Evidence of stream bank failure in the geologic record is surprisingly meager. Observations on modern stream bank failure and on ancient analogues suggest that bank failures most likely to be preserved in the stratigraphic record are those produced by large-scale shear failure—in particular, by rotational slumping.
Although the the frequency of bank failures is much higher along streams of alluvial valleys than along distributaries of deltaic plains, potential for preservation of these large-scale structures is greater in the latter environment. Preservation is apparently most easily accomplished by the type of rotational slumping in which base failure, rather than slope failure, is involved. Base failure is manifest by large slump blocks with surfaces of failure passing below the local thalweg and, therefore, below the depth of active stream scouring; preservation of at least subthalweg parts of slumped masses is, thus, theoretically possible. Areas of active scouring in which banks and thalwegs are cut in very cohesive silt- and clay-rich sediments favor the occurrence of deep rotational slumping and possibly base failure. Such conditions appear to be more common in low-energy deltaic plain or tidal flat environments than in alluvial valleys.
Consequently, fossil displaced bank sediments are predictably rare due to the somewhat restricted environmental conditions under which they can be preserved best, the actual low frequency of bank failure within these environments, and possibly an even lower frequency of true base failure in these same environments.