The Shadows.

In The Sound and The Fury, Faulkner makes a great use of symbolism. His use of the theme of incest makes much more clear his intention with the use of shadows in Quentin's section, especially.
Quentin is always aware of his shadow. He stands on the bridge and watches his shadow in the water. The shadow is drowning. Quentin plans on committing suicide but jumping into the river.
His shadow is the sin that follows him around. His sins, however, are imagined. Quentin has committed no sins. His sins are merely a shadow of his thoughts. He wants to have committed the sins because he believes it is the only way he could have saved Caddy. He wishes that he was impure because he loses Caddy because she has acted differently than what she is supposed to have done.
Quentin's shadow is also the South. His ideas of chivalry are especially prevalent. His actions towards the Italian girl are especially interesting. He constantly refers to her as "sister" and he attempts to make sure she gets back to her home safely. He attempts to just abandon her, but once he sees her again, he accepts that she is with him.
There are also the shadows of the writings that are being referenced by Faulkner. These references build a foundation for recognition. There are numerous biblical references in this section. Particularly, to Genesis and Revelation which represents both the beginner and the end. We are getting the beginning in this section because it is years before the other sections and one of the haunting parts of the Compson family. It is the end because it is last day of Quentin's life.