Fill in the blanks :
1. The …………………….. period is the period for which no written records are available.
2. The period for which written records are available from which history can be written is known as the …………. period.
3. There are various methods of …………. .
4. There are many layers of ………….. deposited one above the other under the surface of the ground.
5. A rough estimate of how many years ago they existed can certainly be made using ……………. methods.
