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Across 4. Any method of determining whether an event or object is older or younger than other events or objects. 6. A principle that states that geologic change occurs suddenly. 12. The largest division of time. 14. The standard method used to divide the Earth's long natural history into manageable parts. 15. A type of fossil that forms when sediments fill in the cavity left by a decomposed organism. 16. The trace or remains of an organism that lived long ago, most commonly preserved in sedimentary rock. 17. A mark or cavity made in a sedimentary surface by a shell or other body. 18. In plate tectonics, a boundary between two plates that are sliding horizontally past one another. 19. In plate tectonics, a section of Earth's lithosphere (crust and upper mantle) that moves around on the mantle.

Down 1. The name Alfred Wegener gave to the large landmass, made up of all continents, that he believed existed before it broke apart to form the present continents. 2. The second largest division of time. 3. The theory that the Earth's lithosphere is divided into tectonic plates that move around on top of the asthenosphere. 5. A principle that states that geologic processes that occured in the past can be explained by current geologic processes. 7. Any method of measuring the age of an event or object in years. 8. A principle that states that younger rocks lie above older rocks if the layers have not been disturbed. 9. In plate tectonics, the boundary between two plates that are converging, or moving toward each other. 10. In plate tectonics, the boundary between two plates that are diverging, or moving away from each other. 11. An arrangement of rock layers in which the oldest rocks are at the bottom. 13. The theory that continents can drift apart from one another and have done