In Fitzgerald's The Great Gatsby, West Egg is the section that all of the new, young money lives in. In order to leave West Egg and head into the city, a person has to travel through what Nick calls "The Valley of Ashes." He describes it as follows:
“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 ash-grey 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."
The Valley of Ashes is an example of the classic "the rich get richer and the poor get poorer." It stands out because it is such a polar opposite to West Egg. West Egg is opulent and garish. It's extreme for the sake of being extreme. It's people showing off the money that they have, and the fact that the valley is right next to it makes the dichotomy even more apparent.
To make matters worse, some West Egg residents enjoy going to the Valley of Ashes so that they can go "slumming it." This is most evident in the character of Tom. He has a mistress, Myrtle, from the Valley, and Tom eventually drags Nick to meet her.
"he jumped to his feet and taking hold of my elbow literally forced me from the car."
It's clear that Nick is not thrilled to be in the Valley of Ashes, but what's telling about how the West Egg people abuse the Valley residents follows shortly in the text.
"It had occurred to me that this shadow of a garage must be a blind and that sumptuous and romantic apartments were concealed overhead when the proprietor himself appeared in the door of an office, wiping his hands on a piece of waste. He was a blonde, spiritless man, anaemic, and faintly handsome. When he saw us a damp gleam of hope sprang into his light blue eyes. ‘Hello, Wilson, old man,’ said Tom, slapping him jovially on the shoulder. ‘How’s business?’ ‘I can’t complain,’ answered Wilson unconvincingly. ‘When are you going to sell me that car?’ ‘Next week; I’ve got my man working on it now.’ ‘Works pretty slow, don’t he?’ ‘No, he doesn’t,’ said Tom coldly. ‘And if you feel that way about it, maybe I’d better sell it somewhere else after all.’ ‘I don’t mean that,’ explained Wilson quickly. ‘I just meant——‘"
Tom shows up at Wilson's garage and Wilson has a "gleam of hope." He's hoping that some West Egg money makes its way to his shop. Then Tom purposely strings him along about the car and makes it painfully obvious that Tom is only going to sell Wilson the car if Tom deems it necessary. Tom could care less, but knows that Wilson could really use it. Instead of being generous with the car, Tom deliberately uses it to be manipulative.
The last bit of evidence for The Valley of Ashes being symbolic of West Egg carelessness is the fact that Myrtle is hit and killed by a West Egg character in The Valley of Ashes. Myrtle is just another victim. A nobody that a West Egg resident can pour abuses on. In this case, the ultimate abuse.