The Sphinx / Rattlesnake Mountain Bouldering Area is a bouldering spot on Shoshone River in Buffalo Bill State Park, WY.
Find your way to the Hayden Arch Road Trailhead. It’s unmarked, but turn off the highway at the green "Bureau of Reclamation" sign and park just before the bridge that crosses the Shoshone River. From there, walk across the bridge and look for a small social trail to your right immediately on the right side of the far end of the bridge. The trail skirts the hillside and goes up to a service road (2ABW). From there, you’ll want to catch the third gully up to where the boulders are. Don’t expect an established-looking trail.