These are interesting, prehistoric gastroliths, found in the stomach cavities of dinosaurs, usually eaten by herbivores to help with the crushing and digesting of their foods.
Now, a little annoyed I didn't know about these when I did my abundant dinosaur hunting in the Drumheller regions, but - unless they were found in-situ, as in the space above, how would you recognize them? Especially given the abundant glacial/river gravels.
Another photo:
Link: Wiki on Gastroliths
And, on a completely unrelated note, did you know that Prospectors in Africa once hunted Ostriches for the diamonds in their Gizzard?
From FascinatingEarth.com:
For the ostriches that inhabited the plains of southwest Africa, choice of stone almost became their undoing. About a century ago a hunter shot a wild ostrich. In preparing it for the evening meal he cut open the gizzard and found several pure gem—quality diamonds among the stony contents. He set out early the next morning to hunt diamond—bearing ostriches. To keep such a find quiet is just about impossible, and word spread quickly. Within a week there was a grand rush onto the plains, and the slaughter began. Prospectors killed the defenseless birds by the thousands. Not all the victims contained diamonds, but some were fantastically rich; in one bird's gizzard 63 diamonds were found.
The ostriches were hunted almost into extinction in this part of Africa. The killing stopped only when too few survivors were left to bother with. The ostrich population has grown since the massacre, but an occasional potential diamond "mine" is still illegally brought down by the ever—present poachers.