Management of Archaeological Sites on Arable Land
Managing archaeological sites on cultivated land presents a particular challenge, since regular cultivation, or only one instance of deeper ploughing, can damage or destroy any hidden remains.
Nevertheless, although these sites have been damaged in the past, and many may be suffering harm now, the process is often partial and gradual. As a result, large numbers of sites under cultivation still remain of great importance and would benefit from action to halt or minimise the impact of ploughing.
All known sites are recorded on Historic Environment Records. Nationally important sites are protected by law from damaging work as scheduled monuments and consent is needed ahead of carrying out works. Local authority archaeologists will always welcome information about new discoveries.
Recognising archaeological sites and remains
Archaeological sites and remains come in many shapes and sizes. Some are easily recognisable, for example earthworks (such as ridge and furrow and burial mounds) and large monuments (such as hill forts). Many more cannot be seen. They are buried below the level of normal ploughing, surviving as pits, ditches and walls, and finds in the sub-soil. Common indicators include:
- scatters of finds brought to the surface by ploughing, such as pottery, burnt clay, flint tools (arrowheads), metalwork (nails and brooches), human and animal bone, and building stone
- patches of stony ground and building materials representing disturbed walls, roads or yards
- darker or lighter patches in a field representing the contents of buried features (pits, ditches and buildings)
- differences in crop growth caused by buried archaeological features
The growth of crops, especially cereals, can reflect the depth of soil and available water. Where there are pits and ditches in the sub-soil, the crops grow better because the soil is deeper and wetter. Conversely, reduced soil depth over walls and patches of stone, together with restricted water, can lead to wilted crops, stunted growth and early ripening.
Why is arable cultivation damaging?
Arable cultivation damages archaeological remains by:
- levelling out earthworks (ancient 'humps and bumps' visible above the surface of the field)
- cutting through and churning up below-ground remains
- eroding protective layers of soil
These problems intensify as the mechanical power of farm machinery increases.
Some types of cultivation are particularly damaging such as:
- cultivation of previously unploughed land
- stone clearance
- growing of root crops and deep-rooted energy crops such as elephant grass (Miscanthus)
Even regular cultivation to the same depth can result in damage to archaeological deposits. This occurs particularly where the management of an arable field, combined with factors such as slope and soil type, leads to thinning of the plough soil and increasing cultivation of the sub-soil.
Situations where this risk is greater include archaeological sites located on:
- light soils vulnerable to soil thinning from water and wind erosion, especially when coupled with deep cultivation and/or autumn sowing
- heavy soils requiring drainage, sub-soiling and deep cultivation
- peaty soils which are especially vulnerable to drying-out and shrinkage as a result of drainage and subsequent wind erosion
- the top or middle of slopes vulnerable to the down slope loss of topsoil
- land where compaction leads to the thinning of soil depth and loss of topsoil through run-off
How to recognise damage
It is often difficult to recognise damage as archaeological remains are hidden. As a general rule, if soil is eroding on an archaeological site, ploughing is the likely cause and damage is likely to occur. Look for fresh archaeological material lying on the surface and fresh subsoil. Both are signs that the plough has cut into previously undisturbed deposits. Examples of archaeological material that may indicate recent disturbance are substantial fragments of plaster or fired clay, large and unworn pieces of pottery, and intact metalwork.
How to prevent or minimise damage
Careful site management can avoid these problems and grant-aid may be available to improve management.
The best way to protect a ploughed archaeological site is to remove it from cultivation. Instead of cultivation, consider putting it down to permanent grass or long-term, non-rotational set-aside. This can also help to reduce soil erosion and provide a wildlife habitat.
Taking land out of cultivation, however, is not always a viable proposition and may not suit all arable systems. Nevertheless, there are steps that can be taken to minimise damage but remember that unploughed land can have buried sites that remain intact as little as 50–100mm below ground level. In order of effectiveness these options are:
- avoid tilling (and bear in mind that periodic ploughing to reduce compaction and facilitate water filtration might also be damaging)
- direct drill
- use minimum cultivation techniques
- maintain the current plough depth to avoid new damage on level land (this is unlikely to work on slopes)
If these options are not possible, these simple steps will give some protection to sites under cultivation:
- take particular care when introducing larger equipment that might increase the plough depth
- watch out for isolated in-field sites (such as burial mounds) under grass, since these are vulnerable to encroachment by ploughing
- avoid sub-soiling, pan busting, stone clearing or new drainage operations
- avoid growing potatoes, sugar beet, energy crops, short rotation coppice or turf
- avoid any harvesting operations that involve rutting, soil removal, significant soil compaction or soil erosion (to prevent soil erosion, follow the advice in Defra's Code of Good Agricultural Practice for Soil)
On known archaeological sites, take care with:
- farm tracks, fences or buildings
- irrigation or slurry lagoons
- vehicle access across sites, especially in wet weather
- ploughing up old pasture
- ploughing too close to standing monuments or earthworks
- planting new field boundaries or trees
- removing historic field boundaries
- digging new drainage ditches or draining wetlands
- altering the soil's chemical balance
- allowing scrub growth or animal burrowing to infiltrate sites (these ungrazed sites should be cut at a minimum two-yearly interval to prevent scrub growth and deter burrowing animals)