Radiometric dating on rock is how long ago it solidified from being molten. Radiometric dating cannot be performed on sedimentary rock since it is ground down and recycled older rock, so radiometric dating would just get the age of bit of old rock tested. However, we can tell which layers are older than others by the order in which they are stacked. We can also establish dates for layers from igneous intrusions which bracket them in. Therefore we can determine the age of a particular layer.
But the sedimentary rock could be much younger than the surrounding igneous rock.
A flood could leave a sedimentary layer-cum-rock around some igneous rocks millions-billions of years after said igneous rocks were formed.
How do you figure a sedimentary layer managed to sneak UNDER an older igneous layer?
Between the sedimentary layer that contains a fossil and the igneous layer below could be hundreds of meters of other sedimentary rock layers ... which means that fossil-layer could have formed a very long time after the igneous layer.
But I guess general geological patterns emerge, which is where index fossils come into play.
Since sedimentary layers (of which I was seeing a lot) consists of older rock that had been ground up and recycled, exactly how are they dated?
I mean, if you date them directly, then you should get a much older age because they consist of much older rock, right?
Right. Furthermore, I imagine sedimentary rock could contain a mish-mash of particles of vastly varying ages - for example, some particles could be only thousands of years old while other particles could be billions of years old.