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.^{[page needed]}

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.

Cairns built in the middle of enclosures can maximize the effect of daytime radiance, since reradiating absorbed sunlight from the stones raises nighttime field temperatures (very slightly) as in walled gardens. Since this effect would encourage crop growth (and potentially reduce the risk of frost) it may have been deliberately intended, though there is no direct evidence that cairns were built for this purpose.

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^{[definition needed]}.

The amount of stone which needed to be cleared from a field depended upon 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. Most surviving cairns are from the Bronze Age but some are considerably older, because field clearance has been practised since the beginnings of agriculture in the Stone Age (such as at the Gardberg site in Vestre Slidre Municipality, Innlandet, Norway).

Cairns may be discrete, in large groups (cairnfields) or as linear formations—linear cairns. Many stones in clearance cairns show plough-marks or ard-marks at various angles to each other, typical of the Bronze Age ploughing methods. These ard-marks indicate that the boulders were obstructing the ploughing process and were in situ for some time before removal.

Other items such as bog-wood and grubbed out tree stumps were sometimes added to the cairns as a result of tree felling and the process of creation of pasture from woodland. Bog-wood stumps and trunks can be thousands of years old, are darker in colour due to the tannins from the peat and do not show typical wood decomposition characteristics due to their preservation within peat bogs. The absence of saw marks indicates felling using axes, providing further information about the age of the wood involved.