What does the Valley of Ashes symbolize?

Before considering the figurative meaning of the ash heaps, one must define what they are:  the by-product of the industrial age, specifically the coal ash left from coal-burning factories.  The valley of ashes in 'Gatsby' lies between the affluent refuge of the Eggs (Long Island's Hamptons) and the busy excitement of Manhattan, signifying the human misery--think of the Wilsons--and environmental cost of the machinery that creates fantastic wealth for a few--people like Tom and Daisy--but leaves most choking on ashes.  The valley, therefore, is meant as a sobering foil to the dreamy opulence of the Eggs, but also as an omen for Myrtle and Gatsby--two doomed characters who pay the price for reaching for the dream