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.
Once these age relations were worked out, another principle fell into place - the principle of fossil succession.
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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.
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.
He concluded that process that are currently operating on the Earth must be the same processes that operated in the past.