Hexagonal Fracture Patterns On Navajo Sandstone Crossbeds At Yellow Knolls, Washington County

Abstract

At this geosite, the main features of interest—remarkably uniform and beautiful fracture patterns dominantly composed of linked hexagons—are present on outcrops of the Jurassic Navajo Sandstone. The Navajo was deposited by large, southward-migrating desert dunes about 200 million years ago, but the fractures that define the hexagons here are just a surficial veneer less than 20 inches (half a meter) deep. The fractures are a weathering phenomenon that developed under climate conditions similar to today’s. Steep thermal gradients develop in the sandstone because it is exposed to solar radiation and changing air temperature. Polygonal fracturing is present in other Navajo exposures in southern Utah, but only in non-bedded (homogeneous) rock. The beautiful, bedding-parallel fracture pattern developed here is very rare; it developed because the bedding planes in the rock at Yellow Knolls are unusually wide-spaced.