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.