A symbol of the inevitable passing of time is Ozymandias. The fact that the statue is broken and the lands are barren shows that even him and his country have been conquered by time and that the most powerful people can not fight against time. The section where he says
'Ozymandias, King of Kings, is my name:
Look at my work, ye mighty men, and desperation! '
Because it contrasts and points this out, it is great.