Geochronologists do not claim that radiometric dating is foolproof (no scientific method is), but it does work reliably for most samples.
An early summary of them is found in Charles Lyell's .
In no way are they meant to imply there are no exceptions.
There are situations where it potentially fails -- for example, in cave deposits.
In this situation, the cave contents are younger than both the bedrock below the cave and the suspended roof above.
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.
The layers of rock are known as "strata", and the study of their succession is known as "stratigraphy".
Fundamental to stratigraphy are a set of simple principles, based on elementary geometry, empirical observation of the way these rocks are deposited today, and gravity.
his document discusses the way radiometric dating and stratigraphic principles are used to establish the conventional geological time scale.