How the geo strata are identified as time periods

To stick my foot in once more: William Smith and several other Englishmen did and wrote about what you're wanting about 190 years ago, Faith. Mr Smith, in particular, built canals for a living. He kept track of the rocks he found while doing this - a particular white-to-yellow limestone might show up at two spots, one 50 miles northeast of the other, and both times with, say, a grey sandstone above it and a black shale below. And the particular set of seashells found in each were very similar - but different from the set in a yellowish limestone that he'd dug through 50 miles to the southeast.Then Smith would go dig a canal elsewhere along the line of the first two above, and predict where he'd hit that same lime with the same seashell assemblage! And he'd never find that assemblage away from that particular line there in southern England, though Frenchmen might find the same set in a similar limestone in the Alps.I'll find the book tonight and get a detail or two.....

I'd be happy to, but it'll be somewhat slow. This $70 oil keeps us hoppin'.

But it can have a great deal to do with determining how quickly the rock formed, and under what conditions. A shale with individual grains all under a micron in size won't be able to form at all in significant turbulence: the particles will never settle. Diatomaceous earth or a limestone made entirely of coccolithophorids is much the same, and also have to wait on the critters they're made from to live, die, and settle to bottom.

Typically you don't need to know the initial ratios: methods like isochrons and concordia diagrams remove that number from the list of variables. Another instance where it doesn't much matter is in dating single zircons. When a crystal of zirconium silicate forms from molten rock, its crystal lattice will exclude any lead very efficiently, but will let some uranium in in the place of zirconium. If you find lead in a well-formed zircon, then, you can bet that it's there as a decay product of uranium, not as an original part of the grain.

Yessir. No more derail.