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					| Geologic Principles |  
					| Lecture 4 2/9/98
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					| |  | Last week we discussed what fossils are and the processes involved in their formation. | 
 |  | This week will look at a primary use of fossils, building the Geologic Time scale. | 
 |  | To start off the discussion we will look at some of the guiding principles of geology. | 
 |  | Three principles form the basis for stratigraphic and paleontologic studies. | 
 |  | These are principles, not laws. There are limitations | 
 |  | Development of the Geologic Time scale relied heavily on these principles and their limitations. | 
 |  | The present is the key to the past | 
 |  | Modern processes operating over long periods of time produced the earth features. | 
 |  | fossils or groups of fossils occur in distinct intervals of rock | 
 |  | Correlation is possible based on fossil assemblages | 
 |  | Subdivision of the rock record is possible based on fossil assemblages | 
 | Environmental considerations | 
 |  | Depositional environments | 
 |  | Laterally adjacent environments are stacked vertically | 
 |  | environment specific fossils | 
 |  | used to detail/confirm environmental conditions | 
 |  | environment independent fossils | 
 |  | used to delineate time units | 
 |  | Time units cross rock units | 
 |  | example: Beach progradation | 
 |  | point out where principles break down. | 
 |  | Rock units are based on environment | 
 |  | Time units are based on fossils | 
 |  | short period of existence | 
 
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