"Sheep in Fog" is a short poem of atmospheric imagery reflecting loneliness. Though Plath herself described the poem as a description of a horse's movement, the experience of reading it is less lucid.
The poem mostly lists seemingly unassociated images. The hills are sloping into whiteness, while "people or stars" regard the speaker sadly. She admits that she disappoints them. There is a train whose smoke trails off like breath. The rust-colored horse is slow, and its hooves are like "dolorous bells." That morning is growing blacker and blacker.
There is a flower that is left out. The speaker feels a stillness in her bones, and observes how the fields "melt [her] heart." Those fields threaten to allow her passage into a heaven that is devoid of stars and a father. It is a "dark water."
"Sheep in Fog" was written on December 2nd, 1962, but substantially revised in January 1963. The changes Plath made to the poem left it a much darker and more foreboding work; this is not surprising, given her despair over her failed marriage and the fact that she would commit suicide about a month later.
Despite the focus on atmosphere over clarity, the poem's setting was quite specific for her - the landscape around her North Tawton home. In a BBC interview, Plath described the poem quite lucidly: "In this poem ['Sheep in Fog'], the speaker's horse is proceeding at a slow, cold walk down a hill of macadam to the stable at the bottom. It is December. It is foggy. In the fog there are sheep."
Considering the pristine landscape and the presence of a horse, it is easy to compare this poem to "Ariel," and yet the tone is extremely different in "Sheep in Fog." The latter poem lacks a vibrant sun and a sense of hurtling momentum. Instead, everything is quiet, ominous, and slow. The white hills evoke sterility over beauty, and the speaker sees not vivacity but disappointment. The horse's hooves are like mournful bells, and the speaker feels "stillness" in her bones.
Above all else, the speaker is overwhelmed by the possibility of being brought to a heaven where she will know neither stars nor her father. Like "dark water," this realm will be dark, unending, and solemn. Where "Ariel" explored rebirth, this poem fears eternal stasis in a type of death. Even the writing is distinct - "Ariel" uses dramatic punctuation like exclamation points and question marks, whereas "Sheep in Fog" feels resigned, hardly directed. Certainly, it is not difficult to see foreshadowing of her suicide here.
Plath uses a couple of significant poetic devices in this poem. First, she uses personification, which was quite common for her. She says that the stars "regard me sadly," and that the train has "breath." Similarly, the fields are threatening her. All of this creates a sense that nature pities her, or finds her presence problematic. She does not belong in it. Secondly, she uses enjambment, a poetic element in which a single sentence is broken across two verses. Her discussion of the horse in the second stanza extends into the third, while the discussion of the morning is split between the third and the fourth. This technique is used to great effect here, as it links all of the disparate metaphors together to create a profound sense of estrangement and uneasiness. In other words, the haunting images are not different attempts at describing the same thing, but rather an overwhelming list of oppressive feelings.
The title of the poem references how Plath feels – she is like a lost sheep wandering in a murky and meaningless world. She feels like she continually disappoints those around her, all while she sees the world blackening. However, the promise of death - this dark heaven - frightens her. There is a paradox here, in that she calls this terrifying place a "heaven." The poem is certainly bleak and hopeless, but all the more sophisticated because it captures a profound ambivalence towards death even in its few lines.