Here on HAZ, there is lively discussion on the importance of cairns, their [ necessity, overuse, size, ] [ correct terminology], [ relationship with LNT ethics ], [ etc. ]. Despite the varying opinions, we all know a cairn when we see it, and this label serves as a repository for photos of cairns of all kinds.

Derived from the Scottish Gaelic term càrn, modern cairns are a man-made stack of stones erected as landmarks. They are found all over the world, and are commonly used in North America and Europe to mark trails or cross-country trail-blazing routes. They are especially useful in terrain above treeline, or other barren areas where a worn path to follow is difficult to establish. In North America, some cairns are called "ducks" or "duckies" because one of the rocks in the cairn protrudes pointing the direction of the route in the form of a "beak".

Cairns often mark trail junctions, noteworthy points such as mountain peaks, or obscured dangers. Some modern cairns are built for decorative or artistic reasons, or to mark a historical site or memorial.

There are historical records of cairns being constructed into Eurasian prehistory. Basically, there have always been cairns. Some cairns in current-day Canada and USA were built as much as 12,000 years ago by indigenous people to mark routes as part of game driving in their hunting pursuits.

Source
Wikipedia