Superposition is a relative age dating principle which states
Thus, in a sequence of layers that have not been overturned by a later deformational event, the oldest layer will be on the bottom and the youngest layer on top.This is the same principle used to determine relative age in the trash pits discussed previously.
Thus, we can deduce that the mudstone and shale are older than the rhyolite dike.
But, since the rhyolite dike does not cut across the shale, we know the shale is younger than the rhyolite dike.
In fact, sedimentary rocks are, in a sense, trash from the Earth's surface deposited in basins.
Principle of Original Horizontality Sedimentary strata are deposited in layers that are horizontal or nearly horizontal, parallel to or nearly parallel to the Earth's surface.
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.
Prior to the late 17th century, geologic time was thought to be the same as historical time.