What are some of the main symbols in Heart of Darkness?

This complex novel is indeed rich in symbolism. Some of the
major symbols include these:

• The Congo River: It is the
  river that takes Marlow deep into the African continent, carrying him away from civilization and
  into ever darker regions of an uninhabited wilderness. The river acts as a stream of
  consciousness as Marlow makes not only a physical journey but a psychological journey, as well.
  As he travels into the primitive Congo, he also travels into the unknown wilderness of his own
  psyche.


• Kurtz: In his brutal, self-obsessed state, Kurtz can be
  interpreted as a symbol of Sigmund Freud's id, the part of the human psyche that is most
  elemental, where instinct and primitive human impulses live. The id is not subject to conscience
  or social restraints, just as Kurtz's behavior is not held in check by any moral or social codes
  of behavior.


• The fence of human heads: This is a powerful symbol
  of Kurtz'sdescent into depravity; it symbolizes how completely isolated he has become from
  humanity.


• Ivory: The European traders' insatiable demand for ivory
  and their horrible, inhuman activities undertaken in pursuit of it make ivory a symbol of naked
  greed.


• The candle: As Kurtz lies dying aboard ship, Marlow goes
  to him carrying a candle. The candle can be interpreted as a symbol of Marlow's humanity. He
  despises the evil that Kurtz represents and the suffering he has imposed upon the natives, but
  Marlow finds compassion in himself for the dying man. The light of Marlow's candle contrasts the
  darkness of Kurtz's inhumanity and depravity.

There are
many other symbols throughout the novel, but these five play an important role in developing
Conrad's primary themes.