Travel The stacks of rocks at national park trails are cairns and NPS wants you to leave them be July 13, 2023Kate Perez, USA TODAYComments Off on The stacks of rocks at national park trails are cairns and NPS wants you to leave them be If you're hiking along a trail in a national park and you see a rock pile, don't touch it: Cairns are often directional tools set up for hikers.