To get the age of the sedimentary rock itself, the material dated has to have formed at the time of consolidation of the rock. The 'age' of an igneous rock refers to the time when the magma or lava from which it formed cooled below a certain temperature.
Consider a flat-floored valley in which a river flows. Then, the ages of the rest of the layers at B may be determined by counting down. Places satisfying these requirements are relatively few. Examples of radioactive parent-daughter pairs and their half lives include: Igneous rocks and highly metamorphosed rocks are the best candidates for radiometric dating because for them, for reasons that won't be discussed here, it can relatively easily be determined whether the initial amount of daughter element present was zero or, if it wasn't zero, what was the initial amount. The age of the lava flow is radiometrically dated as 1 million years old. However, deposition of varves may cease because the glaciers have melted away completely or because the lake basin has been completely filled by sediment.