"Molten lave and glacial ice shaped these unusual rock columns. Basaltic lava more than 400 feet deep filled this narrow valley nearly 100,000 years ago. As the lava cooled, cracks formed on the surface to release built-up tension. These cracks formed into hexagons, one of nature's most efficient and stable shapes. The Cracks deepened as the interior cooled to form basaltic columns. During the last ice age, about 20,000 years ago, a glacier exposed this cliff of columns and polished the top surface. The Postpile continues to be sculpted by weathering and earthquakes that break and change the formation."