Interesting etymology: "sullen" originally meant "solitary," but by the 14th century had picked up the meaning of "morose" or "gloomy," and now only means that. Hey, not all solitary people are gloomy!
I looked it up because Tolkein used the word in describing "sullen hills," meaning gloomy. It struck me as a cool word to apply to hills (rather than a person or a face).