What is the valley of ashes in The Great Gatsby? How did it become this way?

We first see this phrase in Chapter 2:

"...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 gray cars crawls along an invisible track, gives out a ghastly creak, and comes to rest, and immediately the ash-gray men swarm up with leaden spades and stir up an impenetrable cloud, which screens their obscure operations from your sight."

The area is bordered by a "foul" river. The whole description should seem to you very gross and creepy, and you might suspect that there's a good thematic reason why the author bothered to describe it in such detail.

This "valley of ashes" is mentioned again in Chapter 4:

"...as we neared the city. We passed Port Roosevelt, where there was a glimpse of red-belted ocean-going ships, and sped along a cobbled slum lined with the dark, undeserted saloons of the faded-gilt nineteen-hundreds. Then the valley of ashes opened out on both sides of us, and I had a glimpse of Mrs. Wilson straining at the garage pump with panting vitality as we went by."

This second passage seems to suggest that the valley of ashes is also the "cobbled slum" filled with "dark" businesses clinging to the figurative layer of gold that characterized the 1900's. And though the "valley of ashes" is an actual place in the novel's setting, a dirty place where everything is literally and figuratively foul, some readers also interpret this "valley" more broadly and symbolically as a place where American values and morals have gone to die, and/or as a place where poverty and hopelessness are made manifest.

Why did it get this way, though? It's basically a heap of polluted air and land and water, a byproduct of industry. This makes sense thematically, too: the wealthy industrialists in the early 1900's seemed to care only about creating products in the quickest, cheapest ways so that they could earn the most money. They disregarded both the environment and the lower classes by allowing their factories to pollute the river and the valley. Because this is how the "valley of ashes" got this way, you can see how its interpretation as a symbol of social and moral decay is even more apposite.