The Bitter Spring landslide complex, where yesterday's failure along US89 occurred, has been recognized since at least early last century. Davis cited it in 1901. Gregory's 1917 USGS Professional Paper 93, "Geology of the Navajo Country" has this description on p.136:
The La Plata Formation indicated above, is now known as the Navajo and Wingate sandstones. Thanks to Steve Rauzi for passing along this reference.