Why does each eon, era, period, or epoch not begin or end at a specific time?

What is a cordon and where is it found

What is the function of the somatic nervous system? connects the brain and the spinal cord to the rest of the body carry signals to the muscles and glands includes all nerves controlling the muscular system and external sensory receptors controls internal organs like the heart and bladder

Organisms that successfully adapt will leave to grownone of the above