Balanced Boulder in Glen Canyon

October 11, 2016



October 2016 Viewer's ChoicePhotographer: Patti Weeks
Summary Author: Patti Weeks

This gigantic rock appears to be precariously balanced in the National Park Service’s Glen Canyon Recreation Area, below the Vermilion Cliffs in northern Arizona. It tumbled down from atop a nearby cliff thousands of years ago in a process known as cliff retreat. The caprock is composed of Shinarump Conglomerate, a member of the Chinle Formation. It's supported by a pedestal of the softer, less resistant Moenkopi Formation. Eventually, the boulder will topple once again, when the faster-eroding lower rock is no longer able to support it.

The Shinarump Conglomerate is a mid-Triassic rock layer, formed about 200-235 million years ago, comprised of highly resistant coarse sandstone and pebble conglomerate. Shown at bottom is the underlying Moenkopi Formation, which consists of mudstones, sandstones and secondary veins of gypsum. This thicker layer was formed during the early Triassic period (about 247 million years ago.) Both of these formations are found throughout the Colorado Plateau, including the well-known Monument Valley in northeastern Arizona and southeastern Utah. Photos taken on August 20, 2016.

Photo Details: Top - Camera: SONY DSC-RX100M4; Lens: Sony 24-70mm F1.8-2.8; Focal Length: 15.52mm (35mm equivalent: 42mm); Aperture: ƒ/5.6; Exposure Time: 0.0025 s (1/400); ISO equiv: 125. Bottom: same except - Focal Length: 8.8mm (35mm equivalent: 24mm); Aperture: ƒ/4.0; Exposure Time: 0.0050 s (1/200).