Montana Stone Fly This post was last updated on May 12th, 2014 at 12:31 amMontana StoneFly Nymph The Montana Stone is an accurate representation of many different stoneflies across the US. As you can tell by the Name it is popular out west, but don’t discount this fly to be restricted to only the western states. It has its place in the eastern United states as well. Steelhead really like this pattern also.