. These, referred to as ‘milling’, necessitated deep drainage, extensive devegetation, intensive mechanisation, and shallower peat removal but over much wider areas. Additionally, the production cycle was accelerated. The consequences of these changes were severe, with c.70% of Thorne Moors becoming cleared for milling. This was ecologically catastrophic, and Thorne and Hatfield Moors became prominent in the concern for Europe’s threatened mires. They formed the focus of campaigns to defend the last vestiges of lowland English peat from further industrial damage. These campaigns were underwritten by UK and international legislation and obligations, and agreement was reached in 2002 whereby Thorne and Hatfield Moors became part of a Government-funded buy-out for peatland conservation. Over almost the whole of Thorne Moors, peat winning had thus ended retrospectively with the 2001 season, with the last (on Crowle Moor) ceasing shortly afterwards.

From 1971, there has been an evolving conservation presence on Thorne Moors, with full-time staff from 1978. Until recent years, this was alongside the commercial peat interests. Much of the area is now managed by the Government’s nature conservation agency, Natural England, with other parts under the Lincolnshire Wildlife Trust (Crowle Moor) and the Carstairs Conservation Trust (Inkle Moor). Thorne Moors, along with Hatfield Moors, forms the Humberhead Peatlands National Nature Reserve, originally declared in 1995 by Natural England’s predecessor, English Nature. Although Thorne Moors is a shrunken, exploited and therefore degraded mire system, its enduring scientific importance and restoration capabilities were recognized long ago. The resulting European Commission designations are as a Special Area of Conservation (degraded raised mire capable of natural regeneration) and a Special Protection Area (breeding European Nightjars). These ensured its inclusion in the EC Natura 2000 network of prime conservation sites.

Hydrological management for mire restoration is the main conservation objective at Thorne, although with further development of the full range of wetland habitats. However, Thorne Moors is located in a climatically marginal area for the growth of raised mire. It is therefore sensitive to climate change, and requires constant water control to underpin the habitat manipulations. Yet, being a climatically-marginal site increases its informational value for science and conservation. Rodwell (2011) described Thorne and Hatfield Moors in their post-industrial era as showing “signs of change that are not only encouraging in themselves but which also provide us with a fascinating ‘outdoor laboratory’ where ecological processes, given protected time and space to unfold, can be followed”.

Thorne Moors is a Special Protection Area declared in 2000 under the EC Conservation of Wild Birds Directive, for the nationally significant breeding population of European Nightjars. Both species monitoring and management are therefore focused on them, although several other vertebrates as of high interest. It is recognized that there is potential conflict here between nightjar habitat and mire restoration (rewetting). This heightens the need for vigilance, monitoring and flexibility, especially to optimize as much available territory for the birds as possible.


 

3. HABITATS

3.1  Botanical nomenclature

Nomenclature accords with the Check-list of Hill et al. (2008) for bryophytes and The South Yorkshire Plant Atlas (Wilmore et al. 2011) for vascular plants.

 

3.2 Habitat types

A range of habitats is found in the Thorne Moors area, and these can be broadly divided into four types. The first is intensive arable farming, based on high-quality root and cereal crops. This is based on flood-warp, except in the south-west where reclamation largely predated warping. The second is the curtilage of the former Thorne Colliery, with an associated spoil-heap and spoil-dominated wasteland. This is a hostile environment when dry with extensive stretches of relatively bare terrain, colonized by plants of disturbed ground and waste, some unexpected like Bee Orchid Ophrys apifera. Wet areas will support a number of species, notably Common Reed Phragmites australis. The third type is the peat moorland itself, and fourthly there is the unfarmed land between the peat and the fields beyond.

On Thorne Moors, most habitats are acidic and a direct consequence of past peat extraction and subsequent conservation management. However, it should be remembered that during the decades when the deer have recolonized Thorne Moors, the overarching presence of vegetation is only a feature of the recent past. More typical during the 1960s–90s were extensively drained, levelled and devegetated peat flats, allowing machines to create cuttings or scrape the surface. Purseglove (1988) described the “unforgettable spectacle” of Hatfield Moors, with “2,000 acres [800 ha] of totally stripped-out, black, gleaming peat: an area four times as large as Wicken Fen, equivalent to that of a sizeable town”. The situation at Thorne was no different, with green areas on the peat increasingly isolated and contracting. In exploited parts, a fugitive flora consisted of tenacious survivors in chance pockets, and species, like Sheep’s Sorrel Rumex acetosella ssp. acetosella, adapted to thrive on the excoriated surface.

 

3.3 Wetlands

The wetter areas on Thorne Moors are generally associated with recent habitat enhancement work. This is to establish or develop mire vegetation on previously-worked areas of exposed peat. There are, however, parts that remain as open water, although often heavily encroached by Soft-rush Juncus effusus. Wetland habitats on the peat are dominated by species like Downy Birch Betula pubescens ssp. pubescens, Cross-leaved Heath Erica tetralix, Common Cottongrass Eriophorum angustifolium, Hare’s-tail Cottongrass E. vaginatum and Purple Moor-grass Molinia caerulea ssp. caerulea. In addition, eutrophication has allowed the establishment of some invasive species. Stands of Common Reed are the most obvious example. Such nutrient enrichment has often been brought about by mechanical peat thinning and digging into the substrate. However, there can be other causes, like guanotrophication at colonies of Black-headed Gull.

Scattered throughout the wetter peat are water bodies, flooded depressions, wet overgrown peat cuttings, a long-derelict and now barely recognizable canal system, and an extensive network of dikes and drains. In these habitats, the aquatic vegetation can include emergent species, like Purple Small-reed Calamagrostis canescens, Reed Sweet-grass Glyceria maxima, Common Reed and especially rushes Juncus. Several of these may choke watercourses. Other aquatic plants are often less dominant, for example Common Marsh-bedstraw Galium palustre sensu lato and Water-plantain Alisma plantago-aquatica. Submerged ones include bryophytes like Feathery Bog-moss Sphagnum cuspidatum and Floating Hook-moss Warnstorfia fluitans, with flowering-plants like Broad-leaved Pondweed Potamogeton natans. Old overgrown drains, with their rotting tree roots, can be rich in bryophytes, like Silky Forklet-moss Dicranella heteromalla and Nodding Thread-moss Pohlia nutans, and also the aggressive alien Heath Star-moss Campylopus introflexus. More generally, hollows, abandoned cuttings and the old canals may host specialist mire species, especially the peat-forming bog-mosses, but also other specialists like Bog-rosemary Andromeda polifolia and Cranberry Vaccinium oxycoccos. Here, and on other wet areas, Common Haircap Polytrichum commune can be co-dominant.

 

3.4 Routeways and scrub

The trams and canal towpath on the peat, a legacy of the old extractive industry, were often based on, or reinforced  by, limestone chippings, and therefore include neutral or calcareous components like Common Bird’s-foot-trefoil Lotus corniculatus and Yellow-wort Blackstonia perfoliata. They are encroached by domineering species  like Bracken Pteridium aquilinum ssp. aquilinum, Silver Birch Betula pendula, Heather Calluna vulgaris and several grasses including Cock’s-foot Dactylis glomerata, Yorkshire-fog Holcus lanatus and Purple Moor-grass. Further species of these routeways are characteristic of open grassland and bare ground, including Rosebay Willowherb Chamerion angustifolium, Selfheal Prunella vulgaris and Tansy Tanacetum vulgare. The trams often exist on upstanding peat balks. These formed as the flanking resource was progressively removed, leaving the more permanent balks vertically isolated. Such balks and similarly dry tracks support plants often associated with heath, but again especially aggressive ones like Bracken and Heather. In 1989, work began on constructing Fisons’ Road, formed of limestone and other chippings laid on a membrane. This is extensive, and again incongruous plants – like Common Restharrow Ononis repens – appeared alongside acid-tolerant ones.

In all but the wettest areas, birch scrub is widespread. It is stunted where waterlogged or checked by fires, but can develop into mature stands if suitable conditions persist. In these latter parts, Bracken can be frequent. Other species like willows Salix are widely established in damper parts, mostly Goat Willow S. caprea ssp. caprea and Grey Willow S. cinerea sensu lato. Larger species include Crack-willow S. fragilis and White Willow S. alba. Scattered trees and shrubs also occurring on the Moors are as diverse as Scots Pine Pinus sylvestris, Pedunculate Oak Quercus robur and Aspen Populus tremula. In addition, Rhododendron Rhododendron ponticum was introduced in the 19th century, and sometimes forms dense thickets in the south-west and in the Durham’s Garden area.

 

3.5 Edge habitats and peripheral woodland

In undisturbed parts on the fringes of the peat, occasional wet meadows, fenny areas and flooded railway borrow-pits can become dominated by Common Reed, but collectively harbour fen and marsh plants like Meadowsweet Filipendula ulmaria, Wild Angelica Angelica sylvestris and Bulrush Typha latifolia. Aquatic species in the pits, often competing with Common Reed, include Amphibious Bistort Persicaria amphibia, Canadian Waterweed Elodea canadensis and Broad-leaved Pondweed.

The woodland areas and smaller stands at the peripheral peat interface comprise Pedunculate Oak, Silver Birch, Alder Alnus glutinosa, Aspen, willows, Sycamore Acer pseudoplatanus and a variety of other species, sometimes – like the Sycamores – deliberately planted. Of particular richness are areas of mature carr woodland, perhaps up to 80–100 years old, the most important of which are Pony Bridge Wood and – the largest – Will Pits. This latter was an unsuccessful stretch of flood-warp from 1934 that developed into willow carr, with drier birch woodland alongside. Of the main willow taxa there, Grey Willow is the most frequent, although the most impressive can be Hybrid Crack-willow S. x rubens. The significance of the willows is heightened by the epiphytic bryophyes and lichens that grow on them, largely as a result of improving air quality.

The outer margins of the moorland and contiguous vegetated habitats are often quite abruptly demarcated, being frequently delineated by a field-edge drain. The surrounding cultivated land is penetrated by drains and hedgerows, these latter especially of Hawthorn Crataegus monogyna. Overall, these linear habitats have declined and decreased, although there are notable hedges in the Inkle Moor and Durham’s Garden–Jones’ Cable areas.


 

4. REFERENCES

Anon. (1999) Natural Areas in the Yorkshire & the Humber Region. English Nature, Peterborough.

Buckland, P.C. & B.M. Smith (2003) Equifinality, Conservation and the origins of Lowland Raised Mires. The Case of Thorne and Hatfield Moors. Thorne & Hatfield Moors Papers 6: 30-51.

Gaunt, G.D. (1994) Geology of the country around Goole, Doncaster and the Isle of Axholme. Memoir of the British Geological Survey, sheets 79 and 88 (England and Wales). HMSO, London.

Getmapping plc (2001) England the photographic atlas. HarperCollins Publishers, London.

Hill, M.O., T.H. Blackstock, D.G. Long & G.P. Rothero (2008) Check-list and Census Catalogue of British and Irish Bryophytes. British Bryological Society, Northampton.

Purseglove, J. (1988) Taming the Flood. A History and Natural History of Rivers and Wetlands. Oxford University Press, Oxford.

Rodwell, J.S. (2011) Vegetation of the South Yorkshire Landscapes. In: G.T.D. Wilmore, J. Lunn & J.S. Rodwell (editors) The South Yorkshire Plant Atlas. Yorkshire Naturalists’ Union and Yorkshire & the Humber Ecological Data Trust, no place: 28-53.

Wilmore, G.T.D., J. Lunn & J.S. Rodwell (editors) (2011) The South Yorkshire Plant Atlas. Yorkshire Naturalists’ Union and Yorkshire & the Humber Ecological Data Trust, no place.

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