I just happened to run across your description and I wanted to share a few thoughts. I'm a university paleontogist who specializes in track fossils. My guess is that these tracks are from the Fort Union Formation, sedimentary rock that covers large areas of Wyoming and Montana. It's the host sediment for the Powder River coal deposits. The age is Paleocene, approx. 65 million years old. In 1986, a young paleontologist, published a paper in the University of Wyoming publication Contributions to Geology, volume 24.The paper is titled "Paleocene bird and amphibian tracks from the Fort Union Formation, Bighorn Basin, Wyoming. Photos of the bird tracks show them to be in silty sandstone sedimentary rock that resembles the trackways you have found. Based on the age, these would be birds that were wading around not very long after the extinction of the dinosaurs. They are a great find, congratulations on your keen eye as a fossil collector. As far as I know, this is the first discovery of bird tracks in Montana.
George Mustoe
Geology Department
Western Washington University
Bellingham, WA 98225