Footprints to Fossils

Graphic adapted from Lockley & Hunt (1995) by Geoscientist-In-the Park David Tarailo, sponsored by the Geological Society of America, GeoCorps Program, fall 2012.

Zion National Park is rich with information about ancient life, most of which comes from preserved footprints within the rocks. When a dinosaur or other creature steps in soft sediment, like a lake shore or a muddy river floodplain, its footprint is left behind, often with remarkable detail.

 

Graphics by Geoscientist-In-the Park David Tarailo, sponsored by the Geological Society of America, GeoCorps Program, fall 2012.

Imagine yourself, walking on the wet sand of a beach, leaving your footprints. These surface footprints, when discovered in hardened rocks, are known as the "true tracks," and are depressions in a rock surface, just like your footprints on a beach.

What you may not realize is that when your foot presses down the sand surface, the pressure also affects the layers of sand underneath. Your foot squishes those layers down, but preserves less detail than the true track on the surface. These squished layers under the true track are called "under tracks."

If the tracks remain exposed for a short while, the sediment may dry out and harden. The imprint may then act as a natural mold, and if it is rapidly filled in by other sediment, a natural cast may be created. These positive-relief casts are often found on the underside of overhanging rocks, much like under tracks, though usually with more detail.

In Zion, examples of true dinosaur tracks, under tracks, and natural casts have been found in the Moenave and Kayenta Formations.

Learn more about the dinosaur tracks in Zion

 

NPS photo / Bradley Markle

Dinosaur tracks are a type of trace fossil. Trace fossils are not the remains of animal body parts, but instead are features left behind by the activities of ancient animals. Footprints (like those in the photo at right) and insect burrows are examples of trace fossils that can tell us about life of the past.

Learn how other fossils are made

Return to the main Paleontology page