Earlier this week, Rangers from Yosemite National Park shared a Facebook post warning hikers not to build rock cairns, and advising them to dismantle any they happen to find on their adventures. The post was accompanied by a video showing a Ranger kicking over a particularly tall stack of rocks that had been assembled by visitors.
"When used appropriately, rock cairns are great for navigation, safety, and delineating a new or hard-to-follow trail," they wrote. "In general, rock cairns should only be constructed by rangers and trail workers. Please dismantle and refrain from building rock cairns when you visit Yosemite."
However, not all National Parks take the same approach. Although building your own cairns is always a bad idea, you shouldn't always knock down ones that you find. In fact, the general advice is to leave them all alone.
What is a cairn?
A cairn (which comes from the Scots Gaelic word 'càrn') is simply a man-made pile of stones. It is typically used to mark a waypoint, or could traditionally serve as a memorial or burial mound. Some cairns also serve a spiritual purpose. They can be any size, and may be painted or plain.
Modern cairns are usually used to mark hiking trails, particularly above the treeline, in coastal areas. or across featureless glaciers where a person could easily become lost. They are a feature of several US National Parks, include Acadia, where they are used to delineate certain historic routes.
More recently, people have begun building their own cairns to share on social media, which is where things start to get tricky.
What's the problem?
Built sparingly for a specific purpose, cairns aren't a problem. The issue arises when people are constantly adding to them or building new ones for decoration. Sometimes it's a simple question of safety (tall piles of loose rocks could easily fall on children or animals trying to climb them), but there are also less obvious consequences.
As Yosemite's Rangers explained on Facebook, rocks provide an important habitat for insects and invertebrates, and moving them can leave small creatures without a place to shelter from predators.
Removing rocks can cause environmental damage, too. Last summer, Cania Gorge National Park told visitors to stop building cairns after Ranger Cathy Gatley came across a creek where every single stone had been assembled into a pile. These stones would usually slow the flow of water, so removing them could cause flooding further downstream, as well as erosion of the riverbed.
What are the rules?
The National Park Service (NPS) explains that every park has different rules regarding cairns, so it's best to check before you go.
Some parks (including Acadia, El Malpais, and Hawaii Volcanoes) have cairns that are maintained by staff to help hikers stay on the right path, and should be left alone. However, parks like Capitol Reef and Yosemite don't have any official cairns, and hikers who follow any they find could become lost.
Yosemite's advice on dismantling cairns is an exception. Generally, the NPS warns against tampering with any rock piles you come across, neither adding to them nor kicking them over.
"We ask that visitors do not disturb them, knock them down, add to them, or build their own, as that can lead to other visitors getting lost in the desert," said Karen Garthwait, spokesperson for Arches and Canyonlands National Parks, after Yosemite's Ranger shared their video.
"We also ask that visitors not create their own sculptures out of the rocks that they find and collect."
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