Clearance cairn

A clearance cairn is an irregular and unstructured collection of fieldstones which have been removed from arable land or pasture to allow for more effective agriculture and collected into a usually low mound or cairn. Commonly of Bronze Age origins, these cairns may be part of a cairnfield (a collection of closely spaced cairns) where some cairns might be funerary. Clearance cairns are a worldwide phenomenon wherever organised agriculture has been practised.

By removing large and moderate size stones from the surface and sub-surface, ploughing can take place with much less potential damage taking place to the plough blade. Stones also prevent the growth of plants where they physically block access to the soil and their removal allows for a greater surface area for crops to grow and to allow for plants to grow to their full potential; Stones also increase drainage and may therefore deprive plants of moisture. Stones were removed from fields to allow for the efficient use of hand tools, animal powered machines on such fields in the past and for tractors, etc. in more recent times. Some prehistoric clearance cairns may also contain burials. A few clearance cairns may have been additionally created as boundary markers. As ploughing developed and specialised the greater depth tilled and the increased power of mechanised ploughing resulted in more and larger stones being brought to the surface and these too had to be removed.

Field clearance cairns sometimes survive long after cleared fields have fallen out of use and are often the only surviving evidence of past agricultural activity in areas of woodland, upland areas, etc. The existence of these cairns was once regarded as suggesting pastoral farming, many now consider them the result of arable or mixed agricultural exploitation enabling ploughing or hand tillage to work more efficiently.

The possibility exists that the stone piles were built in the middle of the enclosures, with an additional purpose of maximising the effects of daytime radiance; the holding in of heat caught during the day, and its release at night would increase the temperature of the field very slightly. This effect would encourage crop growth and reduce the risk of frost. Walled gardens produce the same effect.

Many clearance stones were used in the construction of defensive structures, houses, farm buildings, walls, drainage ditches, road metalling, etc. Where permanent clearance cairns were formed it was predominately on waste land, such as steep slopes, edges of woodland, field corners, and around earth-fast boulders.

The amount of stone which needed to be cleared from fields depended upon the local geology, glacial deposits, glacial erratics, etc. Many cairns are mainly composed of stones which could have been carried by a single person who would have followed the plough as it turned up small boulders, etc. Some surviving cairns are of considerable antiquity as field clearance has been practised since the beginnings of agriculture in the Stone Age, such as at the Gardberg site in Vestre Slidre, Oppland, Norway. A Bronze Age origin for clearance cairns is more common.

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