Im guessing, but it was around 15/16 years ago when I went on a float trip down the Marias River here in Montana to hunt for fossils. Not a clue if we would find anything but fortunately we came home with some stuff. This one ammonite, Mariasensis turned out to be one very rare ammonite. I made a short little 1:30 minute video for anyone who would like to see it. Enjoy.
RB
https://youtu.be/bM_yZs1uvxI