Glacier Polish

Devil's Postpile, California

This formation is the top of an ancient lava flow. As the lava cooled it fractured into these 5-sided “posts”, the tops of which are pictured here. About 20,000 years ago glaciers moved over this formation carrying with them rocks and sand that acted as an abrasive to score and polish the tops of the posts. The direction of the glacier movement is still clearly visible.