Here I'm not referring to the beautiful city of Cairns in Australia, but to a lonely pile of rocks, often used to mark an event, landmark, or a route of sorts. Here are a few examples that I found in a Wikipedia article:

• They may mark a burial site, and may memorialise the dead.
• They may mark the summit of a mountain.
• Placed at regular intervals, they indicate a path across stony or barren terrain or across glaciers.
• The Inuit erect human-shaped cairns, or inuksuit as milestones or directional markers in the Canadian Arctic.
• In North America, cairns may mark buffalo jumps or "drive lanes".
• In North America, cairns may be used for astronomy.
• In the Canadian Maritimes cairns were used as lighthouse-like holders for fires that guided boats, as in the novel The Shipping News.
• In parks exhibiting fantastic rock formations, such as the Grand Canyon, tourists often construct simple cairns in reverence of the larger counterparts.
• They may be used to commemorate events: anything from a battle site, to the place where a cart tipped over.
• Some are merely places where farmers have collected stones removed from a field.

When hiking on and around Table Mountain you'll be sure to find a few cairns. We spotted this particular cairn in the middle of farmlands, and judging by the size of the individual rocks, it must have taken quite some effort to build it.