With that in mind, it follows that there are basic elements in a landscape that have these angles. They are:
1. The Sky. The sky is almost always the lightest value in nature, as the light from the sun radiates throughout it. This is true even at night.
2. The Ground. Being the flattest plane and directly perpendicular to the light source, the ground gets the most light and is therefore the next lightest value.
3. Slopes. Slopes such as mountains and hills receive the next most amount of light. Because they are slightly vertical, they don't get as much light as the ground.
4. Verticals. Trees and buildings often are vertical in shape and get the least amount of light.
In the below diagram you can see the basic ideas of the theory of angles and how it relates to the landscape.
And here are a couple photos where you can see this in practice in nature. Squint and you can really compare the values.
In the above photo, notice how the angle of the bale of hay (a) darkens as it gets more vertical. Also notice, since light is traveling through the clouds and they are considered to be part of the sky, even their shadows are lighter than the ground plane.
In the above photo, notice how the angle of the smaller trees (3) is different, and therefore lighter, than the trunks of the trees behind them.