The name "Devils Postpile" refers to a dark cliff of columnar basalt.
Radiometric dating indicates the formation was created by a lava flow at some time less than 100,000 years ago.
The source of the lava is thought to have been somewhere near Upper Soda Springs campground at the north end of Pumice Flat on the floor of the Middle Fork of the San Joaquin River, from where it flowed to the site of the Postpile.
Estimates of the formation's thickness range from 400 feet (120 m) to 600 feet (180 m).
The lava that now makes up the Postpile was near the bottom of this mass.

Because of its great thickness, much of the mass of pooled lava cooled slowly and evenly, which is why the columns are so long and so symmetrical.
Columnar jointing occurs when certain types of lava contract while cooling.

A glacier later removed much of this mass of rock and left a polished surface on top of the columns with very noticeable glacial striations and glacial polish.

The Postpile's columns average 2 feet (0.61 m) in diameter, the largest being 3.5 feet (1.1 m), and many are up to 60 feet (18 m) long.
Together they look like tall posts stacked in a pile, hence the feature's name.
If the lava had cooled perfectly evenly, all of the columns would be expected to be hexagonal, but some of the columns have different polygonal cross-sections due to variations in cooling.
A survey of 400 of the Postpile's columns found that 44.5% were 6-sided, 37.5% 5-sided, 9.5% 4-sided, 8.0% 7-sided, and 0.5% 3-sided.
Compared with other examples of columnar jointing, the Postpile has more hexagonal columns.
Another feature that places the Postpile in a special category is the lack of horizontal jointing.
The name "Devils Postpile" refers to a dark cliff of columnar basalt. Radiometric dating indicates the formation was created by a lava flow at some time less than 100,000 years ago. The source of the lava is thought to have been somewhere near Upper Soda Springs campground at the north end of Pumice Flat on the floor of the Middle Fork of the San Joaquin River, from where it flowed to the site of the Postpile. Estimates of the formation's thickness range from 400 feet (120 m) to 600 feet (180 m). The lava that now makes up the Postpile was near the bottom of this mass.
Because of its great thickness, much of the mass of pooled lava cooled slowly and evenly, which is why the columns are so long and so symmetrical. Columnar jointing occurs when certain types of lava contract while cooling.
A glacier later removed much of this mass of rock and left a polished surface on top of the columns with very noticeable glacial striations and glacial polish.
The Postpile's columns average 2 feet (0.61 m) in diameter, the largest being 3.5 feet (1.1 m), and many are up to 60 feet (18 m) long. Together they look like tall posts stacked in a pile, hence the feature's name. If the lava had cooled perfectly evenly, all of the columns would be expected to be hexagonal, but some of the columns have different polygonal cross-sections due to variations in cooling. A survey of 400 of the Postpile's columns found that 44.5% were 6-sided, 37.5% 5-sided, 9.5% 4-sided, 8.0% 7-sided, and 0.5% 3-sided. Compared with other examples of columnar jointing, the Postpile has more hexagonal columns. Another feature that places the Postpile in a special category is the lack of horizontal jointing.
Devils Postpile National Monument