Wind passes over a marsh, the tall grass bends and ripples, and the speaker sees a sea. The poem begins as a simple comparison, "ripples in the marshland grass / Like waves on an enchanted sea", but the "like" doesn't last. By the third stanza the grass is simply "so weird a sea," and the speaker is wondering what might be hidden beneath it. The poem follows a mind that starts by looking at a landscape, then lets its fancy loose on it, and finally comes up against something fancy cannot cross.
The form is tidy. There are six stanzas of six lines, each rhymed *ababcc*, in a mostly regular iambic tetrameter. That steady beat suits the subject, since it rocks like a swell or like grass leaning in the wind. The closing couplet of each stanza works as a small wave breaking, and it is usually where the poem turns toward something darker. Stanza two ends on "What things of the abyss may dwell / Only the fear hushed winds might tell." Some rhymes return like a tide. "Sea" ends a line in the first, third and fifth stanzas, and "green" appears as a rhyme word in both the second and fourth. The poem keeps coming back to the same two things, the water that isn't water and the colour that makes the illusion possible.
The imagery works largely by subtraction. This is a sea defined by what it lacks: "No spray," no splash, "turmoil neither heard nor felt," "No surges break," "foamless surf on sands unseen." Each denial makes the sea stranger, because a real sea would have all of these. The wind itself is a ghost, a figure with "trailing garments" who is "Invisible," and whose footprints are "evanescent," visible only as "shadowed highlight, trough and crest." The only evidence of the wind is a pattern of light and dark on the grass, a neat way of saying that the whole illusion rests on the play of light.
Fear enters quietly. The marsh's "coverts dank" are lurking places, and the line "Only the fear hushed winds might tell" is pleasingly ambiguous. The winds may be hushed by fear, or fear may have hushed them, and either way the speaker is left guessing. In stanza four the sense of hearing takes over from sight. The "elfin murmuring" is "so vague" that the speaker half-wonders whether it is the "far off pound" of surf. The question, introduced by "O can it be," shows the speaker deliberately talking themselves into the fantasy, though the sound is almost certainly just the grass.
The fifth stanza changes the pronoun from the observer's detached view to "we," and the poem begins to sail: "we who sail that eerie sea / Go drifting on a tide of dreams." What carries the travellers is no longer wind or water but "undulating beams / Of sun, dim litten, or the moon / That cringes o'er the farthest dune." The word "cringes" is striking, since it makes even the moon seem nervous about what it looks down on. In the last stanza the sea is "ever changing light," and the speaker admits "whence it bears us no one knows."
The ending is the poem's best moment. We have been sailing, but the final image is of a shore, and the one standing there is not the voyager but "fancy, yearning to explore." Beyond the shimmering green lie "wild chasms of the night," and imagination, which has been so bold, "Pauses, aghast." The verb is held back to the start of the last line, and the poem itself seems to stop short on it.
Its diction is old-fashioned ("adown the lea," "litten"), and it won't surprise anyone who has met a nature poem before. What gives it interest is the discipline of its single metaphor, held for thirty-six lines, and its honesty about where the metaphor leads. An ordinary stretch of grass is enough to start a dream, and the dream is real enough to frighten the person who is having it.