Grades 3–5 · Earth & Space
How can a dinosaur’s footprint become a bump in rock?
A footprint begins as a hollow made when an animal steps on soft mud or sand. If that shape survives, it can record a moment when the animal was alive. This kind of evidence is a trace fossil. It is evidence of an action, rather than a bone or another body part.
In the diagramA footprint can leave a stone castCross-sections through one hollow · not an animal bodyImpressionSoft sedimentFilled + buriedSediment layersCast exposedOlder mud goneThe surviving shape records a step taken by a living animal.
New sediment may settle into the hollow and cover the surface. The fill takes the hollow’s shape. Over time, both the older sediment and the filling sediment can become rock. The hollow is an impression; its matching solid fill is a cast.
Later, the older mudstone may wear away while the filling rock survives. What appears is then a raised footprint shape on the underside of the younger layer. A step that originally made a dent has left a bump. A track found in Denali preserves this kind of cast.
Most ordinary footprints are quickly spoiled by wind, water or another step. Fossil tracks need suitable preservation conditions. Even when the maker’s body is absent, the surviving shape can reveal that an animal moved across that ancient surface.
About this explanation
Starting knowledge: Understand that a foot can make a hollow in soft sediment and sediment can later harden.
Sources
- National Park Service — Discovery of Dinosaur Tracks in Denali
- National Park Service — Trace Fossils