Relative dating with stratigraphy is based on the principle of
CC BY-SA 3.0], via Wikimedia Commons' src=" alt="The canyon is shown with many layers" width="3600" height="2400" srcset=" sizes="(max-width: 709px) 85vw, (max-width: 909px) 67vw, (max-width: 1362px) 62vw, 840px"/The Geologic Time Scale and the basic outline of Earth history were worked out long before we had any scientific means of assigning numerical units of age like years to events of Earth history.
Working out Earth history depended on realizing some key principles of relative time.
This is an example of the Principle of Lateral Continuity This image of Capitol Reef National Park from the ISS shows the Navajo Sandstone, a formation found throughout the western United States.
The rocks are a very light tan here, but can be a reddish color in places like Red Butte Garden in Salt Lake City or in Zion National Park.
This layer is laterally continuous, even though the intervening canyon separates its outcrops on either side by about 18 miles.
These terms may be used in this text and other literature and maps, but the basic unit for naming rocks is the formation Principle of Original Horizontality) and have not been disturbed very much since they were deposited except by a broad regional uplift (there are local exceptions).Lines correlating the strata with equivalent fossil content." width="228" height="167" srcset=" sizes="(max-width: 228px) 85vw, 228px"/ Wikimedia Commons, via Wikimedia Commons' src=" alt="Photo of the Grand Canyon showing expanse of canyon and the various rock layers" width="392" height="261" srcset=" sizes="(max-width: 392px) 85vw, 392px"/This view shows the South Rim separated from the North Rim by approximately 18 miles.The layers of rock lie on top of one another in order from oldest at the bottom to youngest on top based on the Principle of Superposition Sandstone.The discovery of radioactive An atom that has different number of neutrons but the same number of protons.While most properties are based on the number of protons in an element, isotopes can have subtle changes between them, including temperature fractionation and radioactivity.