A cairn is a heap of stones that is conical or pyramidal in shape and that can range from a carefully stacked tower to a mountain. Cairns can be seen at the beach or randomly piled up on the unbeaten path of a hiking trail. The smaller cairns encountered today are often done out of idleness, but, historically, they have significance as the boundaries of a trail or as landmarks for voyagers or travelers. Additionally, they are used to mark burial sites or constructed as memorials or monuments having personal or spiritual significance, so maybe think twice about disturbing one.
Burial cairns date primarily from the Neolithic Period and the Early Bronze Age, and they are sometimes referred to as a barrow (from Old English beorg, meaning "mountain") or tumulus (akin to Latin tumēre, "to swell"), both of which designate a mound of stones or earth over the remains of the dead. Various members of a family or clan might be uncovered when excavating a cairn, or it might be a single important individual, such as a chief or leader. In either case, the body or bodies would normally be found in a vault (again, do not disturb).
The tradition of building cairns goes back many centuries and across continents. The word cairn, however, is a 15th-century English borrowing of Scottish Gaelic carn, referring to a heap of stones. The word may have also meant "horn" in the past, influenced by the Gaulish karn-on. If that is the case, the word would have the connotation of "a cairn on a mountaintop" ("a horn topping the mountain"). But this is etymological digging—not documented fact.