What does it look like and what does it represent?

Literally, the Valley of Ashes is a massive trash dump. The rotting valley represents the rotting American dream. Tom, Daisy, and Jordan, with their endless socials and superficial lives represent how material excess rots their very souls. Their lives, like the valley, are hollow. As time goes on the valley gets ranker as do the lives of many characters in the novel. Self-centered individuals litter the story with meaningless "trash", much like the valley itself.

About half way between West Egg and New York themotor-road hastily joins the railroad and runs beside it for a quarter of a mile, so as to shrink away from a certain desolate area of land. This is a valley of ashes—a fantastic farm where ashes grow like wheat into ridges and hills and grotesque gardens where ashes take the forms of houses and chimneys and rising smoke and finally, with a transcendent effort, of men who move dimly and already crumbling through the powdery air. Occasionally a line of grey cars crawls along an invisible track, gives out a ghastly creak and comes to rest, and immediately the ash-grey men swarm up with leaden spades and stir up an impenetrable cloud which screens their obscure operations from your sight.