An independent voice and a forum for debate for those who find Dartmoor a source of livelihood or inspiration

The Dartmoor Society Research Lecture, 8 November 2013

Exploring Dartmoor’s Skin: from Cut Hill to Hameldown, Drogo & Tottiford – the Soils of Chagford, Moretonhampstead & Beyond

Dr Tim Harrod

Some 70 persons eagerly assembled in the Dolphin Hotel to hear Dr Tim Harrod’s lecture. Notably and shamefully absent were any staff or members of Dartmoor National Park Authority.

Tim Harrod worked for the Soil Survey of England and Wales in south-west England from 1965 until its demise in 1987. He remained with its commercialised successor body doing research and consultancy until his retirement in 2001. The SSEW’s programme of mapping in detail the soils of sample OS 1:25,000 maps in each natural region fell victim to the 1987 cuts with only one survey in Devon remaining to be done, that representing Dartmoor, OS Sheet SX 68/78, around Chagford and Moretonhampstead. Retirement presented him with the opportunity to return to the kind of life he had so enjoyed before 1987 and to carry out the final detailed soil survey of the SSEW programme, an objective he felt was well worthwhile. There is the added benefit that its completion provides the missing link in the story of Devon’s soils, a contribution to both Dartmoor’s and the county’s environmental heritage. 

This lecture was his account of that soil survey, how and why it was done, what it revealed in the way of soil types, why soils matter, finishing with some examples of some unexpected but interesting findings. He began by thanking the Society for its generosity in financially contributing towards the costs of finalising the maps and report to publication standard and for offering this opportunity to explain to members what the survey was all about. The support of farmers and landowners, some of whom were in attendance, and who had allowed access to their land, was also acknowledged. 

The survey area of 20,000 ha, or nearly 50,000 acres, extends from Cut Hill in the west some 20 km (12 miles) eastwards to Heltor and Kennick. A line from East Mill Tor through Raybarrow Pool, Fingle and Clifford Bridges marks the northern limit, with the other edge 10 km to the south running from Horse Hole, through Merripit Hill and by Jay’s Grave, then on to Plumley. A detailed soil survey involves mapping and scientific description of the district’s soil types. The final report also presents information on what the survey means in practice and in terms that both laymen and scientists from other disciplines might find interesting.

To set the scene he described the major natural processes bringing about soil development. These include factors in the geological history of Dartmoor, its climate, vegetation and biological agents, water movement, man’s activities and time. Depending on how these circumstances conspire, different soil types predominate, a theme he demonstrated with a number of photographs. The district’s soils are largely gritty, often stony and light. Where the soil is freely draining and well aerated the soil and subsoil will be brown in colour. Waterlogging, however, excludes the air, turning the subsoils grey or mottled, while peat may form on the top should wetness increase. At first the peat development may be a few centimetres, but under perennially wet circumstance several metres of peat can accumulate. Acidity alone from plant litter or high rainfall also encourages peat formation. The severe acidity (low ph) associated with heathy vegetation dissolves the minerals, notably iron, that give the soils their colour, producing bleached subsoils overlying less acid and iron enriched, deeper subsoils, known to soil science as podzols.

In the drier enclosed country between the Moreton/Chagford area’s eastern limit and the moorland, soils with drab brown topsoils and brighter brown subsoils predominate. Old downland and commons, such as Pepperdon Down, have darker topsoils, while some waterlogged soils form in valley floors. Crossing the cornditch onto the moor often marks an abrupt change to dark, sometimes peaty topsoils. Much of the moorland, up to about 450m above sea level, has the distinctive podzols, often with a thin ironpan seam between the bleached layer and the brighter, deeper subsoil. On flat ground in these areas wetter soils with peaty tops replace the podzols because natural drainage is more sluggish. Above about 450m on all but very steep slopes, the peat thickens. At first it tends to be relatively thin but soon increases to several metres in places. Of the 1,800 measured peat depths 10% were greater than 3.3m (10ft), the deepest near Black Hill being 7.45m (24½ft). The peat of the high blanket bog is composed of fibrous remains of mosses and grasses and typically comprises over 80% water. The thinner peats below the blanket bog are more humified, less fibrous, particularly where their water table has been lowered by erosion gullying or peat cutting. Tors, clitter and scree were also mapped, as was the extensive disturbed ground associated with tin streaming and openworks, along with some very large peat workings between Statt’s House and Wild Tor.

Soils affect the kind and quantity of crops and vegetation, whether for the farmer, the forester or the ecologist. Much of the peat land has been a source of fuel for farms and villages around the moor for many centuries, as have the thinner peaty tops of podzols, which were cut for ’vags.  Soils matter to the environment as they both influence and react to water movement and modulate pathways for pollutants. Sensitive management of the soil helps reduce flood risk by avoiding compaction, while water quality is protected if erosion is avoided or countered. Our drinking water has to pass across and through the soil before it can be delivered to our taps. Patterns of soils are often reflected in the higgledy-piggledy shapes of fields and woods, with hedges sometimes following the edges of soil types, while it was shown how footpaths, in the past key elements of rural life, picked out the dry ground and avoided wet soils. Understanding of soils allows a fuller appreciation of the landscapes we all cherish, whether we earn a living from them, get our water via them, or just enjoy them for themselves.

Several unexpected findings came from the survey and were illustrated in the lecture. In places there are soils with very thick topsoils, some 60 cm (2 ft) or more deep. These may be of historical origin when potato growing was important, the soil being thrown up in ‘lazy beds’ (a method of arable cultivation consisting of parallel banks of narrow ridge and furrow which are dug by hand. The lazy beds can be up to 2.5 m in width, with narrow drainage channels between them).

In places the granite beneath is not the hard, enduring rock that built London Bridge, many local churches and other buildings. Rather its chequered geological past has decomposed it, sometimes to the consistency of a gritty cheese. This produces distinctive landforms with mound-like hills, closer spaced valleys and streams, along with more wet soils. On aerial photos taken at the right time these patterns are particularly distinct. While Dartmoor’s moorland is renowned for its tors, there are few shown on maps of the enclosed in-bye. This is partly because many are shrouded by woodland, with anonymous examples of sizes that put several named moorland outcrops to shame. The soil map reveals the locations of many such tors and clitters.  

Members’ answers to the mystery of how so many large established trees manage first to survive and then thrive perched on boulders or tors were invited. Photographs were shown of candidates as Dartmoor’s highest trees; a Sitka spruce near Wild Tor at about 540m and the oak tree in the rocks by the East Dart at the head of Broad Marsh, some 500m above sea level. The area contains a number of memorials. There is a plaque to the inventor of the jet engine, Sir Frank Whittle, near Chagford, an engraved boulder commemorates the poet laureate Ted Hughes, close to Taw Head, both men of international status. Others remember people little known outside their own circle, but whose memories have been clearly cherished. For Tim Harrod the plaque near Fingle Bridge to Siegfried Marian, a soil scientist, is particularly intriguing. Investigation revealed much about the minor wartime industry of charcoal burning in the Teign Gorge that he organised, along with other interesting facts about Marian, but that’s another story. Perhaps the most striking wartime relic the survey revealed, and sufficiently large to be shown on the soil map as an area of disturbed ground, was the bed of the railway line built by US Army engineers across Mardon Down near Headless Cross, as training for their sappers prior to the invasion of France.

Tim said, ‘I so enjoy seeing patterns in the landscape’, and closed the lecture by repeating his thanks to the Society for its interest and support and answered a number of questions from the members, both formally and in the more relaxed circumstances at the buffet afterwards. He found the warmth of the expressions of support for his efforts in making the survey, that came from both scientific peers and from lay people, both humbling and moving.  

A question was asked about soil in tinworking areas.Tim explained that disturbed ground will clearly show up, and tin mining tailings usually produce a sandy soil containing a high proportion of coarse fine quartz with occasional pebbles of granite and quartz.

Another questioner asked who in their infinite wisdom had decided not to train successors for this vitally important soil survey work? Tim said it was Michael Joplin MP who was Minister of Agriculture and Fisheries 1983–1987. He described the Soil Survey as an ‘expensive nonsense’!

In answer to a question about the effect of soil erosion on salmon eggs, Tim said that where the Fingle Brook meets the R.Teign at the Anglers Rest is an example of pollution. The brook has a clay catchment area, and it very easily produces a polluting plume. This material penetrates the ‘redds’ that salmon make in which to incubate their eggs, which are then deprived of oxygen, which leads to loss of reproduction.

Tim was also asked what he thought caused the gullying in the peat at Broad Down which he had illustrated. Regarding Broad Down and elsewhere, Tim said he was not sure of the exact reason for erosion of peat gullies but said that many of the gullies were ancient as, for example, Charles Vancouver in 1808 (General View of the Agriculture of the County of Devon) mentions crossing ‘frightful chasms’ in the peat on horseback. Tim had studied aerial photographs of Black Hill and its gullies taken in 1975, and compared them to the present-day pattern of gullies, and said there was little apparent difference. But he felt there needed to be proper photogrammetric study of these gullies through time.

Bud Young of the Landscape Research Group said that it is vital that Tim’s work is published with proper funding. This was endorsed by Tom Greeves who warmly thanked Tim Harrod on behalf of everyone present for his splendid lecture.