The valley of ashes is bounded on one side by a small foul river, and, when the drawbridge is up to let barges through, the passengers on waiting trains can stare at the dismal scene for as long as half an hour. There is always a halt there of at least a minute, and it was because of this that I first met Tom Buchanan’s mistress.

F. Scott Fitzgerald

The Great Gatsby, Chapter 2. While riding on the train to the city with Tom, Nick tells us that Tom uses the train stop to meet up with his mistress Myrtle Wilson for a night filled with drunkenness and violence. The ‘ashes’ description symbolizes the valley’s moral depravity and social decline.