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Prior to the late 17th century, geologic time was thought to be the same as historical time.
Archbishop James Ussher of Armagh, Ireland, 1654, added up generations from the Old Testament and determined that Earth formed on October 23, 4004 BCE.
Principles of Stratigraphy Stratigraphy is the study of strata (sedimentary layers) in the Earth's crust.
Thus, if the strata are later uplifted and then cut by a canyon, we know that the same strata would be expected to occur on both sides of the canyon.In the diagram to the right,the fault cuts the limestone and the sandstone, but does not cut the basalt.Thus we know that the fault is younger than the limestone and shale, but older than the basalt above.Once these age relations were worked out, another principle fell into place - the principle of fossil succession.We discuss the 7 principles of stratigraphy first and then see how these apply to fossils.A more modern way of stating the same principle is that the laws of nature (as outlined by the laws of chemistry and physics) have operated in the same way since the beginning of time, and thus if we understand the physical and chemical principles by which nature operates, we can assume that nature operated the same way in the past.Principle of Superposition Because of Earth's gravity, deposition of sediment will occur depositing older layers first followed by successively younger layers.The goal of this lecture is come to come to a scientific understanding of geologic time and the age of the Earth.In order to do so we will have to understand the following: To better understand these concepts, let's look at an archeological example: Imagine we are a group of archeologists studying two different trash pits recently discovered on the Tulane University campus and at the Audubon Zoo (where they all aksed for you).Principle of Uniformitarianism The principle of Uniformitarianism was postulated by James Hutton (1726-1797) who examined rocks in Scotland and noted that features like mudcracks, ripple marks, graded bedding, etc.where the same features that could be seen forming in modern environments.