Metaphor in Kubla Khan

One of the steps in harvesting grain is known as “threshing,” which is a process of separating pure grain from the husk, or “chaff,” using a flail. This process is the origin of the idiom “separating the wheat from the chaff,” which refers to any process of discerning valuable things from worthless things. The thunderous river of the poetic imagination produces “chaffy grain”: plenty of waste along with the occasional gem. The imperfection of the artistic process is one of the poem’s central themes.

These lines draw on a classic metaphor of artistic creation as sexual union and procreation—the “romantic chasm” becomes a womb-like image. The character of the “woman wailing for her demon-lover” gives the poem’s theme of the struggle of artistic creation a tone of sexual longing. The description of the river’s source as “a savage place” and the mention of the demonic lover mark a departure from paradise. The creative act is never ideal.

In the geography of Xanadu, the “deep romantic chasm” is the source of the river. Metaphorically, it is the source of poetic creation. In the poem’s imagery, it is clear that the act of creation immediately mars the ideal garden.

Coleridge subtly maintains the allusion to the myth of Alpheus and Arethusa. Just as Arethusa’s underground escape surfaces in the form of a bountiful fountain, the Alph of Coleridge’s Xanadu rises from the “sunless sea” to feed “five miles of fertile ground” and bounteous gardens.

The metaphorical material of the river continues here. Light and water are the two ancient metaphors for human thought. Water in the form of the sea is our fundamental metaphor for the unconscious mind—the soul, if you will—in all its depths. Light is our fundamental metaphor for conscious knowledge: hence, the “Age of Enlightenment.” Coleridge’s “sunless sea” is a perfect amalgam of the two, a representation of the inscrutable unconscious depths within each person.

The image of the “caverns measureless to man” connects the Alphean myth to one of the poem’s central themes. If the river is the flow of consciousness—a classic poetic trope—the river’s underground flow becomes a metaphor for the unconscious, the source of the creative imagination.

There is no literal woman here. Coleridge uses "woman" only as a metaphor to illustrate or illuminate the "ceaseless turmoil seething" that in the next line leads to the "mighty fountain" erupting.