What type of symbolism is in the book Sula?

A major symbol in Morrison's Sula is the birthmark that is over Sula's eye.  Shadrack says that the birthmark looks like a tadpole, and being an outcast himself in Medallion, it seems fitting that he defines Sula by different terms.  The townspeople, however, see Sula's birthmark as a sign of evil and justify their thought by citing examples of Sula's unconventional behavior.  It is possible that the novel's epigraph, an excerpt from Tennessee Williams's The Rose Tattoo, is linked to Sula's birthmark.  In Williams's play, the protagonist Serafina has much pride and a strong character.  She says that the people do not want to see glory in anybody's heart.  The quotation suggests that people do not want to accept others who are different, preferring to shun them as outcasts rather than trying to accept them as people.  This also happens in Sula, so the birthmark symbolizes Sula's relationship with her community.