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There are several immediate allusions in the connotation of "ROCK" (most of the world, and especially the legal world, draw their "reality" from the shallow realm of dennotation - fortunately, the world of song and of poetry draws its "multi-foleat-realm-of meaning" from connotation.)
One allusion is to the immediate, literal, rock of the mountain on which Mose's stood.
Another is to, Pietra, aka Peter... "The Rock, on which the church was built...."
And still another, (and most likely the closest interpretation) is to the proverbial "rock" (aka..."the building block that was rejected, became the cornerstone of a whole new world." ...Jesus Christ.
Poetry and Song and Scriptures have universal truths...you can clearly hear when they ring "TRUE."