We are getting photos from a variety of sources showing the slump or slide that closed US89 this morning.
Ron Schott posted this interpretation (copyrighted) of the slump on flickr, which shows that it is part of a much larger ancient slump/slide complex.
Phil Pearthree here at AZGS suggests that this is a particular type of landslide that involves rotation of a block of more cohesive material like sandstone over finer-grained deposits and is pretty common along the cliffs of the Colorado Plateau in this general area. They were called Toreva blocks by Reiche (1937), named after a community on the Hopi Reservation. Based on the better photos we have just received, Charles Ferguson, another AZGS geologists says this looks more like a failure in the fine-grained deposits below the cliff.
April 21, 2015
3 hours ago