Arizona State Geologist Lee Allison has two posts on the destruction of highway 89. The first (here) shows an air-photo and cross section of the area. The second (here) shows a video of a news report on the area. The second post also has a link to geologist Wayne Ranney’s blog Earthly Musings in which he shows photos of many scenic views of the area. One photo in particular shows the geology of the area: the highway was built across an old landslide which was apparently reactivated. (See photo below, markup by Ranney).
In the photo, notice the white rock just above the red line and just below the yellow line. This is the Navajo Sandstone. This photo shows an ancient landslide that slid from the red line to the yellow line with displacement of about 250 feet. This occurred sometime before the highway was built in the 1950s. The highway goes along the toe of the ancient landslide, a very unstable position. It was this area than failed in the last few days.
This photo reminds me of a class in Engineering Geology on where NOT to build roads. It will be interesting to see how the Arizona Highway Department repairs the road. Will they build it along the toe of the ancient landslide again, or will they make a very expensive rerouting?
UPDATES from Arizona Geological Survey:
Bitter Spring landslide extends downslope from US89 Includes new video