You can see the pile is in front of a processing plant (it's in the background). Bison bones were ground up into bone meal fertilizer or charred to make bone black, a substance with several uses (one is refining sugar, bone black can be used to absorb impurities and help give the sugar it's white color).

Buffalo Nation: History and Legend of the North American Bison by Valerius Geist has a section discussing the collection and processing of bones.

I won't be able to provide a direct link to the source (because the search engine looks to be down), but this pile of bison skulls was photographed while waiting to be ground into fertilizer. It's a picture from the Burton Historical Collection, housed at the Detroit Public Library. If the search engine comes back to life, I'll provide a direct link.