Here I report a biostratigraphic analysis of 303 genera from 28 North American terrestrial mammalian families, in which all families contain members that are either extant or last appear in Pliocene or Pleistocene deposits. The distribution of these taxa within the Cenozoic rock record is used to evaluate proposed demarcations for the Flood/post-Flood boundary. A pronounced biostratigraphic break is expected at the Flood/post-Flood boundary since the final devastation and burial of pre-Flood nephesh creatures should be stratigraphically overlain by the arrival of post-Flood migrants. It is found that when the Flood/post-Flood boundary is placed at or near the Pliocene/Pleistocene boundary, then a significant number of mammalian genera (23%), and nearly every family (>96%), crosses this boundary. Because numerous boundary-crossing taxa would have to migrate from their North American pre-Flood habitats to board the Ark and return to their same continent of origin in the post-Flood world, it is unlikely that the Pliocene/Pleistocene boundary reflects the Flood/post-Flood boundary. Rather, the Flood/post-Flood boundary should be located below the Pliocene/Pleistocene boundary, at a geological location with a more pronounced biostratigraphic break.