Here are a couple of tangle patterns that are good for learning a little more about shading. It this case, it is 'cast' shadow, the shading that happens when an object blocks the light, and it's outline is cast behind it.
With Bender, the light hits from above, and the cast shadow is beneath. It gives the bars a 3D appearance.
With Dusk, the light is to the right and the cast shadow on the left. Notice how the shadow side of the tree is also dark. Try playing with how dark and how wide the shadows are. See what happens if you detach the cast shadow and draw it a little farther from the bars or trees.
Notice, too, that the bars and trees get smaller the farther away they are from you.