Before the poem even begins, the allusions in its title hint at a story of wild, transcendent freedom. Most famously, "Ariel" is the exuberant air-spirit from Shakespeare's The Tempest. But "Ariel" is also a Hebrew word meaning "lion of God"—and was the name of Sylvia Plath's horse! All these allusions prepare readers for what's to come: this will be a poem about a horseback ride that carries the speaker from physical pleasure to spiritual transcendence.
When the poem opens, the speaker is surrounded by the darkness of the early morning. Her first glimpses of the landscape around her reveal a “substanceless blue / Pour of tor and distances.” The craggy "tor[s]" (or rocky hills) seem to have become misty and immaterial, able to “pour” out in front of her. The imagery here suggests a blurry, dreamlike scene, and gives the impression that the speaker and the horse have moved from "stasis" to a rapid gallop.
But how do readers know the speaker is riding a horse? The short answer is that, at first, they don't! The speaker will only hint that she's on horseback over the course of the poem. Here at the beginning, it almost seems as if the speaker has been swept up on a gust of air—a true "Ariel." First appearing here as a kind of elemental spirit, the speaker's horse will come to symbolize the huge power of instinct and the natural world, a power that will eventually make rider, horse, and landscape into one being.
The poem opens with a series of sentence fragments. These short, broken lines create a sense of urgency, as if the speaker can't arrange her glimpses of the landscape into complete thoughts. And take a look at the enjambment here:
Then the substanceless blue
Pour of tor and distances.
That seamless movement between “blue” and “Pour” mimics the overwhelming rush of images that the speaker describes.
But the first and last lines of this opening stanza are end-stopped, accentuating the natural pause at the end of a line. These end-stopped lines slow the reader down, allowing the imagery to sink in. And because the end-stopped lines create a pause after both “darkness” and “distances,” they accentuate the slant rhyme between the two words—and draw attention to an internal rhyme between "darkness" and "substanceless." Another internal rhyme in “Pour of tor" similarly mirrors the speaker's sense that the landscape is rushing past her in an overwhelming torrent.