Wikipedia defines a cairn as "an artificial pile of stones, often in a conical form". Cairns can have a variety of uses and purposes.
They may mark a significant site, such as the summit of a mountain; or commemorate an event, such as a battle; or they may memorialize the dead (some UK Bronze Age cairns were found to contain small, square stone-built ossuaries or cists).
They may indicate a path - especially across stony, barren or featureless terrain.
They are created by Buddhists for use in religious rites; by Native Americans for cultural and sacred reasons, and for astronomical purposes; and by sculptors like Andy Goldsworthy as environmental art.
They may be formed quite simply and practically by farmers who want to clear their fields of stones (I've seen them in Wasdale in the English Lake District).
Cairns are just one of many different types of petroform. A petroform is a man-made arrangement of rocks or stones in the open air - it may be a stone circle, a dolmen or a menhir for example. The subject of petroforms and petroglyphs (carvings on rock) is a fascinating one - and one I hope to explore some time in a later post.
Between Santa Cilia and Puente la Reina (there are 2 Puente la Reinas on the Camino - this 1st one, Puente la Reina de Jaca, lies 5 km west of Santa Cilia) I passed hundreds of cairns along a path above the river Aragón (see photos) - constructed and added to over the years by pilgrims and other walkers and travellers. I felt compelled to add a few stones myself.
To what purpose? To give some concrete evidence of one's passing? To freeze in time a transient moment of one's particular Camino journey - and of one's life journey in general? To celebrate the beauty of the natural world and acknowlege its sacred nature? To express oneself by creating a primal piece of art as the early cave painters did in Lascaux and Altamira all those 10s of centuries ago? To remember and honour a significant person or event in one's life - similar to lighting a candle in a church? To build a primitive temple to the divine - a kind of spiritual vortex made of stone? I leave you to ponder...