Basalt columns along US Highway 26

This exceptional example of columnar basalt pillars is along US Highway 26 about midway between the Painted Hills Unit and the Sheep Rock Unit of John Day Fossil Beds National Monument. Columnar pillars form when lava cools fairly evenly. They are particularly common in basaltic lava flows from large fissure eruptions. As the lava cools and crystallizes, its volume shrinks. The newly forming rock cracks under the tensional forces generated by shrinkage during cooling. The cracks typically form in the most efficient geometric pattern, hexagonal pillars, which tend to be fairly uniform in shape and size throughout the lava flow (Spry, 1962). Here a stream has cut through the lava flow, revealing the polygon-shaped pillars.