Geology. How do we know about rocks?

Question

I read the book “The Map that Changed the World” about William Smith and the first geological map.

I couldn’t understand quite a few things – what were his insights exactly? Did he just go around England looking at what rocks looked like in different places and then draw that on a map?

The author seems to suggest that he did not only that but formed some theory that allowed him to predict. Can someone explain?

Also – do rock layers appear in the same order around the world? There’s much I don’t understand. And what is an outcrop? Is that just visible rock?

In: 0

Answer 1

Last question first, yes. Mostly. A rock, often of bedrock sticking through soil.

You won’t get a different order unless there is a noticeable mixing of some sort, upliftment and intrusions can happen. But not all layers will be equally represented in all places. Some will have eroded or have other more local effects superceeding the global layering.

Different areas obviously have different deposition events. Somewhere where there is a sea allowing coccolithophore deposits will have different rocks than somewhere getting wind blown sand deposited at the same time.

So this rock layer may go A B C D whole another goes A C E F and another goes B C X Y but you’ll never find something going Y A D B. You’ll never find Pre Cambrian rock laid down above the KT boundary.

Smith did seem to just “look at and pick up” rocks, his insight was some rocks, even ones not connected to others, and that this sandstone with this fossil is the same rock layer as that one miles over there. Using his onsite he made the predictions about what layers would be found where, which validated his theory. This has been expanded from just the UK to the whole world. His methods of relative dating (relying mostly on superposition, ie the younger stuff gets put on the older stuff, and the presence or disappearance of a fossil) means the rocks can be dated relative to each other. Absolute dates through radiographic dating (like argon-argon, Pb-208, Uranium -Lead and even Zircon Crystal dating) allowed absolute dates to be established. This meant worldwide the rocks can be grouped by ages and tracked better.

The whole petro-chemical industry and mining in general relies on the insights some guy had noticing some rocks are the same.