In "The Great Gatsby", what is symbolic about the "valley of ashes", and "the eyes of Doctor T. J. Eckleburg"?

Taken together, these symbolize many things. The eyes are like the image of a forgotten god, looking over things. He should be able to monitor what's happening, but instead, he is empty and forgotten, and so things can go astray and end badly. That happens here, and it aligns with Gatsby's ultimate hollowness. The valley evokes a biblical feel, suggesting a place of death and testing that must be passed through. It also symbolizes the great and ugly gap between rich and poor, and between the sources of wealth and its final face…like Gatsby.