The Mill Canyon Dinosaur Tracksite lies in the Ruby Ranch Member of the Cedar Mountain Formation, a sandstone and mudstone formation in this region that is divided into the Yellow Cat, Poison Strip, and Ruby Ranch members. This rock formed about 112 million years ago, when limey mud on the shoreline of a large lakebed, fed by a nearby river, was walked on by dinosaurs and other animals. Over time, the mud turned to rock and preserved the footprints.