Geologic Time Scale

• The method of reading the order of rocks is called stratigraphy
• Biostratigraphy is the study of the order in which fossils appeared and disappeared through time and rocks
• Fossils can help to match rocks of the same age through correlation
• Most radiometric dating measure the amount of isotopes in rocks using a mass spectrometer
• Index Fossil: lived in one period, easy to identify, abundant, found in many places
• Index fossils must be

  • Distinctive or easily recognizable
  • Abundant
  • Wide geographic distribution
  • Short range through time