Task: Sentence Ordering
Input (English): Steps: (1) A plant dies (2) The remains of the plant are buried very fast in sediment (3) The soft parts of the plant decay (4) The hard parts of the plant are replaced with minerals (5) The rock erodes (6) The fossil appears on the surface of the ground.  
Where does the following step fit into the given list of steps: The sediment turns into rock 
Output (English):  After step 4.