Principles of Stratigraphy Stratigraphy is the study of strata (sedimentary layers) in the Earth's crust.

Geologist in the 1800s worked out 7 basic principles of stratigraphy that allowed them, and now us, to work out the relative ages of rocks.

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.

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.

Sediment deposited on steep slopes will be washed away before it is buried and lithified to become sedimentary rock, but sediment deposited in nearly horizontal layers can be buried and lithified.

Thus rocks that we now see inclined or folded have been disturbed since their original deposition.By carefully digging, we have found that each trash pit shows a sequence of layers.Although the types of trash in each pit is quite variable, each layer has a distinctive kind of trash that distinguishes it from other layers in the pits.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 Inclusions If we find a rock fragment enclosed within another rock, we say the fragment is an inclusion.