Critics have questioned the unity of the poem, noting that the sea of the opening stanza does not appear in the final stanza, while the "darkling plain" of the final line is not apparent in the opening.[25] Various solutions to this problem have been proffered. One critic saw the "darkling plain" with which the poem ends as comparable to the "naked shingles of the world".[26] "Shingles" here means flat beach cobbles, characteristic of some wave-swept coasts. Another found the poem "emotionally convincing" even if its logic may be questionable.[27] The same critic notes that "the poem upends our expectations of metaphor" and sees in this the central power of the poem.[28] The poem's historicism creates another complicating dynamic. Beginning in the present it shifts to the classical age of Greece, then (with its concerns for the sea of faith) it turns to Medieval Europe, before finally returning to the present.[24] The form of the poem itself has drawn considerable comment. Critics have noted the careful diction in the opening description,[29] the overall, spell-binding rhythm and cadence of the poem[30] and its dramatic character.[31] One commentator sees the strophe-antistrophe of the ode at work in the poem, with an ending that contains something of the "cata-strophe" of tragedy.[32] Finally, one critic sees the complexity of the poem's structure resulting in "the first major 'free-verse' poem in the language