Columnar Basalt
Basalt is a fine-grained rock of volcanic origin.  It is an igneous rock, which means it has cooled from a molten state.
Columnar basalt is formed when a thick lava flow cools.  If the cooling is relatively fast, considerable horizontal contraction occurs and causes cracks.  Extensive fracturing forms columns.  Hexagons are the most common because they fit together efficiently without vacant space, but pentagonal, heptagonal, and octagonal joint patterns are also known.
The background photo is the top of Devil's Postpile
Some places you can see columnar basalt are:
Devil's Postpile, California
Devil's Tower, South Dakota
Giant's Causeway, Ireland
Samson's Ribs, Scotland
Snake River Canyon, Idaho
Click to return to the Devil's Postpile photo.