Cairns are small stacks of rocks, typically used as a marker for a hiking trail or a memorial. There are many synonyms for cairns, including rock piles, stone mounds, stone stacks, and rock towers. These various names reflect the different uses and aesthetics of these structures. For example, some cairns are so large they form a wall, while others consist of a single stone balanced atop another. Cairns can be found all over the world, from the hills of Scotland to the deserts of Arizona. They are a popular sight for hikers, who use them to navigate unfamiliar terrain.