Final answer:
The sign that looms over the valley of ashes in The Great Gatsby is a giant billboard with the eyes of Dr. T.J. Eckleburg. It symbolizes moral decay and the loss of spirituality.
Step-by-step explanation:
The sign that looms over the valley of ashes is described as a giant billboard featuring the eyes of Dr. T.J. Eckleburg. The eyes are blue and they seem to watch over the valley, which is a desolate area filled with industrial waste and the ashes of burned-out buildings. The sign represents moral decay and the loss of spirituality in the society depicted in F. Scott Fitzgerald's novel, The Great Gatsby.