Radioactive dating of rock layers
This document is partly based on a prior posting composed in reply to Ted Holden.
My thanks to both him and other critics for motivating me.
These are often characterised as the norm, rather than the exception.
In no way are they meant to imply there are no exceptions.This orientation is not an assumption, because in virtually all situations, it is also possible to determine the original "way up" in the stratigraphic succession from "way up indicators".For example, wave ripples have their pointed crests on the "up" side, and more rounded troughs on the "down" side.The simplest situation for a geologist is a "layer cake" succession of sedimentary or extrusive igneous rock units arranged in nearly horizontal layers.
In such a situation, the "principle of superposition" is easily applied, and the strata towards the bottom are older, those towards the top are younger.They are applied by geologists in the same sense that a "null hypothesis" is in statistics -- not necessarily correct, just testable.