Change is part of Earth’s history. Stratigraphy records and saves these changes in rocks. Fossils reveal many extinctions through time. Geologists first felt these represented catastrophes which wiped our entire groups of organisms. Some felt this was evidence of biblical floods that caused all these extinctions. However, an extinction is more severe than just a one time disaster. For example, in the figure you can see that the river enters into an ocean. Organisms live there. A large monsoon rips up the bottom where the organisms live and kills them. The river then adds new sediment from the land and buries the creatures, preserving them as fossils. With time, the organisms can reestablish their habitat and live until the next storm event. This is not an extinction.