Part of Hall of Planet Earth.
AMNH/R.Mickens As laval cools, the rock contracts. This causes most thick basalt flows to break into cylindrical vertical columns with a characteristic hexagonal cross section. These examples are from the Pomona flow, where columns can run up to 10 meters high and a meter across. This flow erupted from vents near what is now the Washington-Idaho border. Its lavas flowed down the Snake and Columbia River drainages all the way to the Pacific Ocean, some 600 kilometers distant. The Pomona may be the longest flow on Earth.
AMNH Collection