By doing research on the layers of rock, we can find out about its past.

You stop right now and tell me how they date the layers of rock.

He hadn't gone far enough up into the solid rock layers.

A very hard layer of rock can form a temporary base level, until it is cut through.

Getting salt from different layers of rock in a mountain used to be a very dangerous job.

The cave's roof was a layer of rock just above her head.

Any living creature would show up, even behind a layer of solid rock.

No one reported seeing any material brought down, and I'm sure it isn't a simple layer of melted rock.

Another two hours passed, and the layers of rock had been reduced almost to the last barrier.

By doing the right maths, you can then work out where the different layers of rock are.