
Sheep in Fog
The hills step off into whiteness.
People or stars
Regard me sadly, I disappoint them.
The train leaves a line of breath.
O slow
Horse the colour of rust,
Hooves, dolorous bells -
All morning the
Morning has been blackening,
A flower left out.
My bones hold a stillness, the far
Fields melt my heart.
They threaten
To let me through to a heaven
Starless and fatherless, a dark water.
Sylvia Plath
These are the hills that Plath travelled when she was taking her riding lessons. She rode a lazy old horse named "Ariel." Slow and methodic, hesitant to even trot. But mindful of a nice view.

She would mount Ariel and ride sluggishly I am sure, given that horse's age and temperment, up the hill to possibly obtain the summit of the tor. Or maybe not. Tired old steed!
The tor is a metaphor. Back in her day, it would not have been assumed that a woman of even Plath's monumental talent could achieve success.
The sheep are probably very much like those that inspired the poem. They were photographed in Devon where Plath once lived.