How are rock types dated?
* Rock types are not generally constrained to a specific time, eg granites occur throughout geo logical time. Igneous (eg granite, basalt and andesite) and metamorphic rocks (eg schists and gneisses) are dated predominantly by radioactive methods.
Sedimentary rocks are generally dated by the fossil they contain using zone fossils (those which have an internationally determined age range). All rocks can be dated relative to each other to give a local self-contained chronology, using what is called the "cross-cutting relationship" of one rock to another.
Ivan Finney, associate lecturer, geology and sedimentary processes, Open University, Enfield, Middx.
* Buy them a few drinks and enquire after their modelling aspirations.
Vernon Levy, Leeds.