WATCH FOR FALLING ROCK A sedona rock fall
BRIAN F. GOOTEE, AZGS RESEARCH GEOLOGIST
“Watch for Falling Rock” signs aren’t everywhere you need them to be before a rock fall event happens. During the afternoon of June 4th, Enocha Ryan was conducting a healing session with a client at her sanctuary retreat at the base of Indian Point in Oak Creek Canyon, north of Sedona, Arizona. Suddenly everyone heard loud sounds, a mix between thunder and explosions coming from high up on the cliffs. Enocha went outside to identify the source and before she knew it a large boulder was coming right for her. Standing in front of her pump house Enocha ducked just in time to dodge the boulder by only a few inches. The boulder went right through the pump house and continued towards her home. The next victim in the boulder’s pathway was her oldest working hot tub. The boulder took care of that, too. And 20 feet down hill from the hot tub the boulder came to rest in her rose garden, without disturbing a single rose. The five-foot diameter basalt boulder lies there today.
Basalt Boulder of June 4th rock fall event with location map.
Apparently this basalt boulder spalled off a basalt lava flow capping Indian Point approximately 800 to 1000 feet above Enocha’s home. The hundreds of thick shrubs and trees on the hill slope slowed but did not stop the runaway boulder. Retracing the pathway of destroyed manzanita and juniper upslope, I stopped next to an ancient deposit of rock debris levied next to a wash, a debris-flow deposit. I explained to Enocha debris flows are a liquefied flow rich in rock and debris. She asked how common rock falls and debris flows are and if she should be worried. Looking across Oak Creek and pointing to other rock fall and debris flow deposits I explained to Enocha that such events are a fundamental process of erosion in canyons such as Oak Creek Canyon, but relatively uncommon at one location within one’s life span (that is if you’re not a victim of a rock fall). However, the actual events are not predictable with certainty. Enocha inquired about what can cause such events to happen; I explained that water saturation is usually the primary factor, but roots from vegetation, ice-expansion and even vibrations from helicopters (as one flew right over us) can contribute to failure of an already loosened slope. The take-home message of such events hopefully enhances our awareness of the hazards present in our surroundings and the risks associated with them. Fortunately for Enocha she survives to tell about a unique event where she knows the meaning of “Watch for Falling Rock” all too well.
ARTICLE AUTHOR: Brian F. Gootee Research Geologist
Arizona Geological Survey
Phoenix, AZ brian.gootee@ azgs.az.gov