Great rows and piles of bison bone and skull would have been a common sight upon the Great Plains towards the end of the 19th century, and McCarthy’s imagery of windrows and hills offers an appropriate description of a landscape that may have appeared to the eye to consist entirely of bone.
It is estimated that the skeletons of 178,500,000 buffalo were shipped east to be ground for fertiliser, paints and adhesives.
I saw in 1874, the year before the great buffalo slaughter began in earnest, a rick of buffalo bones, on the Santa Fé Railroad right-of-way, and twenty miles ahead of the track from Granada, Colorado, piled twelve feet high, nearly that wide at the base, and one-half mile long. Seven, eight, nine, and ten dollars per ton was realized from that alone.
(John R. Cook, The Border and the Buffalo)