Mountains and Valley

For this painting, I tried replicating a Chrome theme I use. I'm learning how to compose pictures by blocking out values and I find that pictures of nature with its large scale mountains and trees are easier to compose with. When it's people and animals, it seems easier to get lost in the details. 

What kind of references do you prefer to use to learn/practice your composing skills? I'd love to hear from you. Thanks for stopping by!