I am fascinated by cairns. Throughout the history of Homo sapiens, there is evidence that humans have created stacks or piles of rocks for a wide range of purposes. Cairns have served as guideposts to mark trails, boundaries, and locations where food or other valuables have been cashed (including geocashing in more recent years). Other uses have included memorial, commemorative, ceremonial, and astronomical purposes.
The cairn on the left is one I see regularly. It marks the high point of a popular trial in the Boise foothills. The cairn on right is part of a memorial I came across along the Boise River.
Cairns can also serve as an artistic or spiritual form of expression. It seems to me that this purpose gets to the heart of what it means to be human. There is clearly a meditative or zen-like quality associated with carefully balancing stones to create what is obviously an ephemeral work of art.
I photographed this woman building cairns along the shore of Wallowa Lake on July 4, 2019.
My wife an I discovered this cairn at the top of Marys Peak, a few miles west of Corvallis, Oregon. Marys Peak is the highest point in the Oregon Coastal Range.
The cairn in the upper right is a landmark on the trail to East and Hidden Peak in the Wallowa Mountains. The others are obviously balanced rock cairns I photographed at various locations.
In closing, I believe in the concept of "contrast and compare". Having provided some examples of cairns, I offer the image below as an example of a rock that is not a cairn. I suspect it may have some sort of spiritual significance.
Primitive rock art photographed in the hills above Ola, Idaho.