quote:
According to this text book, many times rocks are simply dated by the fossils found in them to give relative ages. It shows a picture of quaternary rock sitting right on top of and next to pre-cambrian rock. It just makes me wonder if the pre-cambrian rock was pre-flood earth and the rock above it was only designated as quaternary deposits because of the modern fossils found in it.
That is because the pre-cambrian rock was at the surface and had no rocks laid on it before the Quaternary rocks were laid down, or younger rock had been eroded away. That does not mean all the rocks laid after the pre cambrian were all the same age. Today you will find rock of all different eras at the surface. Should the land subside and new deposits are laid down, that what you would desribe as an unconformity.
Generally speaking, the same fossils occuring in different rocks is a guide to say those rocks were of the same age. In the UK, the Lyme Regis cliffs in Dorset have Jurassic mudstones and limestone alternating in bands. This area is world famous for Amonites. you can see different species like they have been layered. As one species disappears as you go higher up, so another appears. So if you were to walk 2 miles or so and see the same species appearing in a layer of rock, you would know you are looking at the same aged layer from your start point.
™ Version 4.2
