Clearance cairn

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.

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].

Natural deposits such as so called 'clitter agglomerates', moraines can also on occasion look rather similar to groups of clearance cairns, but their context, position and regular form usually betrays their true origins.

[7] Clearance cairn material is often heterogeneous, typically consisting of both edged and rounded small stones, sometimes mixed with sand, clay or silt, lifted from the field nearby.

[8] Clearance cairns constructed using mechanised methods with horse or motor powered assistance may appear superficially similar, however the stones are generally much larger and they are more randomly tipped rather than hand placed.

Due to the age and 'abandoned' state of many clearance cairns, they are often good sites for the growth and survival of lichens, mosses, ferns and other plants; the actual species being dependent on the rock type.

This clear evidence of previous ancient habitation and cultivation shows that the climate worsened until the farmers were forced to move to lower ground.