CABLE MOUNTAIN ROCK AVALANCHE (RI-281)

By: Jessica J. Castleton and Ben A. Erickson

On August 24, 2019, at approximately 5:30 p.m., a large slab of Navajo Sandstone detached from a vertical cliff face of Cable Mountain above the Weeping Rock area, in Zion National Park, Utah. Utah Geological Survey (UGS) geologists conducted a site reconnaissance on August 26, 2019. An additional investigation including high-resolution photographs and video collected using a small unmanned aircraft system (sUAS), was conducted October 24–25, 2019. Data were also collected to help assess the stability of a prolific rockfall area that has repeatedly damaged the nearby Hidden Canyon trail.  The estimated volume of the rock avalanche was calculated to be 435,712 cubic feet (12,338 m3).  The rock mass was approximately 31,000 tons. The calculated volume and flow-like qualities of the event classify it as a very small rock avalanche. No specific triggering event could be identified for the rock avalanche event; however, it appears thermal cycling is a likely contributing factor.