How does Susan Cooper use imagery in The Dark Is Rising?

Examples of Imagery:

"And though he did not know why, as he stared at the fierce, secret lines of that face, the world he had inhabited since he was born seemed to whirl and break and come down again in a pattern that was not the same as before." (The Sign-Seeker, p. 34)

"But always the Dark was there, swelling and waning, gaining a new Lord of the Dark whenever a man deliberately chose to be changed into something more dread and powerful than his fellows." (Betrayal, p. 107)