How do fossils form layer by layer ?

Explanation

Fossils are seen layer by layer as follows : Some invertebrates on the sea-bed die, and are buried in the sand from about 100 millions years ago. More sand accumulates and gradually a sand-stone is formed under pressure.
Millions of years later, when dinosaurs living in the area die, then their bodies got buried in this mud too. This mud is also compressed into rock, upper layers of rock containing the invertebrate fossils from earlier times.

Millions of years later, the bodies of horse- like creatures dying in that area are fossilised in rocks above these earlier rocks.
Much later, by erosion of soil or by water currets some of the rock weathers and exposes the horse-like fossils. As we dig deeper, we will find older and older fossils.