text: Stratigraphy is the analysis of sediment layers and their correlation to time. Events in a stratigraphy are exceptional layers which were deposited due to a special occurrance - most commonly fire, but also think of the huge ash and lava deposits over Pompeii and Herculaneum, or the Iridium-rich layer on the end of the Jurassic Age commonly connected to the Yucatan meteorite impact.\n\nIn Archaeology or Palaeontology event layers are extremely valuable. Quite often they contain collateral damage of the event which can be dated to the event, and objects which normally wouldn't have been included in the sediment. Schliemann used the ash layers to distinguish several different stages of Troy. (Later research added more...)