A cairn is a human-made pile (or stack) of stones. The word cairn comes from Scottish Gaelic. Cairns have been and are used for various purposes, dating back to from prehistoric times.
In modern times, cairns are often erected as landmarks, some national parks have even asked that hikers stop building them! It can take away from the natural landscape.
Cairns are used as trail markers in many parts of the world, in uplands, on moorland, on mountaintops, near waterways and on sea cliffs, as well as in barren deserts and tundra. They vary in size from small stone markers to entire artificial hills, and in complexity from loose conical rock piles to delicately balanced sculptures and elaborate feats of megalithic engineering.
Most recently, they have been seen as a marking to help you find your way (literally and figuratively.) They are thought to calm a person down and ease their worry.
Keep your eye out next time you are out in wilderness. Can you find a Cairn?