The undulating mountains look like an ink-wash painting.
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山峦 is more literary than 山 (mountain) or 山脉 (mountain chain). It evokes scenic, rolling, layered ranges - common in landscape poetry, travel writing, and descriptive prose.
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峦 stacks 亦 on top of 山 (mountain) at the base - picture peaks piled one above another. 山峦 = mountain upon mountain.