Emma R. answered 06/10/19
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What makes this line difficult to understand is the inversion of the sentence structure. Because the words are not in an order familiar to more modern readers, it is difficult to interpret the meaning. In order to interpret this particular phrase, you need the whole sentence: "Fair scenes, erewhile, I taught, a happy child, / the echoes of your rocks my carols wild." My best rephrasing of the core idea here, is "I was a happy child, who taught my wild carols to the echoes of your rocks." He is literally saying that, as a child, he taught the echoes of those rocks to sound like his favorite childhood songs ("carols"), which he probably sung very loudly and "wildly." What he is describing, less literally, is his childhood self wandering among the echoing rocks, singing loudly and wildly. It is a vivid and nostalgic description of a happy, imaginative child.
I hope this helps! I would love to discuss this poem with you more in-depth, if that would be of help.