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Across 2. occurs when cells are replaced by minerals 5. ______ eon - Earth forms 7. naturally preserved evidence of animal activity (ex. footprint) 9. ______ eon - marine invertebrates appear 14. process of determining the order events happened in 16. scientists who study fossils 17. idea that the same geologic process that shaped the world in the past still happen today 18. principle that states younger rocks lie on top of older rocks 19. ______ period - dinosaurs go extinct 20. ______ era - age of the reptiles 21. ______ eon - early life forms emerge

Down 1. geologist who came up with the idea of uniformitarianism 2. ______ eon - current eon, complex animals and plants emerge 3. remains or physical evidence of an organism preserved by geologic processes 4. created by geologists to divide Earth's history into intervals of time 6. hardened tree sap 8. process of dating an object by determining the number of years it has existed 10. fossils of an organism that only existed for a short time and show up in limited rock layers 11. a surface that represents a missing part of the geologic column 12. ideal sequence of rock layers that contain all known rock formations and fossils 13. idea that geologic changes happen suddenly 15. ______ era - age of the mammals