The rock stacks were created by persons practicing the art of rock balancing. Essentially, it involves placing combinations of rocks in an arrangement whose creation appears to be physically impossible by natural forces. As you might expect, patience is a requirement in order to create some of the more improbable rock stacks that seem to defy gravity.
Some of the more famous rock balancing artists, such as Andy Goldsworthy, have created large free-standing rock structures, such as The Cone:
The Rock People of Bainbridge Island
On Bainbridge Island, along the Winslow shoreline, you can find half-a-dozen rock people. While these rock sculptures are held together by mortar, they show an amazing lifelike human form using a minimal number of rocks.
You can even find a rock person walking his dog!