Couched on the bald top of an eminence;
Wonder to all who do the same espy,
By what means it could thither come, and whence;
So that it seems a thing endued with sense:
Like a Sea-beast crawled forth, that on a shelf
Of rock or sand reposeth, there to sun itself.”
W Wordsworth 1807 The Leech Gatherer
An “erratic” is a boulder, often found perilously perched in a landscape, that has been transported hundreds of miles via glaciers traveling. It often has nothing in common with the local bedrock, but carries with it its own history in that it can provide a map for the historical movements of the earth.
The final casting of the rock, transported from northern Europe, transformed into a body.