Cordilleran Section - 115th Annual Meeting - 2019

Paper No. 25-14
Presentation Time: 9:00 AM-6:00 PM


GLAZNER, Allen F., Dept. of Geological Sciences, University of North Carolina, Chapel Hill, NC 27599-3315

The Devils Postpile in eastern California is one of the world’s best examples of columnar jointing. Although columnar joints are known to produce 6-sided columns, several informal counts by students over the years have repeatedly found more 5-sided columns than 6-sided ones. To see if pentagonal columns really are more abundant than their expected hexagonal cousins, I made a detailed orthophoto and DEM of the upper surface. Contrary to student counts, hexagonal columns dominate the remarkably regular jointing pattern, with chains of pentagonal and heptagonal columns breaking the regular hexagonal symmetry.

The Postpile comprises columns up to 20 m tall that are exposed along the eastern side of the San Joaquin River. The 82 ka basaltic trachyandesite flow was apparently dammed by moraines or ice to a depth of 110 m or more, allowing slow cooling and full maturation of the joint pattern. Column tops form a rolling surface that was smoothed and polished by recent glaciation, providing an exquisite view of the set of columnar joints. A 3D model was constructed from 1274 hand-held, ground-level photos; resolution of the 35x12 m model is a few mm. Of 940 mapped polygons, 53% are hexagons, 27% pentagons, and 18% heptagons. Features that demonstrate highly mature joint development include: (1) columns are uniform in size across the mapped area, averaging 60±16 cm (1 s.d.) in width; (2) interior polygon angles cluster tightly, averaging 119°±14° (1 s.d.); (3) concavities (interior angles >180°) are absent; (4) Voronoi polygons drawn around polygon centroids reproduce the joint system with great accuracy. These qualities all contrast with less mature joint patterns such as those found on Racetrack playa in Death Valley.

Hexagons form chains and clusters that intertwine with complementary chains and clusters of pentagons and heptagons, producing a non-repeating yet organized tiling. Isolated polygons of either type are uncommon, a consequence of the inability to fill a hexagonal hole with a single pentagon or hexagon. The joint network forms a planar, connected, simple, nearly regular graph, and graph theory might provide insight into how nature produces such spectacular examples of self-organization.