(1) A cone-shaped pile of stones built to mark a trail route in open areas where the trail tread is indistinct. Also known as a duck or a monument.
(2) A trail marker of rocks stacked in a way that makes it obvious they were intentionally put there (and not just a natural pile of rocks). Cairns are often used instead of blazes in places where there are few trees, such as in deserts and on bald hilltops and along ridges.
(3) A heap of stones, set up to look artificial, that serves to mark the trail route in dense vegetation, above treeline, or wherever the trail is indistinct.
(4) A cairn is an obviously human-made pile of rocks erected as a trail marker. Cairns are often used above timberline. Each should be tall enough and close enough to the previous one so it can be seen even in heavy fog and above fallen snow.
(1) Also known as: "Duck" or "Monument". A cairn usually has to be rebuilt every year.
(2) The cairn showed us the way. Without it the map would almost have been useless.
(3) If it quacks like a duck but looks like a pile of rocks it could be either an uppity cairn or a bird you don't want to meet in a dark alley.