Two displaced rusty-brown-weathering dolostone (RBWD) members of the Cambrian Bright Angel Shale (BAS) are spectacular evidence for an unusual, ancient, large-scale landslide and cross-canyon runup in the Grand Canyon of the Colorado River, Arizona. Poncho’s Runup, named herein for a local river-runners’ camp, occurs along river miles ~135.9-136.8. The RBWD’s are tracers of the slide path; they have a characteristic weathering, and oolitic and oncolitic lithologies, not confused with other formations in the Canyon.

The participation of the RBWD’s in the slide resulting in Poncho’s Runup is puzzling. They normally crop out in the study area as two horizontal benches ~150-180 m above the River. During the slide event they detached and cut down through the softer shales of the lower part of the BAS, apparently as an intact knife-like sheet that crossed the River and up the opposite slope. The sheet is now represented by 5 erosional remnants that are plastered across the south slope, separated by young drainages. The remnants extend for 1.4 km parallel to the modern River, and laterally upslope as much as 500 m. In elevation they occur from the top of the Tapeats Sandstone cliff, ~60 m above the modern River, to near the top of the BAS, at least 200 m above the River. The rigid RBWD’s were pushed as much as 60 m in elevation above their regional in situ benches, captured the underlying geomorphology of the south slope, and created one of the highest cross-canyon runups (? 200 m) in the contiguous 48 States, The lithostatic load on the failed north slope bedrock is represented by the pervasive ~600 m-high Mauv-Redwall-Supai cliff.

The RBWD’S rode upon a 1-3 m-thick cushion of BAS, plucked river gravel, and Tapeats Sandstone. The moving sheet must have been confined by a thick cover of landslide debris that pressed it to the south slope, kept the fractured segments from dispersing, and created a 200+ m-high dam that was later largely swept away.

The slide has not yet been dated. However, the breccia-filled dolostone fractures are firmly cemented with post-event yellow calcite, suggesting Poncho’s Runup represents the unusual remnant of an ancient large-scale event that was early in the series of retrogressive slides that locally widened the north slope of the Canyon.