Oh trust me I have. I've looked at just about all the swamp related pictures google has to offer. I'm mostly debating whether I want to do an easy background or a hard one.
He drew an outline based of a photo and used a compass to get the proportions right. Then he shaded it by using a sharpened mechanical pencil and making tiny sqiggly shapes. He made them darker and denser for the darker tones and lighter and more spread out for the lighter tones.
I met a guy at an art show that only used a fine point mechanical pencil that he sharpened on rough paper to do all his drawings. No blending stubs or anything. This is one of his drawings he posted online.
Incredible texture.