Mocoular Cue: Light and Shadow

A monocular cue is a depth cue available to either eye alone. One type of a monocular cue is light and shadow, which plays a part on how we perceive depth based on the amount of light or shadowing on an object.
In the picture below, light and shadow play a big part in depicting which tree is farther away. As our eyes move to the right of the picture, we see that the colors of the trees get lighter. From this, we are to assume that because the tree closest to the front is darkest that the lighter trees are farther away than the darkest tree in the front. Shading plays a role in this picture, too. Because some parts of the trees are lighter than some places, and darker in others, we can assume that the trees are 3-D objects instead of flat objects. 
 https://www.google.com/search?noj=1&biw=1280&bih=654&tbm=isch&sa=1&q=light+and+shadow&