Thanks for your responses, billysbrown and tspuckler.

Tspuckler- I'm assuming by 'red-bellied snakes' you mean red-bellied races of Diadophis, like the lovely snake in your photo, and not Storeria occipitomaculata, correct? Head flattening is pretty common among natricines, though I can't recall seeing it in S. occipitomaculata.