Nick describes the valley of ashes as being about equidistant between West Egg and New York City. He calls it
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.
Thus, we can imagine a place where everything is gray and ashen, covered with the detritus of industry. Moreover, everything is covered in the ash: cars, people, homes, the land, everything.
In addition, there is an old, dilapidated billboard, advertising the practice of one Doctor T.J. Eckleburg. As he is some kind of eye doctor, the billboard takes the shape of a giant pair of eyes. They are "blue and gigantic—their retinas are one yard high." They wear a pair of glasses, but no more of the face is represented. Nick speculates that the practice is long-closed and that the billboard has been forgotten.
Moreover, on one side of the valley, there is a "small foul river," and when barges go through, drivers on the road can be detained for several minutes at the drawbridge, allowing them to take in the depressing and gloomy scene. Finally, it is here, in the valley, where he meets Tom Buchanan's mistress, Myrtle Wilson, whose husband owns a gas station and car repairs business.