Tracks are formed when a dinosaur walked along a moist but firm, fine-grained sediment leaving an impression. The tracks then had to be exposed for a short while, allowing them to become drier and harder (and able to resist damage during subsequent burial). These hardened footprints are then gently buried with additional sediment. The most distinct tracks are formed when filled by sediments of a contrasting type (which allows the layers to separate when later re-exposed). After being buried for millions of years and subjected to pressure and head, the original sediment lithofies (turns into rock). Finally, the tracks are re-exposed in modern times by erosion or other forces.