What can u tell from Undisturbed rock layers

Because the rock is laid down over time, the oldest rock (and fossils or whatever) will be in the lowest layer. As you go up, the age decreases - you are looking at younger rocks.

This doesn't tell you the * absolute * age of the rocks - you can't say from this evidence whether they are 100 million or 500 million years old. But you can see changes over history and you can get an ordering in time.