The Hell Creek Formation Is North America’s Legendary Boneyard. See the Top Five Discoveries Found in the Iconic Fossil Bed From preserved plants to T. rex, the material found in these Late Cretaceous rocks has resulted in countless breakthroughs for paleontologists Mountains in Montana’s Makoshika State Park, where some of the Hell Creek Formation lies. Zack Frank / 500px via Getty Images In 1902, professional fossil hunter Barnum Brown excavated a stunning set of bones from eastern Montana’s Cretaceous period rocks. The remains were clearly from a large carnivore—far more imposing than the dinosaurs of the earlier Jurassic—but no one had seen a dinosaur quite like this one before. And it was far from the only prehistoric reptile peeking out of the rocks. The mysterious dinosaur kept clawing Brown back to the area. For years, he and his colleagues returned to find more fossils. He finally named the creature in 1905: Tyrannosaurus rex .…