In The Great Gatsby, why is the the Valley of the Ashes so dirty? What does it mean?

Nick's description of the valley of ashes is so poetic, eerie, and otherworldly that it can leave the reader confused. Ashes growing like wheat, forming into "grotesque gardens," and taking the forms of men? Is Nick, who is usually so specific and prosaic, describing a real scene?

The sense of unreality deepens when Nick goes on to describe "the eyes of Doctor T.J. Eckleburg." Evidently, these are on a billboard or a hanging sign, but Nick never says so. He describes the eyes as if they hover on an invisible face.

Below, I have posted a link to an article (GatsbysTracks) in which the author researches the location of the valley of ashes. Apparently, in Fitzgerald's day there really was "a gigantic trash-burning operation on the road between Great Neck ['West Egg' in Gatsby] and Manhattan." There really was a river, bridge, railroad, and even a store that sold signs. Fitzgerald would have been familiar with this place, and according to one biographer, the sight of it was an early inspiration for this novel. 

Nobody lives near a gigantic trash-burning operation by choice. People live in dirty, unlivable places only when they can't afford to live anywhere nicer. This is the situation for George and Myrtle Wilson, who live in the garage near the side of the road at the edge of the valley of ashes. They are barely surviving economically. Myrtle is discontented with their life; George is beaten down by her discontent. The dirtiness of the valley makes obvious the difference between the Wilsons' lives and the lives of the wealthier characters, who live in lovely East and West Egg, in their mansions with ocean views.  

Basically, the valley of ashes means poverty. It means the harsh realities of life from which the rich can, to some extent, insulate themselves, and the poor cannot.  

It is interesting to note that the richer characters in Gatsby cannot completely escape the valley of ashes.  They have to pass through it on their way to New York City, and Tom's choice to take a mistress who lives in the valley ultimately leads to some very fateful things happening right there in the valley of ashes, in front of George Wilson's garage. Tom, Daisy, and Gatsby might not live in the valley of ashes, but they still have to deal with it.

The valley might mean a little more than poverty. It might mean the brokenness in the world, fallen human nature, or even something worse. In the other link I have posted below (GatsbyDante), see the author's suggestion that the valley of ashes is supposed to give us the impression that the characters are entering hell itself.