Photo by Zooey
I think basalt columns are some of the most interesting geological formations on the planet.
During the cooling of a thick lava flow, contractional joints or fractures form. If a flow cools relatively rapidly, significant contraction forces build up. While a flow can shrink in the vertical dimension without fracturing, it cannot easily accommodate shrinking in the horizontal direction unless cracks form; the extensive fracture network that develops results in the formation of columns.
Go here for some really amazing photos of basalt columns all over the world.