Superposition (Nicolaus Steno, 1638-1687). Within any column of undisturbed rocks, the oldest beds are on the bottom and the youngest beds are on the top.

The "undisturbed" criteria is a very important one since in severe cases sequences of rocks can become overturned so that the youngest beds are on the bottom and the oldest beds are on the top. Geologists have come to recognize certain features within rocks to help them identify the tops and bottoms of beds. These structures are called geopetal structures.