They are used to define periods Geological time.
        
             
        
        
        
Answer:
I don't understand you you now that I'm super girl 
 
        
             
        
        
        
Answer:
Keeping you down to the ground
Explanation:
hope that helps
 
        
             
        
        
        
Answer:
The ozone layer in the stratosphere blocks out the sun's deadly ultraviolet rays. It acts as our planet's natural sunblock.