That redwall was not fully lithified when the canyon was cut is easily
ascertainable at a number of sites in the canyon, but most notably on the
rim of the inner gorge just below O'Neill Butte, where a contorted slab of
Redwall sits perched on the rim of the inner gorge, frozen in time.
Apparently the Redwall was undercut in this area, resulting in slope
failure and a massive slab of the Redwall (about .5 km in diameter and
maybe 50 ft thick) slid down the Tonto slope toward the gorge where it now
sits. The nearest cliff of redwall is more than a mile away. The slab
itself consisting of a few beds was apparently semiconsolidated, because it
is massively brecciated, but recemented in place. This is clear evidence
(as are the hundred or so other places in the canyon where the same
phenomenon can be viewed on a lesser scale) for rapid cutting and erosion
of the section in the canyon. Other places that attest to rapid erosion
and massive displacement of the Paleozoic sediments are at Surprise valley
where a block 5 or 6 miles in length consisting of the entire Paleozoic
section has slumped into the canyon, and at the mouth of Red Canyon where a
similar large mass of the column has slumped riverward, although in this
latter case the fault is along a diabase dike, which may have facilitated
the process. The river was already deeply entrenched when this occurred,
as is indicated by the relationship of the slide block to the present gorge.
Art
http://chadwicka.swau.edu