This is a great graphic on how fossil footprints are formed.
(A) Dinosaur Or Amphibian Steps In Soft Dirt or Mud.
(B) A Cast Of The Dinosaur or Amphibian Foot Is Formed.
(C) The Cast Is Covered Over Millions Of Years.
(D) Weathering Cracks The Rock.
(E) A Fossil Footprint; Concave Type.
(F) A Fossil Footprint; Convex Type.