So Eden sank to grief
An allusion is a reference to another literary work. In this line Frost is alluding to the place of Eden described in the Bible. Eden is a place of perfection. By saying that "Eden sank to grief", Frost is showing that even a place as perfect as Eden cannot stay forever. She tries to hold on but can't. Throughout the poem, Frost is showing how things that are 'gold' or "good" do not last, things change, and even die.