Over long period of time, new layers of rock are formed on the top of old rock, so that the newest stuff is on the top and what we walk on! That means that the oldest rock layer would be the one the farthest down, very deep! It’s almost as if the deeper we dig, the farther we see back in time!

Unless the rocks have been overturned by folding, then the rock layer at the bottom will be the oldest. This is called the principle of superposition. This happens because anything that is laid down must be laid down from air or water, which is on top of any existing rocks, so the existing rocks must be older than anything being laid down now.

The first rule of rock layers is that the bottom layer is always the oldest. Then the other layers were deposited it on top of it at a later time.