Name that era..

The terms familiar from the geological column are only the most basic elements of the story. But these names themselves are worth a visit. Some are derived from the type of rock that characterizes them, as with Carboniferous for strata involving lots of coal, or Cretaceous for the chalky strata. Others come from the area where the rocks were first described, as with the Jurassic (the Jura mountains in Europe) or the Permian (Perm in Russia). Another, the Triassic, is so-called just because it divides naturally into three sub-units. Best of all for any Welsh nationalists reading this are the first three. The Cambrian is named after the Latin word for Wales, while the next two, the Ordovician and Silurian, get their names from ancient Welsh tribes.