At SGDS you can see in an hour what might take a paleontologist a lifetime to discover. You can even touch certain fossils and casts. For example, one “handling table” allows you to hold several ancient treasures. (These fossils were not found at St. George but were brought there to further its educational mission.) The first is a back vertebra from an Ornithischian, a plant eating dinosaur. Next to it is a tail vertebra from a Sauropod. And next to that is another tail vertebra, this time from Hadrosaur, a duck billed dinosaur. Where else but places like SGDS can you hold in your hand a 200-million-year old piece of the past?
SGDS is famous for trace fossils. These are different from body fossils which are, as the name implies, parts of an ancient animal. Trace fossils point to behavior and activity. Running, squatting, and even tail dragging can be deduced from tracks and traces laid down nearly 200 million years ago. As Brian Switek put it in a book review on the subject, “Whispers in stone, trace fossils are moments of ancient life.” [‘Dinosaurs Without Bones’ by Anthony J. Martin. March 7, 2014.]
Trace fossils at SGDS consist of molds and casts. Molds are impressions. Casts are relief. To explain, let’s say you wanted to make a beaver footprint cast down by a creek. With some plaster of Paris, you would fill in a track that left an impression or mold in the soil. Once dry, you would pop out the hardened compound. You now have a cast. A mold can also form when a body fossil decays or dissolves, leaving an impression. So how did all these molds and casts come to be?
How impressions formed is easily described. An animal walked, ran, or dragged part of itself across soft earth or mud. In the case of SGDS, these tracks were usually left along the shore line of prehistoric Lake Dixie. A dropping lake level then helped in the process, allowing a hardening of the mud. These impressions would remain if not weathered out or otherwise destroyed over geologic time. What about casts? We noted how drying mud left hardened impressions. As the lake level rose again it brought sand with it, filling in those tracks. Layer after layer of sand which eventually became sandstone. When sandstone layers are split apart at SGDS they reveal both impressions and casts. A split sandstone block at the site shows both molds and casts and detailed signage explains the process.