Values in a Landscape, part 2

So, did you find all four value groups? They are:
light - sky
medium light - the flat ground
medium dark - the sloping hills
dark - the upright trees.
This brings me to John F. Carlson, a 20th century landscape painter, and his "theory of angles". According to Carlson, and I think he's right, the amount of light any object receives is determined by the "angle" at which it sits in relation to the source of light. The sky, being the source of light, is of course the "lightest". The (more or less) flat ground plane sits basically perpendicular to the sky, giving all it's surface to the light, and thus it receives the most light. Sloping hills sit at a deeper angle to the sky, and so they receive a bit less light. But the upright tree shapes give the least amount of their surface directly to the sky, and so they end up being the "dark" element in a typical landscape.
Why does this help us as landscape painters? If we understand these four basic "planes of light", then we can simplify our approach to painting a landscape by grouping all of our elements into one of these four categories and painting them as a "mass". I can know from the very start that, AS A GROUP, trees will remain a "dark" and I should choose colors and values that remain "in the dark range." A simple value structure will make a stronger statement visually also. I like to look at a black and white image of my paintings now, because taking the color away makes it very obvious where I've failed to keep the planes of light correct. Look at the second image (an older painting). It's a little jumbled looking and doesn't read as well in the black and white version, simply because I've mixed the values too much. The sky looks as dark as the ground. Some of the tree shapes are lighter than the sky! The color version of this painting is much better, because the color helps clue the eye, but I can guarantee that it would have been a stronger, more visually engaging, work had I corrected these value "errors". Compare it to the first image and you can see the difference when the values are kept in their correct range.
Next time: one of the most common errors in painting a landscape.