In Cormac McCarthy's novel No Country for Old Men, how does the author's use of imagery and symbolism create or reveal theme?

One example of an effective use of imagery and symbolism
in Cormac McCarthy’s novel No Country for Old Men occurs near the
end of Chapter I when Moss is hiding in the desert from a truck with armed killers in it
who are pursuing him:

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He flattened himself in the rocks. In addition to
the other bad news his thoughts ran to scorpions and rattlesnakes. The spotlight kept
rowing back and forth across the face of the ridge. Methodically. Bright shuttle, dark
loom. He didnt move.

The
opening sentence here foreshadows Moss’s efforts to hide himself throughout the book.
His hiding here, then, symbolizes his constant hiding later in the novel. The fact that
he hides himself by lying on rocks can be taken to symbolize the pain and discomfort he
will suffer as he tries to conceal himself later in the book. If he were hiding here by
lying on soft sand, his life would seem both literally and symbolically softer and less
hard. His fear of scorpions and rattlesnakes symbolizes the dangers he faces here (and
will face later) from natural threats. However, such threats will prove far less
dangerous to him than threats from other humans. The constant alteration of light and
darkness here may symbolize the alteration of good (associated with Bell) and evil
(associated with Chigurh) in this novel. The idea that light can be dangerous is one of
the many paradoxes evident in this book. Finally, the allusion to a shuttle and loom
symbolizes the theme of fate that seems such a major motif in No Country for
Old Men.