The bones were not near the surface when that deposition began; they were at the base of the sequence.

Reading that stratigraphy required knowledge of the local geology and geomorphology that Orr had spent years accumulating.

He knew the character of the sediments at different levels, the relationship between the alluvial deposits and the marine terraces above them, and the broad chronology of the canyon’s geological history established through his earlier work.

When he saw the bones at that depth, in those sediments, the geological story told him they were old.

The later chronostratigraphic work confirmed and extended Orr’s reading.