Photos

Devil's Postpile near Mammoth Mountain in California is one of the most amazing geological sites I have ever seen. It was formed less that 100,000 years ago when basalt lava erupted from a vent into this valley that was already filled by a glacier. Lava filled the valley from side to side for nearly three miles. At the site that became the postpile it was about 400 feet deep.

The lava flow began to cool at the top from the cool air and at the bottom from the cooler granite bedrock.
Cracks started to appear in the surface and they branched out at about 120 degree angles, which provided the greatest stress relief. Each new crack branched again until polygonal patterns were formed and the cracks continued to reach through the flow until they formed nearly perfect columns.

A subsequent glacier filled the valley again forcing some of the columns to bend under great pressure. It also exposed the sides of these columns and caused many to spall from the face leaving the pile of broken columns in the foreground. The wall is about 40 to 60 feet tall.