A timeline from the past to the future superimposed on a fracture surface
This outcrop of the Aztec Formation is located in the Valley of Fire State Park, in the Mojave Desert of Southern Nevada. It is an eolian arkosic sandstone deposited in the Jurassic Period, about 170 million years ago. Thus begins the fracture’s ancestry.
The fracture surface of this red sandstone is largely covered by dark desert varnish, which is a thin layer of clay containing manganese and iron compounds that forms with the assistance of microbial activity on exposed surfaces of sandstone in desert environments.
Structures on the surface of this fracture show a complex propagation history. Fracture surface features are generally subtle structures that develop along fracture walls and can show directions and stages of their propagation. The prominent arcuate band on the right side of the image is the fringe, which usually forms when a propagating fracture “feels” the presence of a mechanical boundary – typically a bedding surface or a pre-existing fracture – in this case it shows the fracture propagated from lower left to upper right. The subtle texture in the dark area just above and right of the person’s head (you may need to zoom in) is a subtle “plumose” or “hackle” structure that looks like the texture of a feather and indicates local propagation of the fracture from right to left, opposite the direction indicated by the fringe. These contrasting directions of propagation show that the development of this fracture involved several steps that were not consistent in direction.
The figures on the fracture surface are petroglyphs created by Anasazi people who lived here from 300 BC – 1100 AD. They were formed by scratching away the desert varnish surface to reveal lighter-colored host rock. Although many images are recognizable, the meaning they were meant to convey is largely unknown.
Holes in the rock in the lower right quadrant of the photo are vugs that formed during heterogeneous erosive weathering of the rock, and have as much to say about its future as about its past. This beautiful outcrop is slowly disintegrating back to the wind-blown sand from whence it came, ultimately erasing the layers of history that it displays to us today.