These rivets appear to be in leather or at least something flexible. The back of the rivets have a "C" in the center, it might be kind of difficult to make out in the photo.
I find them around one of my sites with a little leather usually attached. My thought was from a saddle or or harness also. At one little area there , I most of dug eight or nine brass buckles and a stirrup. I think a saddle rotted out in that spot.