Basalt Columns beside a Mountain Stream

Columns like these form when a thick lava flow cools slowly and shrinks, cracking into long pillars that are often six-sided, a bit like mud splitting as it dries. A clear mountain stream runs past a mossy wall of them in this basalt jigsaw puzzle of stone, water and fern. The best known example is the Giant's Causeway in Northern Ireland, where tens of thousands of them step down into the sea.